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 Post subject: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 5:33 am 
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Hello. I have an old Sansui receiver (+40 years) in which I will be replacing caps in the power supplies. The largest cap is a 3300 uF / 100wv that charges up to about 85 volts DC. I want to use a resistor with clip leads to discharge it and the other, smaller PSU caps. What values of R and W would you recommend? Thanks

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 5:44 am 
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https://www.electronics-tutorials.ws/rc/rc_2.html


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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 7:23 am 
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If the capacitor needs to be discharged, chances are it's just fine.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 8:05 am 
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hwhall wrote:
https://www.electronics-tutorials.ws/rc/rc_2.html

Thanks, I'll study it.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 8:18 am 
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Barry H Bennett wrote:
If the capacitor needs to be discharged, chances are it's just fine.

Thanks. The 3300 uF is acting like it is intermittent. If I bridge it with a new 3300 uF cap when there is hum in the output, the hum goes away. So I will replace it.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 11:47 am 
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Sounds reasonable. IF it's a screw connection type, maybe try just removing, cleaning, and replacing the screw/lug. I have seen one or two of those over the years that go open, but it's pretty rare.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Thu 19, 2021 5:43 pm 
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Many of these larger caps use the same metalizing techniques. "Dead" shorting a condenser of that construction can cause the metalizing to separate and reduce capacity. The separation can be intermittent and can result in the same cap introducing spikes into the circuit at unpredictable times. Often, hanging a scope on the cap with a trigger set at 25% over the operating voltage will catch that spike to confirm... If the cap is suspect, replace it.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Fri 20, 2021 2:58 am 
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Chas wrote:
Many of these larger caps use the same metalizing techniques. "Dead" shorting a condenser of that construction can cause the metalizing to separate and reduce capacity. The separation can be intermittent and can result in the same cap introducing spikes into the circuit at unpredictable times. Often, hanging a scope on the cap with a trigger set at 25% over the operating voltage will catch that spike to confirm... If the cap is suspect, replace it.

I think it might also be that there is intermittently enough leakage to load the Vcc to an excess current level, reducing the filter's ability to regulate. Just an hypothesis (educated guess?).

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Fri 20, 2021 3:06 am 
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I serviced in-house built production machinery that was powered by pneumatic cylinders. The cylinders were controlled by "Mac" valves 24 volts DC all from a an analog power supply. Despite spike suppression, in 18mo to two years the 5000mf cap on the power supply would fail by loosing capacity. We kept spares on hand but because this was production, the scope check was used to determine the failure. Change out the cap during a lunch break...

IMHO a cap defect in an amp may first show itself as "muddy" reproduction of Bass notes... There-in the bass requiring a quick dump of reserved current from a cap that just can't...

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Fri 20, 2021 3:12 pm 
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To answer your original question, what is a good resister value and wattage to use to discharge a capacitor (so that it is safe to work around).
If you looked at that link, one things that results is that the initial peak current depends on the charge voltage and the resistor value. So if that capacitor was charged to 100V and you used a 100 ohm resistor the peak current would only be 1 amp. A big filter cap like that wouldn't be damaged by that much current, in fact you could go down to 10 ohms no problem (10A). Also it would discharge quicker. On the other hand if you had a large cap charged to 1000V the 10 ohm might be pushing it (100A).
In most cases a value around 20 to 30 ohms would be a good compromise, a 100 ohm could be used but the discharge would be slower, if you leave clip leads on for a time that wouldn't be a problem.
Unless you are discharging a huge capacitor, say well over 1000uf, charged to a high voltage (1000V), the discharge is so quick that as long as the resistor (and wiring) can handle the current, the total power is small. A 5 watt wire wound or larger would work.

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 Post subject: Re: Resistance and wattage values for discharging capacitor?
PostPosted: Aug Fri 20, 2021 4:14 pm 
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This brings up an important point. It is very true that the original "can" capacitors could push amps of ripple current. You have to replace them with new ones that have the same ripple current capacity as the originals. There are much smaller PC board mount 3,300 uF caps rated for 100 volts but they will not have the same ripple current handling ability. If you use those they will have very short lives in service.

It may prove impossible to find the full specifications of the original capacitors. If one can find new "can" style electrolytics with the same uF and voltage ratings, and the same physical dimensions as the originals, there's an excellent chance they will have similar ripple current ratings too. But the large capacitors in equipment like this were usually not standard off-the-shelf parts from the Mouser catalog. They were usually custom ordered by the equipment manufacturer to physically fit and work on that particular chassis. So you may find that available can capacitors with the necessary ratings are too big. In that case what you have to do is put several smaller capacitors in parallel to make up the necessary capacity and ripple current ratings (ripple current is additive when capacitors are placed in parallel).

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