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Registered Member #3124
Joined: Tue Aug 17 2010, 10:25AM
Location: 127.0.0.1, US
Posts: 19
Someone brought me this little capacitor and had no idea even what it was. I have found that it's a capacitor using my multimeter and reads about 293nf. I would like to know what it was used for/in and if anyone can tell me what the voltage rating is on it.
It has nothing printed on it besides "Made In Mexico", has a little clamp thing with a screw hole which says to me that it must have been hooked onto the chassis of something, the outer metal is the negative, and I'm guessing it's not electrolytic since it doesn't appear to have any way to vent it's self.
Registered Member #3124
Joined: Tue Aug 17 2010, 10:25AM
Location: 127.0.0.1, US
Posts: 19
Proud Mary wrote ...
It looks like an ignition coil capacitor from a petrol engine.
Thanks, I was sure someone would instantly recognize it. Do you happen to know the voltage rating these generally run at? I'm assuming since it's for an automobile it probably takes 12 to 16v? Or since it's in some way hooked to the ignition coil, could it be rated for 20kv? I'm no auto mechanic.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
In petrol engine service, the capacitor would typically charge up to 300-350V, so I would guess that the actual voltage rating is probably about 1kV, a generous safety margin to allow for long service life in a hot environment.
But very similar looking capacitors are also used as spark suppressors in electric motors and switching devices, and there is no way I can tell apart from your pictures.
Registered Member #3124
Joined: Tue Aug 17 2010, 10:25AM
Location: 127.0.0.1, US
Posts: 19
Proud Mary wrote ...
In petrol engine service, the capacitor would typically charge up to 300-350V, so I would guess that the actual voltage rating is probably about 1kV, a generous safety margin to allow for long service life in a hot environment.
But very similar looking capacitors are also used as spark suppressors in electric motors and switching devices, and there is no way I can tell apart from your pictures.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
One clue could be the capacity. The ignition capacitors were inside the distributor connecrted across the breaker points, When the points opened the primary of the coil was in series with the 12 volt battery and this capacitor. When the points closed the coil was across the battery. When the points opened, the spark plug that the distributor had selected, would fire. The lug on the wire has that shaped spade to grip the bolt while it is being tightened. One way mechanics tested them was to charge them up and see if they made a spark. But usually a tune-up included 'replacement of the points (the breaker assembly and the "condenser"
Registered Member #3124
Joined: Tue Aug 17 2010, 10:25AM
Location: 127.0.0.1, US
Posts: 19
radiotech wrote ...
One clue could be the capacity. The ignition capacitors were inside the distributor connecrted across the breaker points, When the points opened the primary of the coil was in series with the 12 volt battery and this capacitor. When the points closed the coil was across the battery. When the points opened, the spark plug that the distributor had selected, would fire. The lug on the wire has that shaped spade to grip the bolt while it is being tightened. One way mechanics tested them was to charge them up and see if they made a spark. But usually a tune-up included 'replacement of the points (the breaker assembly and the "condenser"
Maybe I'm misunderstanding the reply, but I really don't know much about the inside of automobiles. I did a little research on distributors anyhow and found that this is definitely the same type of capacitor. Thanks. But I am still unsure about it's voltage. Judging from what I read, this capacitor went before the ignition coil (in the distributor) and therefore was charged to about 12vdc then discharged into the coil. Is this correct?
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
If it was on the distributer, it could have been put in place to prevent the inductive kickback from the ignition coil getting back into your car's circuitry.
The kickback can be pretty significant, a couple hundred volts usually, which could be enough to interfere with other systems in the car.
I dunno about this, just an idea. Not really any other need for a capacitor on the distributer. Unless it was an arc suppressor or something.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
D1G1T4L3CH0 wrote ...
radiotech wrote ...
One clue could be the capacity. The ignition capacitors were inside the distributor connecrted across the breaker points, When the points opened the primary of the coil was in series with the 12 volt battery and this capacitor. When the points closed the coil was across the battery. When the points opened, the spark plug that the distributor had selected, would fire. The lug on the wire has that shaped spade to grip the bolt while it is being tightened. One way mechanics tested them was to charge them up and see if they made a spark. But usually a tune-up included 'replacement of the points (the breaker assembly and the "condenser"
Maybe I'm misunderstanding the reply, but I really don't know much about the inside of automobiles. I did a little research on distributors anyhow and found that this is definitely the same type of capacitor. Thanks. But I am still unsure about it's voltage. Judging from what I read, this capacitor went before the ignition coil (in the distributor) and therefore was charged to about 12vdc then discharged into the coil. Is this correct?
In normal operation of the now obsolete electro-mechanical distributor, the back EMF across the capacitor typically reaches ~300V.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The capacitor is an inherent part of the spark generation, not just a noise supressor.
When the contacts close, current flows through the iggy coil primary, storing energy in the magnetic field. When they open, that current flows into the capacitance across the opening contacts. If there is just the stray capacitance of the coil, the voltage rises very fast and very high into that small capacitance, too fast for the opening gap to withstand, and an arc strikes. Once the arc has struck, the voltage falls - limiting the voltage available to the spark, and the contacts get eaten fast. With the very significant extra capacitance of that doohikey pictured across the contacts, the rate of rise is much slower and to a lower voltage, typically only 300v, which the opening gap can withstand without arcing. The ignition coil transforms that 300 by 1:100 into the 30kV or so to power the plugs.
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What kind of Cap is this?
