I'm trying to get a simple circuit working using an LM319. It's an oscillator which charges a capacitor with a current source, a comparator circuit monitors the output and switches the current source to discharge when the voltage meets a certain point (it's a little more complicated than that, but i'll leave the other details..)

I did have my comparator set up and working but I changed it for the arrangement in the picture attached to get different trip points (I want +-5v). Although it sort of works (you have to wiggle the pots a bit to get an oscillation) the comparator isn't triggering in the right place - on the positive half it triggers at 5v as it should, but on the negative half it triggers at about -10v (attached picture shows asymmetric triangle wave). Looking at the inputs of the comparator, you can see there is a distortion on the negative portion of the signal (also pictured). Why is this happening? how can I fix the problem?

**I did have the +-5v lines connected to the inputs directly without the 10K resistors but it didn't work and i thought i may want something to avoid excess current, that's why they're there...

 

Could we see the rest of the schematic?

 

The only thing I can see which looks wrong is pins 3 and 8 connected to -5V. The comparators will pull their outputs down to that -5V and that is being fed to the '4013 running from +5V and 0V. Something will give when you do that but I don't know what it will do to the circuit. Try connecting pins 3 and 8 to 0V and see if that fixes it.

 

You may be breaking down the base-emitter junctions of the LM319 input transistors. The data sheet shows a maximum differential voltage of +/- 5 volts Absolate max.

This is just a guess without seeing a complete circuit.

A question: Why are you using such a fast comparator with such a slow flip-flop?

 

The only thing I can see which looks wrong is pins 3 and 8 connected to -5V. The comparators will pull their outputs down to that -5V and that is being fed to the '4013 running from +5V and 0V. Something will give when you do that but I don't know what it will do to the circuit. Try connecting pins 3 and 8 to 0V and see if that fixes it.

sorry that's an error - the cd4013 is connected between +5 and -5

 

You may be breaking down the base-emitter junctions of the LM319 input transistors. The data sheet shows a maximum differential voltage of +/- 5 volts Absolate max.

This is just a guess without seeing a complete circuit.

A question: Why are you using such a fast comparator with such a slow flip-flop?

ahh. yes that sounds like you're onto something, i hadn't noticed that

i haven't specced the parts very well, i had a lm319 and a cd4013 lying around. no other reason

 

i haven't specced the parts very well, i had a lm319 and a cd4013 lying around. no other reason

That works for me.

What frequency range are you wanting for the triangle wave?

Knowing your full circuit is important. For instance it might work to attenuate the triangle wave to use smaller voltages into the comparators -- or it might not depending on the rest of the circuit.

 

i've just attenuated the 5v ins and the input signal by the same amount. works fine now

I didn't want to put the whole schematic up as it's big and would have just confused the issue - people tend to start picking apart things that aren't part of the problem. in any case, im quite comfortable with the workings of the circuit, I'm just not all that used to working with the 319!

thanks for your help