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Old 10-28-2021, 05:33 PM   #7
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Default Re: Master power switch for E-bike ideas

Originally Posted by Dannyx View Post
Got a couple of these in my junk drawer....the specs would seem adequate, unless I'm completely missing something.
Might be OK... though I do suggest as well to use an actual physical switch/solenoid/contactor, as stj suggested... because if the motor controller shorts a MOSFET, you may not have a fun time on that bike anymore.

Also, you may not be able to do a slow turn-ON (soft start) with that MOSFET, even though it is rated for 75 Amps. Reason why: look on page 7 at the Maximum SOA (Safe Operating Area) curves to understand why. So long as you use the MOSFET as an actual switch (that is, instantly turn On without trying to do any kind of soft-starting), then the maximum current is pretty much limited by the R_ds(on) resistance. But as V_ds increases, you start using the MOSFET as a linear regulator, and that will start to give limitations on the current you can pass through it. For example, with 50V difference (48V of the battery with a bit of over-estimation ) between Drain and Source, the current the MOSFET can pass becomes limited to about 2.5 Amps DC. That is, if you try to pass more than that, you will overheat the die and blow the MOSFET. However, if your soft-start circuit is active for only about 1 ms before the MOSFET is fully turned On afterwards, then the maximum current you can pass though it is about 20 Amps.

That said, may I suggest a possibly simpler solution to your circuit: instead of doing a soft-start with a MOSFET, how about use a solenoid/contactor or a switch to connect/disconnect the batteries. But to prevent sparking in the switch/contactor, check how much "idle" current the motor controller uses when it is connected to the battery and the motor on the bike not running. Let's say it's just a few mA, like less than 10. Then you can install something like a 1 KOhm, 3-5 Watt resistor across the switch/contactor contacts to allow the caps inside the controller to charge up slowly as you connect the battery. Once they charge high enough, you can engage the switch/contactor, and there would be no or very little sparking in the contactor/switch, because the voltage difference between the almost fully charged caps and the battery should be very small... hence smaller currents.

Last edited by momaka; 10-28-2021 at 05:55 PM..
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