Safely discharging a capacitor

[Palindr☻me]

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

No.

Car wiring is radial from the battery to alternator and from the battery
to load. The impedance of the battery is much lower than that of the
alternator radial circuit - thus transient currents will be met almost
entirely by the battery.

You may like to think what effect the starter motor has on the alternator.

Also, the energy stored in a capacitor depends on the capacitance and on
the square of the voltage. Rather than have a 1F capacitor on the 12v
line, it would be much better to go to the higher voltage power rails of
the amplifier and stick beefy capacitors there.

 

[James Sweet]

nonsense it is clearly an induction motor.

Why do you say that? Most European washing machines use series wound
universal motors with carbon brushes. I've got a Creda washer sitting in
my basement right now awaiting arrival of a set of brushes amoung other
things, no idea how it ended up on this side of the pond but it's a cool
little machine.

 

[James Sweet]

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

They went over the top years ago, those capacitors have been available
for a while.

Just try to find a car CD player anymore that looks at home in anything
but a gaudy racer boy Japenese compact. Seems like they're all fugly and
bubbly, loaded with useless distracting blinky lights and buttons so
small and jumbled it's impossible to operate them safely while driving.

 

[James Sweet]

No.

Car wiring is radial from the battery to alternator and from the battery
to load. The impedance of the battery is much lower than that of the
alternator radial circuit - thus transient currents will be met almost
entirely by the battery.

You may like to think what effect the starter motor has on the alternator.

While this is true, don't forget that the starter motor will never be
operating under load while the alternator is producing any current
unless you're jump starting another car.

 

[Bob Eager]

Why do you say that? Most European washing machines use series wound
universal motors with carbon brushes.

But we were talking about a dryer. They seem to use induction motors
(well, mine does).

 

[powerstation]

Why do you say that? Most European washing machines use series wound
universal motors with carbon brushes. I've got a Creda washer sitting in
my basement right now awaiting arrival of a set of brushes amoung other
things, no idea how it ended up on this side of the pond but it's a cool
little machine.

But its NOT a washing machine he's fixing ! its a tumble drier

 

[powerstation]

But we were talking about a dryer. They seem to use induction motors
(well, mine does).

All the ones I have ever seen are, because they revolve the drum slowly, a
washing machine needs to spin at high speed

 

[The Natural Philosopher]

They went over the top years ago, those capacitors have been available
for a while.

Just try to find a car CD player anymore that looks at home in anything
but a gaudy racer boy Japenese compact. Seems like they're all fugly and
bubbly, loaded with useless distracting blinky lights and buttons so
small and jumbled it's impossible to operate them safely while driving.

Thats because youy get a really good one bult nto any car that chavs don't
buy.

 

[Andrew Gabriel]

Why do you say that? Most European washing machines use series wound
universal motors with carbon brushes. I've got a Creda washer sitting in
my basement right now awaiting arrival of a set of brushes amoung other
things, no idea how it ended up on this side of the pond but it's a cool
little machine.

A European washing machine operates its motor at a large
number of different speeds (and even gradually vary the
speed in some cases) throughout various stages of the
wash cycle, and depending on the wash program selected.
This is most easily done with a universal motor combined
with an electronic speed control board and servo feedback.
(It used to be done with an induction motor and solenoid
operated gearbox 40 years ago, but that's more expensive
and a lot less flexible.)

Tumble driers only have to be able to reverse the drum,
but don't need to change the speed. For this simpler
requirement, an induction motor tends to win.

 

[The Natural Philosopher]

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

its nowhere as bad as what a statrter motor does...alternators are limited
anyway.

 

[James Sweet]

But we were talking about a dryer. They seem to use induction motors
(well, mine does).

Yeah somehow I missed that we were discussing a dryer. The matching
Creda dryer does in fact have an induction motor in it, weird to see a
clothes dryer that will plug into a 15A 240v receptacle, US dryers are
almost universally 4KW.

 

[James Sweet]

Tumble driers only have to be able to reverse the drum,
but don't need to change the speed. For this simpler
requirement, an induction motor tends to win.

Why do they have to be able to reverse the drum? I've never seen a dryer
that did that, obviously they must exist but I'm not sure of the benefits.

 

[James Sweet]

Thats because youy get a really good one bult nto any car that chavs don't
buy.

Would be nice, but I'm afraid back in '84, '87 and '88 respectively,
cars didn't come with CD players of any sort, hence my need to install
aftermarket units in mine.


Not to mention I'm still not aware of any OEM units that will play MP3
discs, an essential feature to me that made all earlier CD players
virtually obsolete. 10 hours of music on one disc, no more fumbling with
CD's in traffic.

 

[Bob Eager]

Why do they have to be able to reverse the drum? I've never seen a dryer
that did that, obviously they must exist but I'm not sure of the benefits.

Loads of them do it[1]...helps to untangle the clothes.

[1] except in the backwards USA?

 

[raden]

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?

It depends what's across it,

stick a screwdriver across it - that'll discharge it

 

[news07]

Nah - that's nancy stuff for kids

.... real man stuff here!

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

Where does one start, any site that had the following in the disclaimer

'DISCLAIMER: the experiments described below are fantastically
dangerous, and they are described without reference to the
many precautions needed to guarantee the experimenter's safety.
Accidentally discharging these capacitor banks through your
body can not only kill, but can explode flesh and bone.'

Followed by

'PARENTS: I supply no detailed plans for reproducing these
experiments. Also, these experiments require large and
expensive lab equipment which is not obtainable by children.
(And the plans for an atomic bomb are safe for children too,
because kids can't afford to buy kilograms of Plutonium!)
If your kids have access to 5,000 volt high-current power supplies,
then they are already in great danger, whether or not they read
about my capacitor discharge experiments below.'

Has my respect!

 

[raden]

Or how about this :-

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

STOP IT STOP IT STOP IT

You're giving me ideas

 

[[email protected]]

nonsense it is clearly an induction motor.

According to my Electrical Engineering professor at university *all*
electrical machines (including transoformers) are really induction
motors.

 

[powerstation]

According to my Electrical Engineering professor at university *all*
electrical machines (including transoformers) are really induction
motors.

Motors with squirrel-cage rotors can be used on single-phase alternating
current by means of various arrangements of inductance and capacitance that
alter the characteristics of the single-phase voltage and make it resemble a
two-phase voltage. Such motors are called split-phase motors or condenser
motors (or capacitor motors), depending on the arrangement used.
Single-phase squirrel-cage motors do not have a large starting torque, and
for applications where such torque is required, repulsion-induction motors
are used. A repulsion-induction motor may be of the split-phase or condenser
type, but has a manual or automatic switch that allows current to flow
between brushes on the commutator when the motor is starting, and
short-circuits all commutator segments after the motor reaches a critical
speed. Repulsion-induction motors are so named because their starting torque
depends on the repulsion between the rotor and the stator, and their torque
while running depends on induction. Series-wound motors with commutators,
which will operate on direct or alternating current, are called universal
motors. They are usually made only in small sizes and are commonly used in
household appliances.

 

[Asimov]

"Andy Dingley" bravely wrote to "All" (21 Dec 05 14:16:40)
--- on the heady topic of "Re: Safely discharging a capacitor"

AD> From: Andy Dingley <[email protected]>
AD> Xref: core-easynews sci.electronics.repair:352066
AD> free.uk.diy.home:47734 uk.d-i-y:545608

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

AD> Stick a big screwdriver across it.

Now why ruin a perfectly good screwdriver?! You should see mine, it's
full of big burned out pits from starting up cars at the solenoid.
Mind that disharging those 50 volt 100,000uF audio amp supply caps is
a little more hazardous than say a 6.8uF AC motor start cap.
(grains of salt ->...)

A*s*i*m*o*v

.... You may be a tech if you're entertained by a 6-pack and sparking HV.