# Solid State Switch for magnet DC Generator

Posted on Feb 4, 2014

Using a flywheel-driven permanent magnet DC generator to charge up a bank of supercapacitors, 35 farad rated to 25V. For safety I limit the max charge to 22V via a comparator and a zener reference. Because I have no stable reference voltage at all in in this system, I rely on the zener for a reference. This small generator, at my max. RPM, put

s out 120V open-circuit and 2. 5A short-circuit. Charging up from cap bank at zero volts, indeed 2. 5A flows into the cap bank and voltage climbs. When the voltage on the cap bank reaches 22V, comparator U1 goes high. U2 serves to invert this signal and turn on PNP Q3. I want to use the signal at Q3`s collector to drive some kind of switch to disconnect the charging current from the DC generator, in order to avoid over charging the cap bank. My question is on Q1 and Q2, my hypothetical switch. Does this approach make sense Starting from zero volts on the cap, I expect Q1 to be on as its gate is connected to is drain through the 1M resistor. As soon as Q3 pulls up Q2`s Vg ~ 10V, I expect Q2 to pull down Q1`s Vg to near ground and effectively shut off charging current to C1. Due to feedback R7 & C2, U1 will not flip back again until ~ 20V. Let`s say that after charging C1 to 22V, Q1 goes into cutoff. Also, the DC generator is at a stop, so Vds = -22V Also, Vgs = -22V. I`m not sure if this condition will wreak havoc with Q1/Q2. The part number ( IP8107N20N3 G ) is spec`d for Vds = 200V and Vgs = +/- 30V but I`m not comfortable with that negative Vds on Q1. Maybe I am going about this switching completely the wrong way. I`ve already burned out an SSR, which used to be in place of Q1, because my amateur experience forgot that when the load is removed from the generator, its open-circuit voltage climbs to 120V and my DC SSR was...

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