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.
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.
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?
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?
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...
...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... 
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?
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?
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.
Seems like an example of unecessary maths to me.
maybe I should check up on my old posts more often.
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)
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)
Stocker said:
I was under the impression that the 0.63 figure meant on charge, after 1 time constant the voltage has reached 63% of supply level, or on discharge, has fallen to 1 - 0.63 = 37% of supply level. Not what your spreadsheet tells me.
In fact, the cap is over 99% flat (or full) after only 5T. Now if you've a big bank of caps with only a couple of volts in them it's going to make a bang if you stick a screwdriver across them, but 250V is down to 12.7V in 3T so there's no danger of electric shock.
http://farside.ph.utexas.edu/teaching/302l/lectures/node89.html
The scientist says "here is new evidence: the paradigm must shift if it is proved".
I'll look into this matter in more detail when I get a chance. Revision to the spreadsheet may be called for. I get a warm fuzzy from the fact that it advises the DIY'er to use a resistor rated twice worst-case max. dissipation... this should be close to ok for the short time even if my time is too slow by 1/2 (or whatever, I can't do maths on a coffee free brain).
I'll look into this matter in more detail when I get a chance. Revision to the spreadsheet may be called for. I get a warm fuzzy from the fact that it advises the DIY'er to use a resistor rated twice worst-case max. dissipation... this should be close to ok for the short time even if my time is too slow by 1/2 (or whatever, I can't do maths on a coffee free brain).
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