I want to make a Capacitor Discharge Tool but I'm confused by all the conflicting advice about the resistor value to use. The range in value is quite startling (and alarming); from 100uF to 10,000uF with a wattage of 5 to 10W.
One fairly convincing video on youtube advocates a 470uF/5W which I was going to go with until someone else used 2 x same in parallel! Another suggests 2k or 4k. Just think of a number and double it?
I'm looking to service/repair electrical gear I find in the home: Stereo amplifier, guitar amplifier, CD player, Turntable, Radio and TV. Nothing industrial.
What would YOU use?
Many thanks,
Mike
One fairly convincing video on youtube advocates a 470uF/5W which I was going to go with until someone else used 2 x same in parallel! Another suggests 2k or 4k. Just think of a number and double it?
I'm looking to service/repair electrical gear I find in the home: Stereo amplifier, guitar amplifier, CD player, Turntable, Radio and TV. Nothing industrial.
What would YOU use?
Many thanks,
Mike
Start with a model and use that to predict the instantaneous peak heat that will be dissipated in the discharging resistor.
P=IV=I²R=V²/R
If your resistor has an instantaneous peak dissipation at the moment of first starting discharging that is below the resistor power rating then you know the resistor will not burn out.
If you model the discharge time constant and use that to determine how long it takes to discharge the capacitor/s down to ~37% of their starting voltage, you can get an idea of how hot the case of the resistor will get.
RC time constant = R*C in seconds
Electronics is a science based discipline. If you are not interested in using the science to arrive at useful answers, then you are in the wrong hobby.
If DIY is not your thing, then buying ready mades would be more appropriate.
P=IV=I²R=V²/R
If your resistor has an instantaneous peak dissipation at the moment of first starting discharging that is below the resistor power rating then you know the resistor will not burn out.
If you model the discharge time constant and use that to determine how long it takes to discharge the capacitor/s down to ~37% of their starting voltage, you can get an idea of how hot the case of the resistor will get.
RC time constant = R*C in seconds
Electronics is a science based discipline. If you are not interested in using the science to arrive at useful answers, then you are in the wrong hobby.
If DIY is not your thing, then buying ready mades would be more appropriate.
You get a range of answers because you have a range of people reporting. There isn't a "right" answser.
An old trick is to clip a ground wire to pin 1 of a preamp tube socket, that drains the B+ line through a (typically) 100k resistor. Takes a while, but doesn't stress the resistor. A lower value resistor is faster, but get low enough and you have to worry about current surge and quick dissipation.
I use 10 watt resistors, not because I need 10 watts dissipation, but because they are physically large enough I can hold it in my fingers and not touch the wire leads.
I just grab one from my drawer and a clip wire from the bench. I used 1000 ohms arbitrarily. COnsidering for the discharge job, I didn't care if it took one second or 20.
But Andrew showed you the math, it isn't hard, figure out a value that serves you.
An old trick is to clip a ground wire to pin 1 of a preamp tube socket, that drains the B+ line through a (typically) 100k resistor. Takes a while, but doesn't stress the resistor. A lower value resistor is faster, but get low enough and you have to worry about current surge and quick dissipation.
I use 10 watt resistors, not because I need 10 watts dissipation, but because they are physically large enough I can hold it in my fingers and not touch the wire leads.
I just grab one from my drawer and a clip wire from the bench. I used 1000 ohms arbitrarily. COnsidering for the discharge job, I didn't care if it took one second or 20.
But Andrew showed you the math, it isn't hard, figure out a value that serves you.
I understand what Andrew is saying but, as we all have to start somewhere, I was hoping that this forum could pass on its collective knowledge to people like myself who are coming through the ranks, as it were. I was just concerned that I could get such diverse opinions on such an important topic as safety. Unless anyone has any concerns, I will use a 1000 Ohm 10w resistor, as Enzo recommends.
Thanks all,
Mike
Thanks all,
Mike
500Vdc stored on a capacitor will try to dissipate 250W into a 1000r resistor.
12Vdc stored on a capacitor will try to dissipate 0.14W into a 1000r resistor.
Do the sums and predict what your situation requires.
12Vdc stored on a capacitor will try to dissipate 0.14W into a 1000r resistor.
Do the sums and predict what your situation requires.
As Andrew has said, you need to do some 'homework' ...we all have had to do the same.
You have some confusion (or typos) with the units you are quoting uF is a capacitance unit, if you are looking for a resistor the unit will be ohms.
If you don't want a home-made discharge device, you can buy something online. But you still need to pay attention to what you are connecting it to. Tube gear often has 300-500 volts in filter caps, so as Andrew points out you want a higher value (more ohms) resistor to 'slow down' the discharge rate, or things can get exciting (blue flash..).
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