choosing a bleed resistor

[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

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-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

Attachment 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...

[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.

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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.