Simple stall over current 12v dc motor protection?

[supak111]

Hey guys, are there any simple circuits out there that can protect a motor from stall over current? I run a 12v dc gear motor from a battery and sometimes something jambs up the motor and I would like some circuit that can sense the motor has stop and automatically turn OFF the power to the motor.

Currently I'm just using a PTC fuse do protect it but they are slow, on average they take about 10-15 seconds of over current before the PTC fuse stops the current from hating the motor.

 

[Alec_t]

Have you tried a slow-blow fuse?
Whatever you use will be a compromise, since it will have to allow the motor to get up to speed from a stopped/stall state.

 

[supak111]

Yea a mechanical fuse isn't really suitable, this happens a lot so I would be replacing fuses all the time. Has to be some kind of smart system that can tell if the motor uses more then say 500ma for more then 3 sec to just turn OFF and perhaps reset back on in 10+ or so seconds later.

 

[CDRIVE]

I'm thinking of a system with a bit more brains. Instead of simply monitoring for stall current how about a system that monitors both motor current and RPM? This way the circuit can differentiate between a stall and startup current.

As with many things these days this is an easy job for a μC.

Chris

 

[Minder]

You just need a small correctly rated circuit breaker, these do not operate immediately but have a small time delay to allow for inrush.
M.

 

[CDRIVE]

You just need a small correctly rated circuit breaker, these do not operate immediately but have a small time delay to allow for inrush.
M.

Now this logic is all wrong! Why make things simple?!

Chris

 

[Colin Mitchell]

Simply detect the brush noise from the motor to turn off the supply.

 

[CDRIVE]

Simply detect the brush noise from the motor to turn off the supply.

Now THIS is really scary! When I suggested testing for RPM I was thinking along the same line! Has the Earth stopped turning on its axis????!

Chris

 

[supak111]

I just looked up mini circuit breakers and I guess they do have them and they're automatic , I had no idea. This would be perfect as its small, cheap $.70 (model OP-01C), and automatic, but I can't seem to find the ones under 2.5A. (PS never mind ,it has a trip time of 5-20sec at 200% current, wont do the job.)

How would I detect brush noise, and have lack of noise turn off the circuit? Sounds like a complicated circuit for someone that is a newbee

 

[CDRIVE]

I typically post spiced (computer simulated) circuits. Unfortunately none of my three spice programs will simulate commutator noise. Since I never actually bench test anything I post on EP and since I don't have your motor I can only give you tips of how I would attempt this.

One of the neat functions of a 555 Timer chip is called "Missing Pulse Detector". The 555's output pin can drive a relay coil who's contacts are in series with the motor.

Alternatively you can rectify and filter the noise spikes to create a DC average voltage to drive a transistor base which controls the relay.

Chris

 

[dorke]

What is the operating current of your motor?
At what current do you wish to "disconnect the motor" and how fast?
You can put a simple current limit in series with your motor,
may do the job of protecting the motor...

 

[CDRIVE]

Yes that should work too but he'll need a short delay to ignore the current limit upon startup of the motor. Not a big deal though.

Chris

 

[Minder]

Have you tried a self resetable PTC fuse?
M.

 

[Martaine2005]

Have you tried a self resetable PTC fuse?

Currently I'm just using a PTC fuse do protect it but they are slow

Minder, Post #1

Martin

 

[CDRIVE]

Minder, Post #1

Martin

Marty, sometimes you amaze me. Whenever I think you've burned out your brain beyond repair you catch stuff like this.

You make your uncle proud!

Uncle Chris

 

[Martaine2005]

Aww, uncle Chris, thank you.
I think the reason I noticed that was because I wasn't on the "grapes"!
Still plenty of time though!

Marty

 

[Alec_t]

How about this?
The current trip is adjustable. The higher the stall current the shorter the delay before the trip.
Automatically resets after a further delay when current drops. R3,C2 determine the delays.

 

[CDRIVE]

Ah dats

Hey guys, are there any simple circuits out there that can protect a motor from stall over current? I run a 12v dc gear motor from a battery and sometimes something jambs up the motor and I would like some circuit that can sense the motor has stop and automatically turn OFF the power to the motor.

Alec, nice design but I think a manual reset would be safer. Ouch!

Chris

 

[Alec_t]

You're probably right. Add a latch to keep the tripped state?

 

[Alec_t]

Here you go: