Capacitor Bleeder Calculator Spreadsheet HERE

[Stocker]

I have been wondering how to bleed voltage off the capacitors in my current project, and there is a lot of confusion and a lot of formulae are referred to but NOBODY has made a calculator.

I made a calculator. It is small, fast and easy. You need an MS Excel compatible program to use it. Somebody tell me if it's got errors and I'll fix and repost it.

Input your rail voltage, capacitance and bleeder resistor values. It will tell you the time constant, the voltage after the first 11 time constants, the voltages, currents and power dissipation involved and it even has a frikkin LASER BEAM...er, a color chart of power vs. time

I really hope someone finds this helpful, because it's an hour of my life I can never get back. Maybe it will save somebody else an hour. Or a life. Be careful with electricity. It can kill you.

Use these data at your own risk, for your own information. No selling my work eh! Redistribution is fine, please only with credit given.

[Cloth Ears]

Hi Stocker,

Great (simple) idea! I just 'unlocked' A4,A5&A6 and then protected the sheet (with no password). Now I don't write over the formulae.

Any idea how to get the time constant (ummm) constant? I'd like to be able to have it at 60 seconds for every value of R&C, but I'm not sure how to work out end voltage (in your sheet you have the time constant being variable and the voltage is adjusted by 0.63 at each iteration).

But, great stuff...

P.S. Just checking, the value for capacitance is for each rail, isn't it?

[pinkmouse]

Simple ideas are the best. Thanks!

[Stocker]

The time constant is the variable ... ...

This sheet is set up to tell you the time constant of the specified values you input. With all the different capacitor and resistor values and ratings, it would be just this side of an infinite number of combinations to achieve any time constant. I guess it would be simple to work up a program or look-up table for it, but I think the current way is good. I am a fan of giving users raw data and lots of control. By the way, if you were to overwrite the formulae, the spreadsheet should always be here for downloading again.

The end voltage is stated after what I thought was "a lot" of time constant iterations. If you need more iterations, input the last voltage as a new beginning voltage. As a practical matter, this spreadsheet will let you know when the voltages have dropped to safe levels. The mathematician says "Each time constant sees a reduced level (0.63) but the voltage will never reach zero".

I think I put a note in the sheet itself; you may have to change resolution or scroll to see it... yes, this is for bleeding one rail to ground. If you want to do two rails, then the maths are pretty simple and I'm pretty sure we can all figure that one out...

[Stocker]

I think I have made a pretty stupid mistake.

1000u = 1m right? so my example 2 000 uF should be 0.002F vice 0.02F in the original file. Corrected in this version. Man, my project just got safe a lot quicker!

[Merlinb]

Why does it matter how long they take to discharge?

[Stocker]

Some of us actually open up the cases and chassis on our electronics...

Sometimes, just for fun, sometimes to repair, sometimes to modify...

DIY remember?

200V can kill you.
A power supply with lower voltage, say an Aleph at 28V and 0.8F, can scare the living daylights out of you, deliver a HEFTY shock AND send a tool flying across the shop (also dangerous, BTW).

So maybe someone would like to know when it is safe to open the cover before sticking their hands into what might have a lethal or dangerous energy potential present eh?

[Merlinb]

That's not really what I meant, Stocker. I mean, what's the point of working out the time constant just for a bleeder? I just pick up a 1M resistor or whatever I have to hand, and put that in the circuit, I know it's going to be mostly discharged after a few minutes that way.
Seems like an example of unecessary maths to me.

[Cloth Ears]

Merlinb,

After 9 hours, with your 1M resister, my caps will be down to 40Volts. They'll discharge faster through the air and through other 'non-conductive' parts of the circuit than through a resistor of that value.

That's why the un-necessary maths.

[Stocker]

I wrote this spreadsheet for the example... 170VDC on 2000uF capacitors is a lot of jou(ice)les to be sticking my hands into. I wanted to be sure I would not be killing myself when I opened the case, and I didn't want to be waiting forever to make a simple single circuit correction. Waiting until tomorrow to make a single solder joint isn't what I call fun. Also, exploding resistors from excess heat is not fun too. I've been an electronics tech. for lots of years and done both but prefer neither.


Plus it's cool to do something nobody else has (AFAIK)