EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: davelectronic on December 23, 2016, 08:55:14 pm
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My Son picked up a freebie scooter in a building. After testing the control pcb is toast. Not wanting to throw £££ at it i was thinking a simple relay to momentary switch hack. As the inital current and start is likley to be huge, i was looking for a soft start idea if possible. Its not worth me throwing pounds at it, but the motor spins up with 12 Volts applied. The SLA is 7 Ah i can't find a rating on the motor, but its been reduced down with gearing. Any ideas to a soft start with a high current relay and momentary switch would be great. Thanks, any ideas appreciated.
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If you're looking for something really quick and dirty (as in your button and relay) then a high power series resistor is the old-time DC motor starter solution. That would limit the stall current but have less effect as motor speed increases and current drops. If you took 1 Ohm as a starting point then that would limit motor current to 12A. Whether you go up or down from there depends on the motor size, mass you're moving, jerkiness etc.
It's not elegant, or battery efficient, you need a chunky resistor - it would disipate 140W stalled (less in practice with the thing running). Watch the burn risk! Also not very efficient in battery terms. Got any old wirewound heating elements etc?
Your other alternative might be to look on ebay or somewhere for a MOSFET PWM DC motor controller. You probably want to be looking in the 30A range.
Edit: Maybe something like this:
http://www.ebay.co.uk/itm/1500W-DC-12V-50V-PWM-30A-DC-Motor-Adjust-Speed-Controller-Potentiometer-TE565-/142081401882?hash=item2114b6241a:g:wsYAAOSw6n5Xq2b7 (http://www.ebay.co.uk/itm/1500W-DC-12V-50V-PWM-30A-DC-Motor-Adjust-Speed-Controller-Potentiometer-TE565-/142081401882?hash=item2114b6241a:g:wsYAAOSw6n5Xq2b7)
I see 10A ones too, but that might be cutting it close, not knowing the motor. Should be an easy job to move the pot external - presumably there's already some sort of hand control in place.
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You could try modifying a soft-start circuit for audio amplifiers, like this from the excellent Rod Elliot's site, (http://sound.whsites.net/project39.htm) dispensing with the transformer, rectifier and high voltage protection, which would simplify things a lot. The basic idea is setting up a timer which acts upon one relay: normally closed to some power resistors in series with the load, and normally open to a short to the load. The timer keeps the relay closed for a fixed time, the switches to full loading.
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I know this is a bit more than you were thinking, but it might not be too hard to put together a PWM controller.
A POT, microcontroller, and a couple of FETs can do the trick. Sure, there is a little more to it than just that, but once you understand it all it isn't too bad. If you wanted to skip the POT you could simply have the microcotnroller ramp up the duty cycle to help limit starting current spikes.
Good luck with whatever you figure out.