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Some power supplies have a light that comes on when current limiting is active. How would I design a circuit or add a circuit to a linear regulator that could light an LED when it is current limiting?

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  • \$\begingroup\$ I edited the question since shopping questions are off topic. \$\endgroup\$
    – Voltage Spike
    Jan 17, 2017 at 20:16
  • \$\begingroup\$ First define the limits for V , I and duration, t , then a good answer is possible. \$\endgroup\$ Jan 17, 2017 at 20:20

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You could put a comparator on your Vreg output.

If its output voltage falls because more load current is drawn, the regulator will increase its current output to keep the rail voltage at the required level. If the regulator can't deliver more current because it is already at maximum, the rail voltage will fall.

So if you set a comparator and voltage reference to trip when the rail is just below the lowest voltage its spec' says it should maintain in regulation, you can drive and LED. It's worth connecting the regulator output to the comparator input through an RC low-pass filter, in case your application's load (far away, noisy etc) gives you an off-putting flickering LED; you can set the filter as suits it.

More saying this for the fun of designing it but if you use a dual comparator, you can make your LED flasher out of the second comparator using a capacitor and two-ish resistors.

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  • \$\begingroup\$ Interesting way. This method looks like it would be usable on pretty much all types of regulators. What frequency would you set the LPF at? \$\endgroup\$
    – cosmarchy
    Jan 18, 2017 at 19:35
  • \$\begingroup\$ @cosmarchy, hope it helps. The filter depends on your application, as per the answer. Maybe start with RxC=0.1 a go from there, you can tune it as you try it. \$\endgroup\$
    – TonyM
    Jan 18, 2017 at 19:39
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You might try looking for a device with a power-good output, rather than a current-limiting output.

Most simple voltage regulator ICs do not actually limit current directly, although they are advertised with ratings like "1A LDO voltage regulator". Instead, they supply as much current as possible until they overheat and go into thermal shutdown. A regulator like this would turn off its power-good output if it went into thermal shutdown (but you would also lose power).

If you need a precise current limit -- i.e., operation in constant-current mode -- those devices are also available. For example, the Linear LT3050 has both current limiting and an output that indicates when the device is supplying the maximum output current.

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  • \$\begingroup\$ power-good is the kind of thing I'm after. The LT3050 doesn't provide much current although there are others in that range that do and still provide suitable outputs. \$\endgroup\$
    – cosmarchy
    Jan 18, 2017 at 19:36

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