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I've got a power supply that'll deliver 0.5-1A (depending on a later design decision), and a motor that will normally draw about 150mA, but its stall current is a whopping 2.1A. In my application I shouldn't stall very often or for more than very brief periods of time, but I may stall nonetheless, and it's very important that I don't draw too much current from my power source. I can only assume this is a common problem with a simple solution that I'm unaware of because I'm a total newb. My instinct is to provide a secondary source of power that can supply 2.1A momentarily, like a battery or a big juicy capacitor, and something to sense current from the normal power supply and cut it off or limit it if it gets too high. I'm really rather radically unprepared for coming up with a way to do this on my own, however.

Any suggestions?

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  • \$\begingroup\$ What's the motor's rated voltage? \$\endgroup\$
    – m.Alin
    Aug 1, 2011 at 8:50

2 Answers 2

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Some power supplies are designed to safely 'fold back' the voltage when the current demand goes too high. This means that once you reach the max current, if the load resistance decreases so as to demand more current, the supply voltage just decreases so as to keep the current from exceeding the maximum.

If you have that kind of supply, you should be fine. If you don't have that kind of supply, then it's simply under-rated and you need one that either (a) has the fold-back feature, or (b) can supply the 2.1A, with a comfortable safety margin.

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  • \$\begingroup\$ Thanks. I spent some time trying to get a power source that can supply what my motors needed. Then I just decided to spend a little money to get better motors that stall at a lower amperage. \$\endgroup\$
    – jsn
    Aug 8, 2011 at 18:21
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A stall condition can pull an enormous amount of current if you use large capacitor banks without any type of current limit control and can cause damage to the power electronics. Usually you'd have current sense feedback that would allow you to monitor the current and turn off your power mosfets (or transistors or whatever) under such a condition. This typically involves a low value resistor (or the on resistance of a mosfet) and a difference amplifier or other amplifier circuit feeding a hardware shut down or a microcontroller that can respond quickly. The response must be fast, especially if your control logic is running off the same supply. The drawback to setting a lower, more conservative current limit is that start up conditions usually pull more current. So setting the current limit too low will limit your start up acceleration.

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  • \$\begingroup\$ This is probably the most useful advice for someone who can't change their power/engine specs and knows what they are doing more than I do. \$\endgroup\$
    – jsn
    Aug 8, 2011 at 18:20

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