choosing a bleed resistor

[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-star...wer-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

[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-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]

Why build a second power supply just to run the relay?

Feed from the secondary?, that's just too easy!

[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]

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...