choosing a bleed resistor

[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.







Verstuurd vanaf mijn SM-A320FL met Tapatalk

[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]

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.

[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.