choosing a bleed resistor

[the great waldo]

Hello All
I am slowly getting my power supply built (unregulated linear) I wll be using 50 volts after the rectifier 3 x 4700 uf 100v Capacitors. I was just wondering what the correct value for the bleed resistor should be. One post http://www.mycncuk.com/threads/7942-What-Toroidal-parts-do-i-need?p=62015#post62015 gave a formula that gave me about 350 ohms somewhere else on the internet made use of 2.2 k ohms. I was wondering what the pitfalls are using the wrong value resistor. As always any help gratfully accepted.

Cheers

Andrew

[m_c]

Lower resistance = faster voltage drop after power removal, but more heat generated.
Higher resistance = slower voltage drop after power removal, but less heat generated.

Once you have drives attached to the power supply, they'll help reduce the voltage. The bleed resistor is more a safety measure for if there are no drives attached, as otherwise the capacitor(s) could retain quite a high voltage for a considerable amount of time.
I've just checked what I've used previously, and for my digitiser (around 40VDC IIRC) it's a 270ohm 15W, and the mill a 680ohm 25W (around 55VDC IIRC).

[the great waldo]

Hi
Does the resitor in parallel with the caps alter the voltage coming into the controller whilst running the motors and controller at all? If so in which way?

Cheers

Andrew

[Doddy]

Hi
Does the resitor in parallel with the caps alter the voltage coming into the controller whilst running the motors and controller at all? If so in which way?

Cheers

Andrew

With an unregulated PSU, the bleed resistor will lower the terminal voltage slightly. Provided the PSU is specified sufficiently for the steppers then the resistor effect will be negligible.

I'd be looking to dissipate 2->5W just to keep the temperature down and keep the costs low.

R = \frac{{{V}_{RMS}}^{2}}{P}

[driftspin]

After switching off primary power supply the drives drain the caps quickly. Even with no steppers attatched.

You can check it only takes 10 seconds or so.

Please check your setup with a dvm

Grts Bert.





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[the great waldo]

After switching off primary power supply the drives drain the caps quickly. Even with no steppers attatched.

You can check it only takes 10 seconds or so.

Please check your setup with a dvm

Grts Bert.





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Hi Bert

Do you think it's not worth bothering with the bleed resistor ?
cheers

Andrew

[Neale]

One of the crazy things about specifying a bleed resistor is that it is based on things like heat dissipation at the time it is not doing anything useful, just sitting there wasting energy. The amount of stored energy that it actually needs to bleed if the input voltage fails or is switched off is very small. I chose to forget it altogether - I didn't want all that extra heat and wasted energy when, in practice, the drivers do the job very effectively. The only time a bleed resistor would be useful is under unusual conditions, during testing, for example. I'm happy to be aware of the issue at such times - it's no worse than avoiding the mains connections, for example. I'm not criticising people who do fit them, but for me it falls into the same category as a "cabinet open" switch. Commercially you should fit one of these in case some clown opens the cabinet and puts a finger where they shouldn't. For home workshop use, I want to be able to run with the door open sometimes - for example, to measure the power supply voltage to see what happens when it is turned off!

I have three e-stop switches around the machine and limit switches on all axes (except lowest position, of course). For me, that is a reasonable set of safety features; there's an MCB where the power enters the cabinet, a safety relay, and a fuse on the motor PSU (because it was fitted when I bought it). The only failure I have had on my machine was that fuse failing - through age, it seems, as there was no overload. Other people fit MCBs, fuses, etc, all over the place. Opinions vary!

[A_Camera]

Do you think it's not worth bothering with the bleed resistor ?

Bleed resistors are pointless, unless you have a very high voltage and you must work with the equipment, for example a camera flash, which is potentially dangerous unless you drain the capacitor, or measure the voltage and KNOW that the capacitor is discharged, which can take a very long time in case of a flash. In normal power supplies it will be drained after maximum a few minutes, even without any bleed resistor because you most certainly have some things, like a driver, or other electronics connected to it. Bleed resistors are just waste of energy when the PSU is ON. In your case, 50VDC isn't a risk anyway, unless you are easily scared. You may feel it, but nothing more. Besides, it can't be charged to more than 50V anyway. The fact that you are using 100V capacitors says nothing about the stored voltage, only that the capacitors can be used up to 100V. Your capacitors will never be charged over 50V if the PSU is 50VDC out after the rectifiers.

What I think is MUCH more important is that you have a slow starter circuit, because toroidal transformers draw a lot of current at start up and they can blow the fuse, or start with a very loud bang. Slow starter circuits are placed on the primary side and they limit the current for a short period (typically about 2 seconds), while the large capacitors are charged. This is necessary because the large capacitors short circuit the secondary side for a short time when they start to charge. The current rush cause the large bang and can also blow your mains circuit breaker, or fuse in the wall plug if you have one.

[the great waldo]

Bleed resistors are pointless, unless you have a very high voltage and you must work with the equipment, for example a camera flash, which is potentially dangerous unless you drain the capacitor, or measure the voltage and KNOW that the capacitor is discharged, which can take a very long time in case of a flash. In normal power supplies it will be drained after maximum a few minutes, even without any bleed resistor because you most certainly have some things, like a driver, or other electronics connected to it. Bleed resistors are just waste of energy when the PSU is ON. In your case, 50VDC isn't a risk anyway, unless you are easily scared. You may feel it, but nothing more. Besides, it can't be charged to more than 50V anyway. The fact that you are using 100V capacitors says nothing about the stored voltage, only that the capacitors can be used up to 100V. Your capacitors will never be charged over 50V if the PSU is 50VDC out after the rectifiers.

What I think is MUCH more important is that you have a slow starter circuit, because toroidal transformers draw a lot of current at start up and they can blow the fuse, or start with a very loud bang. Slow starter circuits are placed on the primary side and they limit the current for a short period (typically about 2 seconds), while the large capacitors are charged. This is necessary because the large capacitors short circuit the secondary side for a short time when they start to charge. The current rush cause the large bang and can also blow your mains circuit breaker, or fuse in the wall plug if you have one.

Hi A camera

I thought that might be a problem and bought one of these
https://www.conrad.at/de/renkforce-622412-einschaltstrom-begrenzer-fuer-grossverbraucher-weiss-ip20-622412.html

I assume that should do the job, although I would like to build a power surge reducer into the case to be a bit more self contained, if anyone has a straight forward circuit diagram for such a device i would like to see it. Thanks in advance.
Andrew

[Neale]

I am using a 650vA toroidal transformer (transformer rating is a good guide to the amount of inrush current you get at startup) and I have a 10A MCB where the mains enters the cabinet. I used a C curve MCB in place of the more usual B curve, which is a bit more tolerant of short-term surges. Never had it blow - unless you have a much larger transformer you probably don't need inrush protection.

[driftspin]

Hi Bert

Do you think it's not worth bothering with the bleed resistor ?
cheers

AndrewEh... well when nothing is connected it takes forever and a day to get rid of the lethal voltage levels...

So.. you can check voltage level vs time after switching off.

I would say 10 seconds to 10 ish volts is fine...

If it takes more time then what you are comfortable with put a resistor in.

50VAC 50hz / 120VDC was once considered the critical level for safety under normal conditions.

I would say all the metal near raises risks a bit.


So... then the energies stored in the caps is the next thing to worry about.

Caps discharge at high amp rate on shortcircuit... like a welding torch...


So big caps are always dangerous.

The above is just my opinion.
More safe is always better.

So never work on live systems, always check your measuring device for good to go before use.
Then check for 0 voltage and then check your measuring device is still in good working order after that.

https://youtu.be/EoWMF3VkI6U

This guy explains it well

https://youtu.be/sI5Ftm1-jik


He is more fun though [emoji848]


Grtz Bert


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[the great waldo]

Ok cheers Bert

I'll check that out. I had a few jumps years ago with my old Marshall valve amp ,and that was after it was turned off for a week. I became fairly wary of capacitors after that.

Cheers

Andrew

[A_Camera]

Eh... well when nothing is connected it takes forever and a day to get rid of the lethal voltage levels...

So.. you can check voltage level vs time after switching off.

I would say 10 seconds to 10 ish volts is fine...

If it takes more time then what you are comfortable with put a resistor in.

50VAC 50hz / 120VDC was once considered the critical level for safety under normal conditions.

I would say all the metal near raises risks a bit.


So... then the energies stored in the caps is the next thing to worry about.

Caps discharge at high amp rate on shortcircuit... like a welding torch...


So big caps are always dangerous.

Nonsense.

Large capacitors in combination with coils (like inside a camera flash) are dangerous, but NOT always. Large capacitors alone, charged to low voltage are NEVER dangerous, but yes, they can be used to create visual effects, sparks, burns and so on. Can be spectacular when shown to people who are easily impressed, scared or don't know better.


The above is just my opinion.
More safe is always better.

So never work on live systems, always check your measuring device for good to go before use.
Then check for 0 voltage and then check your measuring device is still in good working order after that.

https://youtu.be/EoWMF3VkI6U

This guy explains it well

https://youtu.be/sI5Ftm1-jik

He is more fun though [emoji848]


Sorry, but this is what happens when you don't understand the facts, just find something on the internet which suits your purpose. Your videos just creates confusion and fear. Both are attention grabbing nonsense. None of those videos show anything of value and DEFINITELY not prove the motive for bleed resistors. Even the second one only barely touches the subject, not really explaining anything.

The first one shows that a large capacitor can hold a lot of energy when fully charged, which is true, but... that does not make it dangerous. The capacitor the guy is playing with is 2.5V and that's NOT enough to cause harm. It is enough to create sparks, burn wires or copper tracks and "weld" but will NOT kill you, not even feel it. The voltage is simply too low. What kills you is the current through your heart. The current is proportional to the voltage and the RESISTANCE of what is put in between the + / - terminals. When you have bare a wire between 2.5V (the voltage of that capacitor when fully charged) the resistance is basically zero (extremely low but lets assume 0.001 Ohm) so based on Ohms law ( U / R = I) the current will be almost infinitive ( 2500A with the example of 0.001 Ohm ) and because of that very high current the power will be very high ( 2500 * 2,5V = 6250 W ) so the copper wire is burned and sparks are generated. This is theory, in real calculation you will need to add the internal resistance of the capacitor also, so the current will be lower, but still very high.

Now, what kills you is as I said, the current through your heart, and you only need about 20 mA is needed to kill you. But it is not that simple, unless you connect the electrodes directly to the wet heart. Normal human body has a resistance of over 2 M Ohm (measure with wet fingers) so with the above example this gives 0.00000125A which is EXTREMELY far from any danger. That is one reason why you can operate even an electric welder without risk of dying. Those are DC with very high current also, yet they are safe to operate. Also, if stored energy in a large capacitor would be a killer factor you would not be able to handle a battery before discharging it. Do you connect a bleed resistor to any battery? No? Why not if you are so safety concerned? Isn't it obvious that they are NOT dangerous to touch? They contain also a lot of energy and they can also burn wires, create sparks and even cause fire if the terminals are short circuited, but don't worry, you won't get electrocuted if you touch the terminals...

The second video is a little nearer to the truth but unfortunately the guy is spending more time in his acting and playing stupid than explaining facts. Of course, if he was not playing stupid he wouldn't have had so many views, so he is interested in making money, not delivering facts...

He mentioned that AC is more dangerous than DC and he also mentioned that the capacitors should be discharged before touching, but he did NOT mention that it is because his capacitors are charged to 170V, not 2.5V as in the first video. This makes a HUGE difference because using the above example it will result in 0.000085A through your heart which is 68 times more than in the first video, but still VERY far from dangerous. Of course, since current flows the shortest route, the way he touches the circuit at the end may result in larger current because he may short out the circuit between two tracks, separated by only a few mm, so his skin resistance may be pretty low. Anyway, in the second case, discharging the capacitor is a good safety measure but connecting a bleed resistor is still waste of energy and not necessary at all.

This thread is about a power supply which is generating 50VDC after the rectifier. The 100V capacitors will NEVER be charged to higher voltage than 50V. 50V is NOT dangerous, but without doubt can create sparks, weld or even cause fire if the current is high enough. Using bleed resistors is pointless in this case. If you absolutely want an indication of voltage it is a better idea to add a simple LED circuit. It will at least have a function and at the same time act as a bleed circuit.

[A_Camera]

Ok cheers Bert

I'll check that out. I had a few jumps years ago with my old Marshall valve amp ,and that was after it was turned off for a week. I became fairly wary of capacitors after that.

Cheers

Andrew

It's a good idea to be careful, but unless you know what is what and why, it is easy to fall for nonsense on YouTube. Valve amplifiers use high voltage in the whole circuit, very far and different from what you will use. Also, if you absolutely must handle the capacitor you can always touch the terminals with a resistor first so that they discharge through the resistor, or measure the voltage across the terminals before touching them. The other thing is that you can add a LED on the DC side which will indicate if there is enough voltage, and act as a bleeding circuit. Just get a LED, connect a 5k resistor in series and that's it. It will be your power on indicator, and your capacitor will be discharged after a few seconds to so low level that you don't need to worry at all.

A slow starter on the primary side is more important than bleeding resistor. The bleeding is fixed by the circuitry which will be connected to the rectifier. The drivers work well as a "bleed resistor", they will quickly discharge your capacitors when you power off, so there is nothing to worry about on the secondary side. Take care of the primary side because toroidal transformers have a large inrush current, even without anything connected to the secondary side, so large transformers should have a slow starter circuit.

[A_Camera]

Hi A camera

I thought that might be a problem and bought one of these
https://www.conrad.at/de/renkforce-622412-einschaltstrom-begrenzer-fuer-grossverbraucher-weiss-ip20-622412.html

I assume that should do the job, although I would like to build a power surge reducer into the case to be a bit more self contained, if anyone has a straight forward circuit diagram for such a device i would like to see it. Thanks in advance.
Andrew

Hi,

Here is a simple circuit which will limit the current during about 1.5s which should be enough.

http://electronics-diy.com/soft-start-for-power-supply.php

Even the one from Conrad which you linked will work, but it is limiting between 0.4-1s only. The circuit needed is pretty simple, but of course, if it is a commercial product then it has a price. Never the less, well worth investing in it. I have one built in my PSU, but an external one works just as well.

[the great waldo]

Hi A_Camera

Thank you for the link that looks like just what I need.

Cheers

Andrew

[A_Camera]

Hi A_Camera

Thank you for the link that looks like just what I need.

Cheers

Andrew

You're welcome, but be careful if you plan to build one. As opposed to the 50VDC, the 230AC is very dangerous and should be handled accordingly and taken seriously.

[the great waldo]

Thanks
I'm always wary of mains power. Just out.of interest what is the amp rating of the rectifier in the circuit ? The round housing limits the options a bit. There seem to be different ones listed on the net. My electronics knowledge is self learnt. Am I right in thinking that the resistor capacitor network is the timer for the circuit. The article is a bit vague in the description of the circuit. I assume the rectifier is only to supply a dc to the relay so does not need to be high amps ? If I'm thinking wrong I'm happy to be corrected!!

Cheers
Andrew

[cropwell]

Hi,

Here is a simple circuit which will limit the current during about 1.5s which should be enough.

http://electronics-diy.com/soft-start-for-power-supply.php

Even the one from Conrad which you linked will work, but it is limiting between 0.4-1s only. The circuit needed is pretty simple, but of course, if it is a commercial product then it has a price. Never the less, well worth investing in it. I have one built in my PSU, but an external one works just as well.

If you put in a DPCO relay you could use it to drop in a bleed resistor when the power is removed, after all this is what this thread started out as :nonchalance:

[the great waldo]

Hi Cropwell.
That seems to be simple/sensible solution. Kill 2 birds with one stone. I'm surprised no one makes a pcb or kit like that.
Cheers
Andrew

[john swift]

looking at PCB art work
http://electronics-diy.com/soft-start-for-power-supply.php
it looks like its a project take from the Dutch: Elektuur magazine

John

PS

found it
the delay is from the July August 1997 issue of Elektor

24522

the B250C1500 is a 1.5A 500V bridge rectifier
I would of thought a 1A bridge rectifier would be OK
or four 1N4007 diodes

[the great waldo]

Hi John.
Thanks very much, saved me some time this evening hunting for the article.
Cheers
Andrew

[Doddy]

Or use a NTC thermistor.

BTW, that design - the resistor wattage values are wrong - if, as claimed, it's intended to limit the current to 5A then the resistors should be rated at 250W (P=I^2 x R). I'd also caution that C1 is specified at the limits of its operation - failure of that would result in the explosive failure of C2 C2 and likely destruction of R1 and RE1.

Treat with care.

Oh, you dissipate about 6W in that capacitor and relay.

And A-Camera, electric shock isn't the only hazard of a charged capacitor. The biggest hazard at this potential are burns and physical injury resulting from a spontaneous reaction to a sudden discharge. You mention batteries - of course you don't use a bleed resistor on batteries - that's a divisive argument. But the first instruction in any vehicle maintenance manual is to remove the -V lead before any work. Why?

[Neale]

Why build a second power supply just to run the relay? How about driving a 24V relay from the existing power supply output via a suitable zener? With a zener chosen to drop enough volts to give nominal relay voltage when PSU is at working voltage, the relay will operate a little earlier than that, but by then the initial surge will be over. Even simpler, use a series resistor to give nominal relay voltage at PSU nominal output. I'm not convinced that surge protection is needed at all (my system works fine without but I chose my input MCB carefully so nothing trips, and that's the only reason why you need to bother) but if it is, make it as simple as possible. I've never used the NTC approach, partly because it looks as if you have to choose carefully to match PSU current draw, which is pretty variable driving a CNC machine.

[cropwell]

I would like to know what the design input voltage is to this soft tart circuit in Elektuur. We are assuming 50v as that is what we need, but it has 40v caps and a 24v relay.

[Doddy]

Nope, it's 220-240 on the primary of the toroidal, something that should be borne in mind for our US readers.

It uses a potential divider across the C1 capacitor and the relay coil. It works as-is, but is fraught with problems when people start substituting components.

[cropwell]

Sorry Doddy, I was trying to delete my post as I realised it was mains input. Just had a senior moment !

[Doddy]

Why build a second power supply just to run the relay?
Feed from the secondary?, that's just too easy!

[driftspin]

Nonsense.

Large capacitors in combination with coils (like inside a camera flash) are dangerous, but NOT always. Large capacitors alone, charged to low voltage are NEVER dangerous, but yes, they can be used to create visual effects, sparks, burns and so on. Can be spectacular when shown to people who are easily impressed, scared or don't know better.



Sorry, but this is what happens when you don't understand the facts, just find something on the internet which suits your purpose. Your videos just creates confusion and fear. Both are attention grabbing nonsense. None of those videos show anything of value and DEFINITELY not prove the motive for bleed resistors. Even the second one only barely touches the subject, not really explaining anything.

The first one shows that a large capacitor can hold a lot of energy when fully charged, which is true, but... that does not make it dangerous. The capacitor the guy is playing with is 2.5V and that's NOT enough to cause harm. It is enough to create sparks, burn wires or copper tracks and "weld" but will NOT kill you, not even feel it. The voltage is simply too low. What kills you is the current through your heart. The current is proportional to the voltage and the RESISTANCE of what is put in between the + / - terminals. When you have bare a wire between 2.5V (the voltage of that capacitor when fully charged) the resistance is basically zero (extremely low but lets assume 0.001 Ohm) so based on Ohms law ( U / R = I) the current will be almost infinitive ( 2500A with the example of 0.001 Ohm ) and because of that very high current the power will be very high ( 2500 * 2,5V = 6250 W ) so the copper wire is burned and sparks are generated. This is theory, in real calculation you will need to add the internal resistance of the capacitor also, so the current will be lower, but still very high.

Now, what kills you is as I said, the current through your heart, and you only need about 20 mA is needed to kill you. But it is not that simple, unless you connect the electrodes directly to the wet heart. Normal human body has a resistance of over 2 M Ohm (measure with wet fingers) so with the above example this gives 0.00000125A which is EXTREMELY far from any danger. That is one reason why you can operate even an electric welder without risk of dying. Those are DC with very high current also, yet they are safe to operate. Also, if stored energy in a large capacitor would be a killer factor you would not be able to handle a battery before discharging it. Do you connect a bleed resistor to any battery? No? Why not if you are so safety concerned? Isn't it obvious that they are NOT dangerous to touch? They contain also a lot of energy and they can also burn wires, create sparks and even cause fire if the terminals are short circuited, but don't worry, you won't get electrocuted if you touch the terminals...

The second video is a little nearer to the truth but unfortunately the guy is spending more time in his acting and playing stupid than explaining facts. Of course, if he was not playing stupid he wouldn't have had so many views, so he is interested in making money, not delivering facts...

He mentioned that AC is more dangerous than DC and he also mentioned that the capacitors should be discharged before touching, but he did NOT mention that it is because his capacitors are charged to 170V, not 2.5V as in the first video. This makes a HUGE difference because using the above example it will result in 0.000085A through your heart which is 68 times more than in the first video, but still VERY far from dangerous. Of course, since current flows the shortest route, the way he touches the circuit at the end may result in larger current because he may short out the circuit between two tracks, separated by only a few mm, so his skin resistance may be pretty low. Anyway, in the second case, discharging the capacitor is a good safety measure but connecting a bleed resistor is still waste of energy and not necessary at all.

This thread is about a power supply which is generating 50VDC after the rectifier. The 100V capacitors will NEVER be charged to higher voltage than 50V. 50V is NOT dangerous, but without doubt can create sparks, weld or even cause fire if the current is high enough. Using bleed resistors is pointless in this case. If you absolutely want an indication of voltage it is a better idea to add a simple LED circuit. It will at least have a function and at the same time act as a bleed circuit.Hi A_Camera,


What did i say wrong to make you throw up that many words?
I am not here to challenge anyone's knowledge, or but heads.

The mentioned purpose of the bleed resistor is getting rid of the charge ... at some point ... after shutting down.


Limiting in rush current is a good idea to keep cb in turned on position at power up.




Now for the playing down of electrical dangers.


Uncontrolled / unintential discharge of capacitors can lead to damage and can potentially hurt you in the process.. at any voltage.


If you look at those videos you must have been able to spot that.



Higher voltage levels have higher potential to disrupt hart rhythm AC or DC , when it makes current flow through the body and meets the hart along its route.


Being in close contact with well grounded metal surfaces clearly does not help to make it more safe.


Higher amp rates burn stuff like metal and can vaporize it.
Heat burns the skin last time i checked.



So what i would like you to do is not under estimate and be mature about dangers of electricity.



So from my point of view clearly YOU don't understand.



But there is of course, potentially, the Darwin award for anyone who does not work and or design in a safe way.




My 2cents worth.

Grts Bert.







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[the great waldo]

Nope, it's 220-240 on the primary of the toroidal, something that should be borne in mind for our US readers.

It uses a potential divider across the C1 capacitor and the relay coil. It works as-is, but is fraught with problems when people start substituting components.

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

[Clive S]

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

Personally I don't think you need one. I have a 625vA one that runs without any trouble and 3 caps is ample.

[the great waldo]

Hi Clive
I'll put one I found on ebay https://www.ebay.de/itm/131927219743 in just not to strain my wiring. It's an old house in the country here in Austria with a new distribution box wired up about 15 years ago which happens to be in the workshop as well so cable runs are not too long. A little peace of mind is worth the added expense for me.
Cheers

Andrew

[driftspin]

Personally I don't think you need one. I have a 625vA one that runs without any trouble and 3 caps is ample.Hi Clive S,

You helped me with the parts list for the diy psu i have.

500va 2x25v.


No problems here either on 16amp B type CB.


Andrew, do you have issues on the mains power side?



In rush limiting is ok but not really needed when mains side is no problem.




Grtz Bert.



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[the great waldo]

Hi Bert
I haven't put the supply together yet but it's an old house and I don't want to overload the the circuit breakers so it's really just for a bit of added protection.

Cheers

Andrew

[cropwell]

Why build a second power supply just to run the relay? How about driving a 24V relay from the existing power supply output via a suitable zener? With a zener chosen to drop enough volts to give nominal relay voltage when PSU is at working voltage, the relay will operate a little earlier than that, but by then the initial surge will be over. Even simpler, use a series resistor to give nominal relay voltage at PSU nominal output.

Interesting idea ! Why not use a filament light bulb as the inrush protection resistance. If the bulb fails nothing will happen and if it is OK it will go out when you are at operating voltage. Just an idea that 'flashed' through my addled brain.

[A_Camera]

If you put in a DPCO relay you could use it to drop in a bleed resistor when the power is removed, after all this is what this thread started out as :nonchalance:

If bleeding is what you want, I still think a simple LED and a 5k resistor after the rectifier is the way to go. It will provide power on/off indication and also drain the capacitor when you power off.

[A_Camera]

looking at PCB art work
http://electronics-diy.com/soft-start-for-power-supply.php
it looks like its a project take from the Dutch: Elektuur magazine

John

PS

found it
the delay is from the July August 1997 issue of Elektor

24522

the B250C1500 is a 1.5A 500V bridge rectifier
I would of thought a 1A bridge rectifier would be OK
or four 1N4007 diodes


So the guy stole it and put his own copyright message on it... I have my book shelves full of old books and magazines, even several years of Elektor (which I think is German from the beginning) but googling is faster than going through all of those.

Anyway, good detective work.

[A_Camera]

Or use a NTC thermistor.

BTW, that design - the resistor wattage values are wrong - if, as claimed, it's intended to limit the current to 5A then the resistors should be rated at 250W (P=I^2 x R). I'd also caution that C1 is specified at the limits of its operation - failure of that would result in the explosive failure of C2 C2 and likely destruction of R1 and RE1.

Treat with care.

Oh, you dissipate about 6W in that capacitor and relay.

True. I haven't build that circuit, so no warranty about it's functionality.


And A-Camera, electric shock isn't the only hazard of a charged capacitor. The biggest hazard at this potential are burns and physical injury resulting from a spontaneous reaction to a sudden discharge. You mention batteries - of course you don't use a bleed resistor on batteries - that's a divisive argument. But the first instruction in any vehicle maintenance manual is to remove the -V lead before any work. Why?

...because of explosion risk...? Lead batteries can LITERALLY explode in your face and a car battery explosion is extremely dangerous because of the acid inside. Apart from the actual battery explosion, the heat it generates if short circuited is more than enough to set everything on fire. If none of that happens, the sparks which are/can be created are so large and powerful so the fuel can be set on fire, and if there is the exact right mixture of fuel (oil and/or petrol/diesel fumes) and air then the whole car will be blown to pieces, taking you with it... Removing the car battery is indeed important, especially after an accident, but maybe not any work. I don't remove the battery for work I do on my cars, like "normal" maintenance, filling of fluids, changing of tires and so on. Everything else is left to the experts, and they aren't always removing the battery for the normal service.

You can't compare car batteries with capacitors or smaller batteries. Also, cars contain other circuitry as well, not just rectifier and capacitor, like a simple PSU. ...but I mentioned that there is a slight risk for surface skin burns.

The argument about the batteries is still valid. A simple power tool battery, or even a lithium camera battery contains more charge than a capacitor and NOBODY of sane mind would think about bleeding resistor.

After a few accidents I ALWAYS discharge the main capacitor in camera flashes before I open one. A tiny flash using 3V battery power makes your heart beat a few extra beats if you are not careful when you open a flash... :toot:

[Neale]

If you look at those videos you must have been able to spot that.


I watched those videos - the first one was quite entertaining. But did no-one spot the value of the capacitor he was using? 2600F! Farad, not microfarad. That's about 3,000,000 times bigger than the usual smoothing capacitors we are talking about! Yup, quite a lot of stored energy. Not entirely relevant, though. My machine is wired according to good practice with separate feeds to each stepper driver from the PSU. In effect, I have four separate permanently-connected bleed devices connected. A bleed resistor in this situation is a waste of energy, quite literally. Those capacitors will not hold charge after switch-off except in an extraordinarily unlikely combination of faults. The only time, in practice, that a bleed resistor might be useful is when testing off-load, and in this case you just need to be aware of the possible problems. The mains input connections are a much bigger danger if you poke around with a finger or have loose trailing wires.

And as several people have said, certainly up to about 625VA toroidal transformers, a 16A B-curve or 10A C-curve MCB is perfectly happy without any inrush limiter.

The engineer says, if you don't need it, don't put it in!

You are more likely to get problems with things like the RFI filter on the VFD. These can cause out-of-balance live-neutral currents that trip RCDs.

[A_Camera]

Hi A_Camera,

What did i say wrong to make you throw up that many words?
I am not here to challenge anyone's knowledge, or but heads.

The mentioned purpose of the bleed resistor is getting rid of the charge ... at some point ... after shutting down.

Limiting in rush current is a good idea to keep cb in turned on position at power up.

Now for the playing down of electrical dangers.

Uncontrolled / unintential discharge of capacitors can lead to damage and can potentially hurt you in the process.. at any voltage.

If you look at those videos you must have been able to spot that.

Higher voltage levels have higher potential to disrupt hart rhythm AC or DC , when it makes current flow through the body and meets the hart along its route.

Being in close contact with well grounded metal surfaces clearly does not help to make it more safe.

Higher amp rates burn stuff like metal and can vaporize it.
Heat burns the skin last time i checked.

So what i would like you to do is not under estimate and be mature about dangers of electricity.

So from my point of view clearly YOU don't understand.

But there is of course, potentially, the Darwin award for anyone who does not work and or design in a safe way.

My 2cents worth.

Grts Bert.

Verstuurd vanaf mijn SM-A320FL met Tapatalk

What I did was explained facts. Sorry if the number of words are above your standards, but that's how it is. Anyway, to read my many words should take less time than watching those videos you linked. Never the less, I watched both of them, which is why I commented them. As for not understanding those... well, sorry, but the reason I wrote what I wrote is that I understood them very well but apparently, I don't think you did.

[A_Camera]

Hi Doddy
I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.
cheers

Andrew

If I were you I'd buy the one from Conrad, or something similar ready made. In my opinion it is not worth the effort and is safer and guaranteed to work. The reason I linked to that circuit is because you asked for an example. Anyway, if you decide to build it then in your case I think it needs some modification, so I'd modify the following:

1. The four resistors, as Doddy mentioned already. It is better to use more powerful type, even if the 5W may work for such application because of the short time current if flowing trough. In fact, I think a single resistor would be better, something like this:

https://www.conrad.at/de/ate-electronics-rb101-47r-j-hochlast-widerstand-47-axial-bedrahtet-12-w-5-1-st-420251.html

That one is 47 Ohm, so the current will be limited a little more, to 4.9A Instead of 5.75A with the 40 Ohm, but that doesn't matter.

2. The relay in the example is 8A, I'd take a more powerful one, for example this one:

https://www.conrad.at/de/printrelais-24-vdc-16-a-1-wechsler-finder-406190240000-1-st-502934.html

3. The fuse should be 10A slow type.

4. The art work in my opinion should have more powerful tracks, especially on the K1, K1 and the relay part. No need to waste copper by removing more than what's necessary to provide isolation.

5. Resistors R1, R2 and R3 should be metal film, 0.6W or 1W type. Even if the current trough is low, it is good practice to use better resistors than coal film type.

https://www.conrad.at/de/metallschicht-widerstand-470-k-axial-bedrahtet-0207-06-w-1-yageo-mf0207fte52-470k-1-st-1417688.html

https://www.conrad.at/de/metallschicht-widerstand-220-axial-bedrahtet-0207-06-w-1-yageo-mf0207fte52-220r-1-st-1417626.html

[A_Camera]

I watched those videos - the first one was quite entertaining. But did no-one spot the value of the capacitor he was using? 2600F! Farad, not microfarad. That's about 3,000,000 times bigger than the usual smoothing capacitors we are talking about! Yup, quite a lot of stored energy. Not entirely relevant, though. My machine is wired according to good practice with separate feeds to each stepper driver from the PSU. In effect, I have four separate permanently-connected bleed devices connected. A bleed resistor in this situation is a waste of energy, quite literally. Those capacitors will not hold charge after switch-off except in an extraordinarily unlikely combination of faults. The only time, in practice, that a bleed resistor might be useful is when testing off-load, and in this case you just need to be aware of the possible problems. The mains input connections are a much bigger danger if you poke around with a finger or have loose trailing wires.

And as several people have said, certainly up to about 625VA toroidal transformers, a 16A B-curve or 10A C-curve MCB is perfectly happy without any inrush limiter.

The engineer says, if you don't need it, don't put it in!

You are more likely to get problems with things like the RFI filter on the VFD. These can cause out-of-balance live-neutral currents that trip RCDs.

I mentioned how irrelevant the first video was because of the type of experiments he makes (basically, playing like a kid, as I played with capacitors when I was 7-8 years old), and also because the very large capacitor, which is actually not only large, but also only 2.5V, totally irrelevant and nothing but entertainment.

The second video was a little more facts, but too much monkeying. The guy is too busy playing an idiot instead of explaining facts. At the end of the video, the guy drops the PSU, pretending he received an electric shock from the capacitors... but what he forgets to mention is that it is because his PSU is 170V, so of course, one should be more careful there, but still, a bleed resistor is not necessary because the capacitors will normally be discharged after a few seconds, or a few minutes later if something wrong with the circuit after the rectifier. Playing an idiot is popular on YouTube, it generates MANY views. Boring facts are not as popular.

I agree, what's not needed should not be put in, but I disagree regarding the slow starter. I hate the bang which the toroidal transformer cause without the current limiter, so in my opinion, that is necessary in these type of PSU.

The engineer in me doesn't understand why bother with a dedicated bleed resistors at all in a PSU, because normally there are other electronics connected to it, which will automatically ALWAYS discharge the capacitors after a few seconds maximum.