Re: OT: Condenser Capacitance Value

Posted by KEB on April 20, 2007 at 10:28:47 from (192.91.171.42):

In Reply to: OT: Condenser Capacitance Value posted by VinnieNC on April 20, 2007 at 07:57:48:

Vinnie,

A better, more commonly used approach is to use a diode as a "back-emf" suppressor on the load you're switching. Simply connect a rectifier diode, like a 1N4004 or something similar, across the load (not across the contact points), with the banded end connected to the positive side of the load and the non-banded end connected to the negative (grounded) side. If you really want to use a capacitor across the contacts, any disk capacitor in the 0.01 to 0.1 uF range would work, but in general its better to suppress the source rather than try to protect against possible effects.

The inductive "kick" from switching an inductive load is always the opposite polarity of the applied power, so the diode shorts the inductive spike but does not normally conduct.

What you're trying to do isn't quite analogous to a kettering ignition. Here, you want to suppress the effects of the back emf spike. In a kettering ignition, you want to use that spike to make a spark in the cylinder. In addition to limiting wear on the points, the condenser in a kettering ignition is part of a resonant circuit which "rings" when the points open, resulting in a hotter spark than you would get from the collapsing magnetic field in the coil alone.

Do a web search on "back-emf suppression" for more information than you'll ever be able to use. Actually, I seem to recall automative horns coming with suppression already installed, but can't be sure.

By the way, the comment below that a current surge will cause a capacitor to explode is not true. What typically damages a capacitor is a VOLTAGE spike high enough to puncture the dielectric material between the capacitor plates. Once this path is established, how much current flows depends on how much current is available from the source of the voltage spike; if the source impedance is low enough, it may be possible to drive enough current through the capacitor to cause it to overheat and rupture. It is possible to overheat a capacitor by trying to force too much alternating current through it, but this generally requires a sustained current source, not a surge.

Keith

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