Posted by Sherman in Idaho [24.32.202.166] on Thursday, July 11, 2013 at 01:41:19 :
In Reply to: Condenser capacitor value? posted by derek [68.124.177.75] on Thursday, July 11, 2013 at 00:00:14 :
Lots of cheap DVMs like you get from Harbor Freight for $20 have a capacitance measuring range these days. So, get a known-good new condenser and measure it. Or open the can and read the numbers (sometimes color-coded stripes) off of whatever's inside. Once upon a time, the whole can WAS the condenser, full of rolled up paper and aluminum foil and soaked with oil, but nowadays they just stick a standard commercial capacitor in there.
If you order one up from Mouser, I would go for a "film" capacitor, probably 100 volt rating. Most electronics these days uses ceramic capacitors because they have much more capacitance in a smaller space and are cheaper for the same value, but their capacitance varies all over the place with voltage, they can be mechanically fragile, and in this case, size doesn't matter anyway. Film (mylar or polyester) capacitors are a roll of plastic film and foil, sort of a modern version of the old oil and paper kind. They have much better voltage/capacitance characteristics (the audiophiles like them) and they're still plenty cheap enough for our purposes. 0.1 uF or 0.22 uF are standard values that would probably be about right. You want the high voltage rating because in an automotive system, it's easy to get spikes of 50V or more. Film capacitors are "self-healing" in the sense that an overvoltage will burn through the film at one point, and burn the foil back from around the hole so that the capacitor continues to function, rather than shorting out (which is a nice feature), but each time it does this, it loses a bit of its capacitance. If the capacitance is too small, the distributor points will start to burn.