Comparator problem

[AxleD]

Hi All,

For my first project I was looking at building a simple linear power supply based on the LM317 chip. I built up the circuit yesterday (breadboard) and the voltage, relay and current controls are all working as expected.

As discussed in a previous thread, to determine if the supply is in current limiting mode, I am using a differential amplifier to the look at the voltage drop across the first 317 (in the attached circuit) and then compare the output to a reference voltage. If it’s greater than 2V in this case, the comparator lights a LED.

The circuit works well but I have a small issue with the current limiting LED turning on briefly whenever the power switch is flipped (on or off). I have rebuilt the circuit without the relays and it still occurs. I also have added a separate voltage regulator for the opamp and comparator but still no luck. I also added some hysteresis to the comparator but I still get the issue.

Any ideas on what this could be? Any guidance on what I should look for would be appreciated. I am currently using a cheap plug pack to power the system and I wonder if that could be the issue?

Thanks

[Simon]

You need to post a diagram of what you have done.

[AxleD]

Thanks i will try and post a better circuit diagram.

[Zero999]

This is happening because the capacitors on the output provide a short circuit as they charge up, causing it to briefly go into current limiting mode.

This can be fixed by reducing the bandwidth of the differential amplifier (bypass one of the feedback resistors with a capacitor) so it no longer responds to brief transients.