Sawtooth generator

160

03:09:16

This circuit should produce a sawtooth waveform at capacitor outout, but the simulation doesn't show this behavior. Anyone got a clue why it isn't working?
Thanks,
Source : https://www.youtube.com/watch?v=2a1I1X3RV0g

published 12 years ago

seanrwillie

12 years ago

Slow down the simulation speed and add a switch to the supply. There are still some problems though.

UncleRick

12 years ago

Well... slowing down the simulation isn't going to do anything. If there was going to be a sawtooth there, it would work in real-time.

rbrtkurtz

12 years ago

If you slow it down to 5ms/s, there's something kind of, almost, sorta resembling a sawtooth.

UncleRick

12 years ago

I see what the video guy is trying to do. The dual transistor is acting a an SCR but the voltage isn't charging the cap high enough to trigger that SCR circuit. I've been pulling my hair out trying to balance the values, but haven't found the right combo yet. I think thr SCR circuit needs a little over 4 volts to trigger. Not certain about that yet.

UncleRick

12 years ago

The SCR circuit needs nearly 5 volts to trigger. The charge on the cap needs to reach 5 volts before the SCR will do it's part. Then the falling current needs to drop far enough to allow the SCR circuit to release. Then the sawtooth will work repetitively.

UncleRick

12 years ago

Yeah, rbrtkurtz. I still can't wrap my head around that simulation speed thing yet. It doesn't seem to work the way it seems like it should in my head. (My bad.)

crada666

12 years ago

Thx for all the comments. I managed to get something close to a sawtooth by reducing the simulation to below 1ms/s.

UncleRick

12 years ago

I wonder... might the sim speed adjustment be slowing the simulation down enough to allow the CPU enough time, to perform all the calculations it needs to do? That is something I had never considered until this moment. I had always figured it was nothing more than a simple speed adjustment to allow us to see what the simulator is doing.

rbrtkurtz

12 years ago

I slowed the sim speed down to 250us (what the dude in the video had his scope set at), and it actually looks very similar. The problem is that at seemingly random intervals, the cap doesn't just discharge down to a few mV, it goes down to more than -2V. Crada666 made the circuit correctly, so I'm not sure what is causing this. Those transistors switching is going to happen pretty damn fast, so it could be a simulation/CPU issue.

rbrtkurtz

12 years ago

That's it. It's just too much to simulate the circuit at high speeds. If you turn the sim speed down to 100us/s it becomes very stable. At 50us/s, it is painfully slow to watch the sawtooth, but you can see the switching at the PNP much better. I think that's where the hangup is at. So to answer your question, @Crada666, it's working, you just have to run it at a very slow sim speed. We're not exactly working with the most powerful circuit simulator ever, but that fact that you can do it with a phone is still pretty amazing.

UncleRick

12 years ago

Thanks rbrtkurtz, for the confirmation. I wish that Igor had had the time to put one together. I might have learned about the operation of the sim speed adjustment a long time ago. Thank you for your assistance with this circuit as well. It's been too long since I dealt with some of the ideas used here.

UncleRick

12 years ago

...put one togethet. A manual, I meant to say.

UncleRick

12 years ago

Also thanks to you as s well, Crada666. It is appreciated..

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