Current Limiter

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Andrew_Hibberd
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Current Limiter

Postby Andrew_Hibberd » Thu Feb 03, 2011 3:37 pm

I have been thinking about a grabby weapon using a sanyo HP motor. However having had gears strip in the past I am looking at a current limiter on the weapon motor. A typical set up is shown here:
Image
My concern is that the HP motor will pull over 500mA before the gears start stripping and the transistors in this circuit aren't going to take the heating effect.

Does anyone have any experience/ advice about current limiting motors in this way?

I am open to other suggestions of ways to stop the gears stripping

Thanks
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BeligerAnt
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Postby BeligerAnt » Thu Feb 03, 2011 8:10 pm

From memory a stalled 12mm motor is around 5 Ohms?
The sense resistor will dissipate 600mV x 500mA, about 0.25W
The motor voltage will be about 500mA x 5 Ohms = 2.5V
So the transistor will dissipate about 500mA x 4.9V, about 2.5W

The transistor may need a small heatsink, but nothing too dramatic.

A better method might be to detect when the motor stalls, then turn the motor off completely for a (short) while, then try again.
This reduces the power dissipation in the transistor and resistor (the transistor is only a switch). The motor saving is down to the on/off ratio of the circuit. The electronics gets a bit more complex...

Alternatively, some sort of clutch mechanism?
Gary, Team BeligerAnt

Andrew_Hibberd
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Postby Andrew_Hibberd » Thu Feb 03, 2011 8:25 pm

I was looking at making some of the chassis from ali to take the forces involved so I could just bolt some TO220 transistors to that, which wouldn't be too painful.

Sensing the stall then turning off the motor would be a neat solution but looking arround its not too easy to do. Prehaps a strain guage on the arm to measure the bending force, or a force sensor under the tip may be easier.

I really like the idea of a clutch, but making one small enough to take the forces involved may be difficult, has anyone see anything comercial avaliable?
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Postby Andrew_Hibberd » Tue Feb 08, 2011 2:09 pm

I have been playing arround with the design and may have a solution to my problem using an external gearbox.

The motor I had gears strip on (and I believe pete w has too) was a 298:1 HP motor. Has anyone ever stalled a HP motor with 10,30,50:1 HP gear ratios and had gears strip or has any other issues?
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BeligerAnt
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Postby BeligerAnt » Tue Feb 08, 2011 9:49 pm

We have certainly gone through a few gearboxes, not sure of the ratios though.
These were used for drive, but have suffered several days of Modelworld plus AWS, RRC, RFRC etc before giving up.
Even on the smaller ratios the gears (and hence the teeth) are pretty small, so there's a limit to how much they will put up with...
Gary, Team BeligerAnt


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