Cap. Discharge Res.

[ray13]

Power equals (voltage squared) divided by resistance. P=E*E/R

 

[ian field]

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?

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.

 

[Arfa Daily]

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?

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

 

[M.Joshi]

Hello,

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

 

[TimPerry]

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

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.
http://www.kpsec.freeuk.com/capacit.htm

 

[Arfa Daily]

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.
http://www.kpsec.freeuk.com/capacit.htm

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

 

[M.Joshi]

Hi Arfa,

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

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

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

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

 

[TimPerry]

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?

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.

 

[ray13]

Normally they have a resistor across either the motor run or motor
start capacitor, here in the states we're talking 470K ohms at 1 watt,
in a 220-240vac world more like 1meg at 2 watts.. Really, if you don't
exceed the wattage or voltage rating of the resistor you will be ok.

 

[Arfa Daily]

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?

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

 

[M.Joshi]

Having opened up the appliance, I discovered that there is an X2 0.1µ
275V interference suppression capacitor across the supply which has n
load present when the appliance is in idle mode. Hence, the capacito
remains charged when the plug is removed.

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

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

 

[Arfa Daily]

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.

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