LM393 comparator output does not attain 'high'

I guess that something is oscillating.

Does the LED light when it shouldn't?

A little positive feedback round the comparator might help?.

Nigel Goodwin said:

A little positive feedback round the comparator might help?.

Click to expand...

Say 10k between pin 1 and pin 3?
But pin3 never exceeds +/-6V. Pin 1 would be stuck there?

Oscillation: I check on my scope this afternoon and feed back here.

earckens said:

Say 10k between pin 1 and pin 3?
But pin3 never exceeds +/-6V. Pin 1 would be stuck there?

Oscillation: I check on my scope this afternoon and feed back here.

Click to expand...

I would suggest higher than that - try 100K or 1M and see if it makes any difference, if not try lower.

Is it me? I don't do analogue, bu!!! Vref path is sitting at 70uA R1 and R2 should equal around 14k Will this work with 10 times lower current?

Sitting down now just in case I get blown out the water!!!

Diver300 said:

I guess that something is oscillating.

Does the LED light when it shouldn't?

Click to expand...

I just put the scope on pin 1: indeed, plenty of oscillations: 400ns period, 5vpp on pin 1.

When pin 3 is low (0V), then no oscillations, and the LED turn (correctly) bright.

I set up the LM393 in lab conditions, with LED and series 4k7, and then it behaves as it should: clean on or off. And pin 1 all the way to near Vcc.

Pin 3 (input +), connected to pin 4 (LM723) cannot be interfered with (because pin 4 is crucial to current sensing circuit in the LM723);
There is of course a difference in LED brightness when LM393 pin 1 is low, versus oscillating.

Hard to see for me how to solve this right now.

There is no power supply decoupling around IC1. Depending on the IC's internal circuit, a high source impedance for Vcc (and "high' in this case can be just a few ohms) can create either a positive or negative feedback path backwards through the circuit, disrupting normal operation. One chip can put enough noise on a power rail to disrupt another chip.

Power supply circuits need heavy decoupling. Each IC should have a 0.1 uF ceramic or stacked metal film cap between its power pins, as close as possible with short leads. In parallel with this add a 10 uF to 47 uF electrolytic.

ak