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Sorry to bring this up again but I overlooked the buying of the resistors. Turns out my source of resistors didn't have any 1.2 ohms or anything that could be used to make a 0.4 ohm total.
I went looking on Farnell and RS but I'm now not sure of the number of Watts I'll be dissipating across each resistor. This is regarding the 3 1.2 ohm resistors in parallel. The 100 ohm MOSFET resistor power is tiny and not a problem.

Using 10A and 4V I came to 13.3W per resistor, but this seems rather high and any resistor with this kind of rating seems way too big for what I'm doing. I mean most of them have heat sinks attached and look like they should be controlling something 10x the size of my motors.
Have I made some silly mistake with the 13.3W or is this correct? If so, am I just failing at finding resistors?

Thanks a lot, once more smiley
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If you use a single 0.4 ohm resistor (or 0.39 ohm, which is the nearest standard value), then it will indeed dissipate around 14W when the motor stalls. If you use three 1.2 ohm resistors in parallel, then each one only takes one third of the 10A current, so they dissipate just under 5W each.
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If you use a single 0.4 ohm resistor (or 0.39 ohm, which is the nearest standard value), then it will indeed dissipate around 14W when the motor stalls. If you use three 1.2 ohm resistors in parallel, then each one only takes one third of the 10A current, so they dissipate just under 5W each.

At 10A the PD across each of the resistors would be 4V (0.4 * 10)? Therefore the power dissipated would be 4 * 10/3 = 13.3W. I thought I'd already compensated for the 3 resistors by dividing 10 by 3?

Thanks for the quick reply.
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My mistake, you're quite right. The total dissipation in the 0.4 ohm resistor will be 4v * 10A = 40W and this gets split between the three 1.2 ohm resistors. The power dissipation in the motor will also be around 40W in the stalled condition.

This demonstrates the advantage of active current limiting. The principle is to sense the current in the motor and adjust the PWM fast enough to keep the current below a set value, rather than use series resistors.
« Last Edit: April 20, 2012, 04:18:23 am by dc42 » Logged

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My mistake, you're quite right. The total dissipation in the 0.4 ohm resistor will be 4v * 10A = 40W and this gets split between the three 1.2 ohm resistors. The power dissipation in the motor will also be around 40W in the stalled condition.

This demonstrates the advantage of active current limiting. The principle is to sense the current in the motor and adjust the PWM fast enough to keep the current below a set value, rather than use series resistors.

Sorry for the late reply, I was trying to ask a friend of mine for advice but he hasn't been able to get back to me. Looks like I'll need to go with active limiting as the resistor route isn't going to work.
I've looked up Active Current Limiting on Google, though there seem to be a few different things of varying complexity. Could you point me in the right direction of what I need to do for active limiting?

Thanks again.
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There are a few possibilities:

1. Use a transistor to monitor the current and reduce the gate drive to the mosfet as necessary. Then you don't need the power resistors (apart from a current sense resistor), however it is then the mosfet that has to dissipate the power. So it will need a good heatsink. The attached schematic uses a 0.1 ohm current sense resistor, available from Farnell http://uk.farnell.com/vishay-draloric/ac05000001007jac00/resistor-5w-5-0r1/dp/1735117, to limit the current to around 6.5A (the transistor will conduct when its base-emitter voltage reaches around 0.65v).

2. As (1) but also detect when current limiting is occurring using an Arduino input, and use it to reduce the PWM factor. The mosfet will then only be dissipating much power for short periods, so a smaller heatsink will suffice - as long as the Arduino software is working correctly.

3. Active current limiting in hardware. You can sense the current in a series resistor and when it reaches the limit, turn off the mosfet for the remainder of the current PWM cycle. The simplest circuit I have managed to come up with for this uses a comparator, a 74HC00 logic gate package, and a few resistors.


* Scan 82.JPG (61.52 KB, 1654x1166 - viewed 22 times.)
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