choosing a bleed resistor

**the great waldo** · 10-07-2018

Originally Posted by A_Camera

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-6...20-622412.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** · 10-07-2018

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.

**A_Camera** · 11-07-2018

Originally Posted by the great waldo

Hi A camera

I thought that might be a problem and bought one of these
https://www.conrad.at/de/renkforce-6...20-622412.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-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.

**the great waldo** · 11-07-2018

Hi A_Camera

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

Cheers

Andrew

**A_Camera** · 11-07-2018

Originally Posted by the great waldo

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** · 11-07-2018

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** · 11-07-2018

Originally Posted by A_Camera

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** · 11-07-2018

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** · 11-07-2018

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