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I looked at another question here and saw someone talking about different LM317 current limiting methods, and in one of the comments he uploaded this image:

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But no one answered the comment and I'm also really curious if this would work? I tried it on a breadboard and it worked exactly as i wanted it too, Any downsides with using this?

Link to other question: Smartest way to use current limit using LM317?

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Ronald Donald
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1 Answers1

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The only real downside is that you're using the Vbe of the transistor as your voltage reference. This will have a strong temperature coefficient.

Basically it works by using the transistor as a kind of variable voltage limiter that varies from a minimum of about 0.6 V to a fairly high arbitrary value. By raising the voltage across the transistor, you reduce the maximum voltage across the 0.5Ω resistor, thereby limiting the maximum current.

Note that only about half of the pot's range will be useful. If you turn it down too far, raising the transistor's voltage limit above 1.25 V, the only current that reaches the output will be current that passes through the pot itself.

It would make sense to put a 470Ω resistor between the leg of the pot and the emitter of the transistor; then the full range of the pot would be useful.

Dave Tweed
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  • Oh i understand, thanks for your detailed answer, I just want to avoid heating up the pot, a transistor on a heatsink seems like a better option! – Ronald Donald Mar 11 '16 at 22:29
  • Curious, how did you calculate to get the 470Ω? – Ronald Donald Mar 11 '16 at 22:38
  • Simple -- the maximum useful voltage across the transistor is about double the minimum voltage, so I just made it approximately equal to the value of the pot. – Dave Tweed Mar 11 '16 at 22:39