Strange output with RC one shot and comparator - Page 1
Forum section: electronics / projects

author: aiq25
author_rank: Regular Contributor
timestamp: March 17, 2021, 11:06:00 pm
content: I’m working on a simple RC based one shot that goes into a comparator to provide a 40-50 ms reset pulse during board start up for a digital IC. I’m using a 4.7uF and 10kOhm resistor that goes into a comparator with a 2.5V threshold. The circuit is powered from a 5V voltage supply. Sorry I don’t have the schematic handy right now but will try to upload it later.In simulation everything works fine. The comparators outputs a nice stable digital signal. When I prototyped the circuit I used a “fast” push-pull comparator but my output has a slope to it, almost 20-30 ms to raise to 5V. This is strange behavior since the output should just be a square wave. Did anyone else have this issue? I tried two different comparators.I also tried to use a dual op-amp, with one acting as a buffer amplifier but the results were the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2021, 11:09:22 pm
content: Waiting for circuit.And, why not use a regular reset generator like MCP102?Tim

author: aiq25
author_rank: Regular Contributor
timestamp: March 18, 2021, 01:17:13 am
content: Quote from: T3sl4co1l on March 17, 2021, 11:09:22 pmI was trying to use the parts we already had in the lab. Plus this circuit apparantly is used in a production design, so it should've worked but I'm not sure what is happening.Attached is the schematic. I don't have the oscilloscope screenshot, so here is a crude drawing of the output.I used a MCP6541 in the prototype.

author: uer166
author_rank: Super Contributor
timestamp: March 18, 2021, 01:42:04 am
content: Do you expect the comparator to work as a comparator at all voltages, including 0 and as it rises to the nominal levels?

author: aiq25
author_rank: Regular Contributor
timestamp: March 18, 2021, 01:45:54 am
content: Quote from: uer166 on March 18, 2021, 01:42:04 amThe reference voltage has a time delay and I will try removing the 100nF. I didn't expect the output wave to have a raise, since it is a comparator I expected the output to reach 5V once it hits the reference voltage of 2.5V, otherwise the output should be 0V. What I see at the output of the comparator is a raise in voltage, over 20-30ms. Is this assumption wrong?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2021, 07:13:49 am
content: What is the "5V" from, how long does it take to reach 5V?Tim

author: Zero999
author_rank: Super Contributor
timestamp: March 18, 2021, 09:17:14 am
content: Quote from: aiq25 on March 18, 2021, 01:17:13 amGet rid of C1 and put a 1M resistor from U1's output, to the +input. A diode across R4 is also a good idea to protect the comparator's input, when the power is removed and enable the circuit to reset quickly, by providing a discharge path for C3. I'd also increase R4 and decrease C3: timing capacitors should generally be as small as possible.

author: aiq25
author_rank: Regular Contributor
timestamp: March 18, 2021, 11:18:51 am
content: Quote from: T3sl4co1l on March 18, 2021, 07:13:49 amIn this case the 5V is coming from a lab power supply. I haven't timed it but it has a fairly fast response (no where near the ramp up of the output).

author: aiq25
author_rank: Regular Contributor
timestamp: March 18, 2021, 11:20:22 am
content: Quote from: Zero999 on March 18, 2021, 09:17:14 amThanks! Will try that.

author: aiq25
author_rank: Regular Contributor
timestamp: March 20, 2021, 02:39:04 pm
content: Turns out the circuit design was fine, it was the power supply ramp up. Not sure how I missed that.