Adjustable triangle waveform generator with diode switch

229

02:40:16

Recently, I was looking thru YouTube and came across a sawtooth generstor on chan w2aew vidieo no.104 Aug 2013. I thought it was interesting enough to try out on the EC simulator. Some important notes.

The relay in the EC simulation is not required in the actual hardware. It is being used to isolate the output of the comparator from the other active devices in the overall feedback loop. Without the relay isolation, EC will stop with a convergence error (No Solution Found). I have encountered this problem before with EC whenever I try to add active devices as part of the feedback loop of the OA. I suspect it's the result of the EC implementation  tradeoff made between an easy to use GUI or adding Spice simulation complexity.

Although the configuration of the 4 diodes in the center may not be easily recognized, they are just configured as a standard full wave bridge being used as diode switches.

 In the actual hardware, remove the relay and all it's connections and connect the output of the comparator directly to the pn junction of the left set of diodes. A high at this control point will switch in the current source (ramp up - charge cap) while a low will switch in the current sink (ramp down - discharge cap). The sawtooth symmetry can be adjusted by changing the source and sink currents. Frequency is most eaisly changed via the capacitor at the bridge output.

For a more complete explanation of the circuit, see the YouTube video mentioned at the beginning.

published 7 years ago

2ctiby

7 years ago

Does a 200k resistor (adjustable) help instead of the relay http://everycircuit.com/circuit/6387405453262848

leed124

7 years ago

2ctiby, thanks for your suggestion. The answer to your question is both yes and no. Yes, because the EC simulation can now start and finish without a convergence error, However, No, because the control node now follows the charge and discharge cycles of the triangle waveform. This does not faithfully represent the control node of the actual hardware which always follows the voltage levels of the comparator output, which, the relay EC simulation workaround does. The 200k resistor workaroud changes the comparator output from a stiff voltage source to more of a current source and the blocking diodes are no longer truely isolating the control node from the capacitor charge and discharge . A smaller resistor helps by making the comparator output look more loke a voltage source, but the control node now developes glitches on the rising and falling edges at the diode control node.

2ctiby

7 years ago

Yes, I noticed that...it is why I chose that resistor value. I have just adjusted that effort but I don't know if it is better, or going off-track. Good luck.

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