Cap. Discharge Res.

What are you trying to accomplish? If you need a safe way to dissipate the charge on a mains IP reservoir cap, I find a salvaged NTC inrush surge limiting thermistor from a scrap monitors PSU section works very well. It starts off with a high resistance before you apply it to the cap terminals and is heated by the energy discharging from the cap so the resistance decreases to fully drain the cap.

Any cap that is placed across the mains in a piece of equipment, should not in theory require any resistor to discharge it, since the load presented by the equipment itself should do that. The value of any such caps is usually very small anyway, so any residual charge from the last half cycle of the mains that appeared across it, will be more or less *instantaneously* discharged by the load.

Arfa

Hello,

I was wondering if there is a method of calculating the value and wattage for the resistor that goes across a mains capacitor to discharge it once the power has been removed?

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M.Joshi

Arfa, some of the equipment i work on used high voltage supplies to power tubs power amplifiers. large bleeder resistors are place across the HV filter capacitor in the event of a tube failure (or some other type of failure) that would leave a HV cap charged up.

these are often 100 k ohm 200 watt wirewound placed in series/parallel. the ides is to discharge 7,300 volts in about two or three seconds down to a "safe" level (at least safe enough to open the doors without a giant arc when the safety shunts engage.

other units switch the bleeders in when the doors open.

M.Joshi, the resistance is set by taking the voltage and capacitance into consideration and deciding how fast you want to discharge the cap. then wattage is determined by E max squared / R then multiply by whatever safety factor you feel comfortable with and round off to the nearest standard value.

here's a place to read about time constants.

Hi Tim

Agreed. Everything you say is absolutely valid, but I didn't think that was what the OP actually asked. He specified " mains capacitor " which I took to mean a capacitor across the mains, ( ie line power in the US ) which many items of equipment have for transient suppression purposes, or as part of a filter network to prevent crud from the equipment going back up the mains.

If he actually meant a resevoir or smoothing cap, or some other HT decoupling cap on the back side of a rectifier, then that is altogether a different matter. The only thing I would add to your explanation of determining values for such a discharge resistor, is that the value of the voltage that will be across the cap / resistor combination, is critically important in this case. Everybody knows that resistors have ohmic values, and power ratings, but many do not know that they also have a voltage working rating, which with many resistor types, is not very high at only 2 or 3 hundred volts. If the intended application exceeds the maximum working voltage of the selected resistor type, then the value should be halved, and two of the same value placed in series, across the cap to be discharged. This will double the effective working voltage.

I have also seen 4 resistors used ; two 2-resistor chains, in parallel across the cap to be discharged. Presumably, this gets a single resistor, of twice the voltage rating, and twice the power rating, with a built in safety factor that if any one resistor goes open, there is still a discharge path, allbeit a slower one, across the cap.

Arfa

Hi Arfa,

You are spot-on, I am referring to the capacitor across the mains/supply (Usually termed X-type).

We have a kitchen appliance (Moulinex - I think) which has a motor and is connected by a switch which automatically disconnects when you stop using the machine.

I have found that if the plug is removed, you get a small electric-shock if you accidently touch the live & neutral pins. I tested this by plugging the appliance into the mains socket, then removing the plug and shorting the two pins together. You can see and hear a small spark!

I am assuming that either the appliance has a faulty resistor connected across the supply capacitor or they failed to include one at manufacture?

Arfa Daily Wrote:

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M.Joshi

oh, a juicer!

don't worry about it. the charge contained will be too small to damage anything.

unless it a UK term that i have not heard of there is no such thing as a "mains capacitor". you are referring to an RFI/EMI filter. i doubt anyone would put power factor correction (PFC) in a small appliance.

any modification involving the addition of a resistor to the appliance would likely increase the potential for fire hazard. removal of the filter would possibly increase buzz in you sound system or interfere with radio/TV reception when the appliance is in operation.

You might want to just open the thing up and have a look to see if there is a resistor across the cap, or that it's not a special CR combination in an encapsulated package, as they sometimes are. I think it likely you will find that there is no such resistor present, and the manufacturers felt it safe for it to be like that. It will have undergone stringent safety tests at the design and prototyping stages, and will have had to have shown compliance for safety regulations to have gained its CE approval. Moulinex may well self-certify of course, but it is more than they would dare do to 'fudge' this.

If there is no resistor present, I wouldn't advocate adding one, because as another poster has suggested, this would quite possibly compromise things like fire safety. Bear in mind also, that any resistor across the mains, has got to be capable of withstanding at least the peak value of the UK mains, which is up around 360v worst case, so would realistically want to be rated to withstand 500v.

What you are encountering is by no means uncommon with this sort of equipment, but does seem to be 'normal'. I would guess that the amount of energy stored in the relatively small value of the cap, is probably considered harmless.

Arfa

Having opened up the appliance, I discovered that there is an X2 0.1µF

275V interference suppression capacitor across the supply which has no load present when the appliance is in idle mode. Hence, the capacitor remains charged when the plug is removed.

I'm sure I have seen resistors across X2 capacitors in-circuit usually in the Mega-Ohm range?

Looking at the Maplin metal film 2W resistors, they can handle 500V.

Arfa Daily Wrote:

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M.Joshi

Well, I guess that one of them of say 1M, is probably not going to do any harm. It will only result in a peak leakage current of around 360uA, and negligible power dissipation ie heat in the resistor. It should discharge the cap to a safe level in less than a tenth of a second, so much less time than it would take you to get the plug out of the wall, and touch the pins. Go for it, if that is your inclination. I think it would probably be safe enough, provided you do a decent job of the soldering, and pay attention to insulation and spacing issues.

Arfa