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Old 10-19-2010, 02:12 PM   #3853 (permalink)
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Originally Posted by adamj12b View Post
First, What current are the zener diodes rated for? That have to be able to take the same amount of current as the MOSFETS can put out.

Also, with no capacitors near the fets, the controller will only last but a few seconds. Maybe not even that. The capacitors are in place to absorbe the voltage spikes from switching to protect the MOSFETS, not to help the batteries. They need to be placed as close to the switch as you can get it, with the smallest inductive loop.

Zener is 1n5388bgos. You will have to clarify your statement that the diodes "have to be able to take the same amount of current as the MOSFETS." The diodes are basically a short-circuit for inductive feedback voltages so the peak current they receive is directly proportional to the inductor size and inductor current and inversely proportional to the mosfet turn-off time. I don't think the size the diodes need to be can be accurately determined by simply looking as mosfet current. These diodes could indeed turn out, with further testing, to be too small but not for the reason you suggested.

Regarding your statement that capacitors are needed to prevent voltage spikes on the mosfets. If you look at the diagram for your controller, you will notice that the capacitors are simply placed from battery positive to battery negative. That configuration does very little (or almost nothing) to smooth out "voltage spikes (on the drain) from switching." The voltage spikes occur on the DRAIN of the mosfet and these are caused by inductive feedback from the motor (inductor). This inductive feedback is caused by a magnetic field that is generated around the inductor (motor) (while the mosfets are in the ON state) that collapses suddenly (due to mosfet opening abruptly). There are no capacitors attached to the DRAIN, thus there is no protection for the mosfets by the capacitors (which have been placed across the battery terminals only).

Last edited by princeton; 10-19-2010 at 03:17 PM..
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