Current flow controller for DC Motors

**mikebar** · - 8th October 2008, 15:16

Hi to all.
I'm in the process to control a DC brushed motor 24V 5A continuous (so, at start it will draw much more current).
The motor specifications says that that motor never have to be feeded with more than 5A.
I'm planning to drive it with a single logic level mosfet (60V 45A) on the low side, with hardware PWM
My question is:
there are some suggestions (even better a drawing) for a circuit that control the current and allow more than 5A let's say for 2 seconds, and then if this condition is still there for more than 2 seconds, the power is automatically adjusted not to feed more than 5A?

Even a short-circuit control circuit could be ok, even if I prefer to regulate the current not to exceed 5A.
Any solution will be ok. With PICs or without PICs. Honestly, better without PICs for tasks like that.
Anyone have some suggestions or sources to refer from?

Thanks in advance.

**mackrackit** · - 8th October 2008, 15:37

The motor will draw the amps needed. So if the driver circuit will handle the starting amps you are OK.

When the motor is running, less amps are required, the amps will drop. The greater the load the greater the amps. An amp meter can be used for feed back to tell how much of a load is on the motor.

I do not think the extra circuitry is needed.

**mikebar** · - 8th October 2008, 16:09

Originally Posted by mackrackit

The motor will draw the amps needed. So if the driver circuit will handle the starting amps you are OK.

When the motor is running, less amps are required, the amps will drop. The greater the load the greater the amps. An amp meter can be used for feed back to tell how much of a load is on the motor.

I do not think the extra circuitry is needed.

Thank you very much mackrackit for the answer.
Actually, what I'm looking for it is some "automatic sensor" that does not allow more than 5A at 24V for the motor. This kind of circuitry could also reveal if the motor's shat it is locked.
I wish to avoid the use of fuses, because one the fuse is blown it need somebody that replace it...
Any idea?

**mackrackit** · - 8th October 2008, 16:34

What about an automobile/truck circuit breaker. They reset on their own and normally will handle a bit more amps than rated for a short time.

But if the shaft is locked then you do not want it to keep trying? So a latching relay tied in with the circuit breaker might do the trick.

**mikebar** · - 8th October 2008, 17:01

Originally Posted by mackrackit

What about an automobile/truck circuit breaker. They reset on their own and normally will handle a bit more amps than rated for a short time.

But if the shaft is locked then you do not want it to keep trying? So a latching relay tied in with the circuit breaker might do the trick.

Uhmmm..
It could work.
I was looking for some more sophisticated solution (like transistors that regulate the current flow right to the thereshold)...

skimask · - 8th October 2008, 17:18

Originally Posted by mikebar

Uhmmm..
It could work.
I was looking for some more sophisticated solution (like transistors that regulate the current flow right to the thereshold)...

It's called 'foldback' in a regulator circuit.
http://en.wikipedia.org/wiki/Foldbac..._supply_design)
Totally useless link, but it might get you started...

EDIT:
http://www.national.com/appinfo/power/files/f4.pdf
Better link...

**Acetronics2** · - 8th October 2008, 17:41

Originally Posted by mikebar

Thank you very much mackrackit for the answer.
Actually, what I'm looking for it is some "automatic sensor" that does not allow more than 5A at 24V for the motor. This kind of circuitry could also reveal if the motor's shat it is locked.
I wish to avoid the use of fuses, because one the fuse is blown it need somebody that replace it...
Any idea?

did you look at Raychem RUE Polyswitches ...

much better than your PIC ... as Dave raised !!!

lol

I think Microchip has a similar App Note about servomotor ... just take the relevant part ...

Alain

**mikebar** · - 8th October 2008, 18:47

Thank you guys.
It is a starting point.
I'll post some other questions and when I reach the final result, I'll post the schematics.

Thank you, for now.

**sayzer** · - 9th October 2008, 07:33

If your PIC has comparator, then you can use it to detect current as you wish.
The example below shows the use of an external comparator, but if your PIC has an internal one then you are good to go.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2892&d=1223533980" >
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