choosing a bleed resistor

[john swift]

looking at PCB art work
http://electronics-diy.com/soft-star...wer-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



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