At 11:39 AM 4/18/2010, Janne Ruohomäki wrote: >Isn't this something what the diodes installed in parallel with the >relay coil are for, in transistor controlled circuits ? Hi Yanne -- yes, that's what the diode is for. It's reverse biased against the coil drive voltage, so it doesn't short out the coil; but when the circuit opens the collapsing magnetic field in the coil drives its input terminal negative trying to keep the current flowing. The diode is then forward biased and discharges most of the stored energy in the coil. A capacitor would lower the voltage of the spike, but store the coil energy which would then pass back and forth, stored as an electrical field in the cap and as a magnetic field in the coil as it gradually used up the energy. This is how the old Kettering (points-type) ignition system works, and why the points burn up rapidly if the capacitor fails open or is missing. Yours, David |
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