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Old 03-27-2012, 07:06 PM   #48 (permalink)
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Originally Posted by mechman600 View Post
In my field circuit, am I correct in assuming that the voltage spike is very low amperage? I am basically electronics illiterate. Can you give me the part number of a suitable diode?
Hi mechman,
The field winding is an inductor. A voltage across the inductor causes a current to flow and the inductor stores energy in a magnetic field, it's like a flywheel all spun up. When the voltage is removed, opening the field switch for instance, the current continues to flow, at a falling rate depending on the total amount of energy stored. If the voltage is removed abruptly the full current that was flowing now sees a very high resistance, and E=I*R so the voltage spikes. If the circuit was hooked up with the battery positive and then a switch and then the field coil and then the battery negative, just when the switch is opened the current was flowing through the coil to the battery negative. That current continues at the same rate and direction, so the voltage at the switch end of the coil goes way negative (compared to the battery positive voltage that it was when the switch was closed). The diode needs to conduct the full current that the coil carries. The voltage spike won't be high because the diode resistance is low (and so the spike is suppressed). The highest voltage the diode sees is full battery voltage. For up to 144 volts, a 200 volt diode with a forward current rating higher than (like twice) the maximum field current will do.

Do you know the field resistance or maximum current and voltage?

The diodes you link to are Schottke, which are low forward voltage and fast, both things you don't need and cost extra. Schottkes aren't bad, just unnecessary. I'd imagine the maximum field current is more than 5 amps, so something like this might be better. But you don't need 10. Also you can find cheaper diodes locally, I bet.

Also, as an aside, Daox refers to transient suppression diodes which are made exactly for this job, except they are expecting electronics type voltage, like 12 or 15 volts.

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