Over-current Protection Circuit

[TerryJC]

I've done some more tests with a longer timebase. In each case the yellow trace shows the power-on reset at Pin 13 and the blue traces show the gate I/Ps and O/Ps:

Pins 8 and 12 (the feedback):

TEK0000.JPG (187.25 KiB) Viewed 174 times

Pin 9 (the comparator O/P; low at startup until enough current flows)):

TEK0001.JPG (187.97 KiB) Viewed 174 times

Pins 10 and 11 (opposite feedback and driver for the LED):

TEK0002.JPG (189.99 KiB) Viewed 174 times

As you can see the signal at Pin 11 (which should remain high to keep the LED off), goes low after about 100 ms. I can't see why the circuit isn't latching at the initial state.

[TerryJC]

I've sussed it. Penri, the other day I said that the design of this circuit is tried and tested. That was true, but at the time (August last year), I was using a FET as a cheap and cheerful comparator:

FET_Comparator.png (15.58 KiB) Viewed 172 times

As you can see the default state is active low, so the Latch gets a High until the current exceeds the limit. I hadn't noticed that change and designed the new comparator to be active High.

There are a number of ways round this, but the simplest is to change the comparator circuit. I'll look into this tomorrow.

[Penri]

You are right, I was going though the circuit and asking myself why the comparator output was low, it should only go low under an over-current condition.

[TerryJC]

The Over-current Protection Circuit is now tripping reliably at approx 1.5 A. This Topic should therefor be complete, so I will continue this narrative at viewtopic.php?t=314.

[Penri]

Excellent news!