I have a motor setup to spin an object. The object is top heavy so it spins with an irregular motion. On the upswing the little sucker draws quite a bit of current but on the down swing it uses very little. The motor runs off of two 6v batteries in series, i.e. a 12v power supply.
What I'm curious about is how bad the downswing is assuming it may actually be generating power. I have a flyback diode that should protect against voltage spikes but I'm curious if I should have a capacitor as well to smooth the power flow.
If so how should I connect it? Just stick it straight on the ve+ line or on the motor + and - terminals?
Much depends upon the current involved. For low-ish currents, you could just put a diode in series with your power supply to prevent current from flowing back into the batteries. How much is "quite a bit of current"?
The capacitor is not a bad idea, especially if you do put a diode in series with the batteries. You would indeed put it in parallel with the power source to your motor driver (+ve to +ve and -ve to -ve).
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I don't think that I would worry about this question at all only , IF the motor driving circuit is of a " purely resistive " type . ( only the motor , batteries and included wiring )
Have you ever heard of " dynamic braking " ? The back EMF of the motor creates a reversing current ( Faraday ) WHEN the motor's input leads are strapped with a resistor . Inertia of the system coupled to the motor does the braking thru the motor . This App is along the same lines as far as I can tell .
As has already been said, a diode in series with the motor will prevent current being fed back to the battery. What sort of non-rechargable battery are you using?
Given the mechanical losses in the system I doubt the motor puts more than 10% of the energy back in on each down-stroke, which I don't think is going to cause problems, but I might be wrong. I worry there is a v. small risk of pressure build-up in such a situation - perhaps manufacturers datasheet has info?
Perhaps the safest advice is replace them with appropriate rated rechargeables? (A good idea anyway in long run?)
If you blow out your batteries, it won't be from stray voltage generated on the downswing, it'll be from trying to source more than they can handle - though I've never heard of anyone managing this with disposable batteries. As previous people have mentioned, you can add a diode to your system to keep electricity flowing in one direction only. How many amps are being sourced on the upswing, and what sort of battery are you using?
I use a diode to prevent reverse voltage into the transistor (mosfet) that drives the motor which should work fine. I also went ahead and hooked up a cap from +ve to -ve but I haven't looked at it on an oscilloscope yet to see the difference.
The draw is at max about 500ma on upswing (i.e. pulling frame's full weight up and away from gravity). On the down-swing (i.e. inertia moving the frame towards direction of gravity while motor still working) my multi-meter reads as low as 80ma but I'm not sure if that is accurate since it is digital and the reading latency is somewhat high.
Your batteries will only be "recharging" when the input voltage is greater than their static voltage. Likely your motor has to spin quite fast to generate 12 V. When your gig is running, measure the voltage across the battery terminals. If it is greater than the battery voltage when the motor is not running, the batteries are being recharged.
Without seeing your rig, I don't think you need to worry. If you want peace of mind though, a diode won't hurt.