Cut power to motor under peaked load

Community Projects
1

The system is for my the rear shock on my motorcycle. The shock had an adjustable preload. Basically a small hydraulic piston that preloads the spring on the shock. The shock comes from another make of motorcycle, so I don’t have the computer that would normally read the preload position and stop the motor when the hydraulic piston reaches the end of its range.

I control the bi-directional preload motor with a DPDT switch through two relays on my PCB. Both relays have GND to the NC pin, and positive to the NO pin. When I toggle the switch up or down, the corresponding relay opens and sends power to one of the motor’s poles, turning it in one direction or the other.

The issue I have is that without the computer, I don’t have any protection against bottoming out the motor. The hydraulic piston rides a screw drive. I bottomed it out once, which pushed the load on the motor and blew my 5a fuse. I haven’t measured the normal operating load of the motor, but I’m told it’s about 2 amps. What I’m hoping to do is have a cut off that stops the motor if the load exceeds a certain threshold. Maybe it would disconnect power to the two relays. Does that sound reasonable? I’ve been searching online, but I can’t seem to get the right combination of terms.

Thanks for any advice you can give.

2

It sounds like you need a resettable fuse for the motor’s maximum current.

3

Brilliant! I didn’t know those existed. Looks like an E-T-A 5A circuit breaker could do it for me. Just replace the regular 5A fuse I’m already using.

Thansk!

4

I was thinking more along the lines of a PTC electronic resettable fuse (basically a thermistor that switches off when it gets hot enough), but the ETA breaker should work also.

5

PTCs (polyfuses) and fuses and breakers have very different characteristics and intended end-use applications.

Fuses are current-interrupt devices where the time-integral of the over-load is important.

PTCs are typically used to provide a very high impedance to DC current after a few seconds or minutes of current that exceeds the rating. The self-heating that causes an increased DC resistance a very slow reaction relative to a fuse. Hold current ratings vary significantly with ambient temperature, so the ‘distance’ between the hold and trip ratings of the device can vary an order of magnitude.