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Old 10-24-2021, 07:42 AM   #1
Dannyx
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Join Date: Aug 2016
City & State: Constanta
My Country: Romania
Line Voltage: 230VAC 50Hz
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Posts: 3,248
Talking Master power switch for E-bike ideas

Good day folks. I recently upgraded my bike with a sketchy motor and battery set I picked up from Ali. I say "sketchy" because the motor is rated 1.5kW (at least on paper), so such a retrofit is borderline illegal here and would require registration and possibly a type-A driver's license to operate on public roads, but looking past all that, it's a lot of fun and does go very fast....so fast in fact that the thing does wheelies on its own if I gun it hard enough

There's one thing I'm looking to improve: no master kill-switch on this thing at all. The 48v battery pack is permanently connected to the control box via an XT60 connector. Of course, I could just disconnect the connector when it sits unused for long periods of time, like during winter now, but I noticed there's a big spark inside the connector upon re-connection, which would ruin the connector pretty quickly. This tells me the control box must have some large-ish caps in it and no inrush limiter, so I thought I could combine an on-off feature and an inrush current limiter.

A while ago, I briefly discussed such a circuit in my thread HERE, when I was dealing with an amplifier board which also exhibited arcing upon power-up and someone suggested a simple circuit based around a MOSFET which I actually built and tested and it appeared to work relatively well, so I thought I could adapt this circuit and beef it up enough to work with my bike.

Here's the schematic I adapted from the other thread: the values for the components may not be correct (in fact I'm certain they're not and will require adapting), but the concept should be the same. The 48v battery is on the left and the rest of the bike, the load (control box + motor) is on the right. With SW1 open, the gate of M1 is pulled low to the GND of the battery by R2, so it is "open" - no spark can occur upon connection. Afterwards, SW1 closes and the C1 charges up slow-ish via R1, initiating the "soft-start" state, thus limiting the current should SW1 already be in the "closed" state upon insertion of the XT60 connector. D1 clamps the maximum possible voltage to the gate of M1, since 48v would immediately destroy it. I was told D2 discharges C1 through the "load" when the switch is opened or the battery is unplugged, so the soft start operates upon the next power-up, otherwise M1 would still be on, nullifying the soft-start effect. Alternatively, I could make SW1 a SPDT so it throws the cap either to the "charge-up" resistor when "on" or to a "discharge resistor" when "off" to drain it...that's the theory at least. M1 would have to be pretty beefy AND as low an RDSon as possible to withstand both the high inrush current of the cap when it's in that linear region when it turns on AND THEN pass the required current to the motor when running, which although I haven't measured myself, I estimate to be around 30-40A according to the control box's max specs, upon take-off from a standstill, when the rotor is at 0 RPM and the current draw is at its maximum.

There's another thing I haven't taken into account and it just occurred to me: this thing has a regenerative braking feature which uses the battery as a load to slow the motor down (yes, I tested it and it actually works - not just marketing BS ).....would this cause issues when pushing current back INTO the battery through the FET ?

Let me know if you have any suggestions and whether this is a viable way of switching a high current DC circuit....
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