DC motor reverses when lost of power

You haven't said how long the motor has to run after power is shut off.

When you say 'power is lost' does that mean someone threw a switch, or is it really a loss of power at the source. If the former, put some logic between the switch and the power supply to the motor---a lot of digital cameras are smart enough to retract the zoom lens before shutting down after you move the switch to 'OFF'.

If it's truly a power loss, you might consider a 3V rechargeable lithium battery and some logic.

Mark Borgerson

Exactly, to do work (like reversing the motor) there HAS to be some sort of energy available. Be it electrical or mechanical. Maybe he can get with that EER guy and he can pull the power from the ether ;). But seriously, he can try a bigger cap. Like one of the larger backup caps??? Jim

Hi, friends, Thx for your advice. Maybe the problem needs more clarification. I now have 2 devices, device A & B. A is with motor, but no power. B has power. Whenever A and B contact, the motor on A is power by the B. Now what i want is when A left B (A is lost of power), the motor on A could spin back for 10 turns before it stops. Actually H bridge or some logic circuits with big cap could do this. But space is a problem for me. I am just exploring new ways to rotate back the motor. Just thinking of using magnetic machanical force may help. Montoya

Montoya wrote: [...]

Still not enough information, but here's a try:

Use a torsional spring and a clutch that slips after 10 turns. The motor has to be strong enough to overcome the clutch friction, and the spring has to be selected to wind up 10 times before the clutch slips - maybe not possible.

Use a bigger CAP ? - above a certain size, such CAPS can be called Batteries.

-jg

Using a torsional spring is good idea, but unfortunately my motor's max torque is only in terms of mW. which makes it hard to overcome some friction.