Safely discharging a capacitor

[paulfoel]

Going to attempt to change the capacitor on my tumble dryer... Its a 8
micro-Farad one.

I think its dead anyway but I'm assuming I can check with a multimeter?
What sort of voltage/current is one of these likely to have?

Assuming it is still ' live' how do I discharge it to make it safe?

 

[The3rd Earl Of Derby]

Going to attempt to change the capacitor on my tumble dryer... Its a 8
micro-Farad one.

I think its dead anyway but I'm assuming I can check with a
multimeter? What sort of voltage/current is one of these likely to
have?

Assuming it is still ' live' how do I discharge it to make it safe?

Does your meter support capacitance? if not you're wasting you're time,
these are only a few pence anyway in a maplins outlet.

 

[Andrew Gabriel]

Going to attempt to change the capacitor on my tumble dryer... Its a 8
micro-Farad one.

I think its dead anyway but I'm assuming I can check with a multimeter?
What sort of voltage/current is one of these likely to have?

Assuming it is still ' live' how do I discharge it to make it safe?

Most likely it is discharged through the motor windings unless
the switching is such that it gets disconnected, although
relying on the motor windings being intact is not smart when
the thing is known not to be working properly. Secondly, it
probably has a bleed resistor built in, although those can
fail too. I would probably just short it out with a screwdriver,
although that can in theory damage the capacitor. Leave it for
at least 5 minutes since last powered up, and then short out,
and you'll be fine. Making up a proper resistive discharger for
this one occasion just isn't worth it.

 

[vortex2]

fail too. I would probably just short it out with a screwdriver,
although that can in theory damage the capacitor. Leave it for
at least 5 minutes since last powered up, and then short out,
and you'll be fine. Making up a proper resistive discharger for
this one occasion just isn't worth it.

I was in the local Maplin shop the other day and saw that you can purchase 1
farad (huge) capacitors for power supply smoothing chav's in car audio
systems.

Must be interesting when you accidently short one of these!

http://search.ebay.co.uk/search/1-farad-capacitor_W0QQfclZ4QQfnuZ1


David

 

[Dave D]

Going to attempt to change the capacitor on my tumble dryer... Its a 8
micro-Farad one.

I think its dead anyway but I'm assuming I can check with a multimeter?
What sort of voltage/current is one of these likely to have?

Assuming it is still ' live' how do I discharge it to make it safe?

Does the motor have brushes and a commutator? If so, change the brushes
before you start looking elsewhere, they're seldom expensive and are a
consumable item. Capacitors may or may not last the life of an appliance but
brushes are certain to wear from day one and are usually the first suspect
in a misfiring/intermittant motor.

With brush type motors, the brushes wear down and consequently less pressure
is applied by the spring, causing a higher resistance/poorer connection
between brush and commutator. An arc is caused, which heats the brush and
can deform the casing causing the brush to stick, making the problem
increase exponentially. It also causes the commutator to become blackened
which makes matters even worse.

I'd take out the brush carriers and check the brushes move freely and that
there's plenty of length left on them. Also check the commutator is clean.

If the motor does not have a brush/commutator arrangement, I'd check all
connections are secure and check the control board for dry/cracked solder
joints before suspecting component failure.

Dave

 

[Slurp]

Must be interesting when you accidently short one of these!

Nah - that's nancy stuff for kids

..... real man stuff here!

http://www.amasci.com/amateur/capexpt.html

 

[Andy Dingley]

Assuming it is still ' live' how do I discharge it to make it safe?

Stick a big screwdriver across it. This is bad advice for capacitors in
general, but for these motor capacitors used on 240V AC only it's OK.
There is minimal likelihood of there being any charge left in there,
you're only shorting it to make _sure_ before you touch it by hand. If
it _had_ been charged up by the Workshop Pixies beforehand, then it just
costs you a screwdriver and a pair of trousers.

If you're fooling with HT DC on capacitors, then fit proper bleed
resistors before you start and make yourself a discharging stick with a
resistor in it.

 

[powerstation]

Does the motor have brushes and a commutator? If so, change the brushes
before you start looking elsewhere, they're seldom expensive and are a
consumable item. Capacitors may or may not last the life of an appliance
but brushes are certain to wear from day one and are usually the first
suspect in a misfiring/intermittant motor.

With brush type motors, the brushes wear down and consequently less
pressure is applied by the spring, causing a higher resistance/poorer
connection between brush and commutator. An arc is caused, which heats the
brush and can deform the casing causing the brush to stick, making the
problem increase exponentially. It also causes the commutator to become
blackened which makes matters even worse.

I'd take out the brush carriers and check the brushes move freely and that
there's plenty of length left on them. Also check the commutator is clean.

If the motor does not have a brush/commutator arrangement, I'd check all
connections are secure and check the control board for dry/cracked solder
joints before suspecting component failure.

Dave

nonsense it is clearly an induction motor.

 

[[email protected]]

Or how about this :-

http://www.electricstuff.co.uk/marxthree.html

 

[Dave D]

nonsense it is clearly an induction motor.

Probably, but I'm not at the OP's house to take a look and be certain this
is the case, and as the symptoms are classic signs of worn brushes, I
thought I'd mention it in case.

The rest of my advice about looking for bad connections before replacing
components is perfectly valid as well.

Dave

 

[Sam Goldwasser]

I was in the local Maplin shop the other day and saw that you can purchase 1
farad (huge) capacitors for power supply smoothing chav's in car audio
systems.

Must be interesting when you accidently short one of these!

http://search.ebay.co.uk/search/1-farad-capacitor_W0QQfclZ4QQfnuZ1

A capacitor with a digital readout? Now they've gone over the top!

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[paulfoel]

nonsense it is clearly an induction motor.

Yep. It is.

 

[Ron(UK)]

I worked for many years in the domestic appliance repair trade. To
discharge motor caps I just used a neon mains voltage tester, the type
with a croc clip to complete the circuit. thatgets the voltage down to
just a few volts almost instantly, whence you can safely short the cap
if necessary with a screwy or spanner whatever. Shorting out a fully
charged 8 or 10 muff cap with a screwdriver doesn`t do the cap,
screwdriver or customer any good In the absense of a neon device you
could use a light bulb or a resistor of a suitable value.

btw, I`ve never seen a tumble dryer in the UK with a brush type motor,
always induction motors with either capacitor or relay start.

Ron(UK)

 

[powerstation]

Yep. It is.

There is a TB regarding capacitors on White Knight TD's, the fault is caused
by the capacitor being affixed too close to the motor, the spare part now
includes a bracket to move it further away. Sometimes even when the part is
replaced the motor subsequently burns out if the windings have been damaged
by a stalled motor.

 

[Andy Dingley]

Probably, but I'm not at the OP's house to take a look and be certain this
is the case, and as the symptoms are classic signs of worn brushes, I
thought I'd mention it in case.

How many tumble driers use brushed motors?

How many brushed motors also have a capacitor?

Now this assumes that it _is_ a capacitor of course. If it does have a
brushed motor, I wouldn't be surprised if it had a line filter instead
and these can look very similar to capacitors.

 

[Ralph Mowery]

I worked for many years in the domestic appliance repair trade. To
discharge motor caps I just used a neon mains voltage tester, the type
with a croc clip to complete the circuit. thatgets the voltage down to
just a few volts almost instantly, whence you can safely short the cap
if necessary with a screwy or spanner whatever. Shorting out a fully
charged 8 or 10 muff cap with a screwdriver doesn`t do the cap,
screwdriver or customer any good In the absense of a neon device you
could use a light bulb or a resistor of a suitable value.

The neon tester will only get you to around the 70 volt level. Still not a
bad idea. That way you can see if it is charged to the full voltage. If
you are going to use a light bulb, you may as well use the screwdriver as
when the bulb is cold the resisitance is very low.

 

[Ron(UK)]

The neon tester will only get you to around the 70 volt level. Still not a
bad idea. That way you can see if it is charged to the full voltage. If
you are going to use a light bulb, you may as well use the screwdriver as
when the bulb is cold the resisitance is very low.

In theory, but a neon strikes around 70/90 volts, it maintains down to
a much lower voltage and will drain much of the charge. try it. Using a
bulb, you get a flash of light, using a screwdrive you get a big bang
and a flash of hot metal - better watch your eyes.

Ron

 

[Sam Goldwasser]

I worked for many years in the domestic appliance repair trade. To
discharge motor caps I just used a neon mains voltage tester, the type
with a croc clip to complete the circuit. thatgets the voltage down
to just a few volts almost instantly, whence you can safely short the
cap if necessary with a screwy or spanner whatever. Shorting out a
fully charged 8 or 10 muff cap with a screwdriver doesn`t do the cap,
screwdriver or customer any good In the absense of a neon device
you could use a light bulb or a resistor of a suitable value.

btw, I`ve never seen a tumble dryer in the UK with a brush type motor,
always induction motors with either capacitor or relay start.

Not sure how you'd get down to a few V if going through a neon tester.
They typically drop out at around 50-60 V.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

 

[CJT]

I was in the local Maplin shop the other day and saw that you can purchase 1
farad (huge) capacitors for power supply smoothing chav's in car audio
systems.

Must be interesting when you accidently short one of these!

http://search.ebay.co.uk/search/1-farad-capacitor_W0QQfclZ4QQfnuZ1


David

Does anybody worry about the inrush current to these, and what that does
to one's alternator?

 

[CJT]

Not sure how you'd get down to a few V if going through a neon tester.
They typically drop out at around 50-60 V.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.

Indeed. And they draw so little current that it'll take forever to
discharge a big capacitor using one.