Discharge through an Op-Amp/Comparator output? - Page 1
Forum section: electronics / beginners

author: luissizzle
author_rank: Newbie
timestamp: August 03, 2021, 01:07:44 am
content: An ideal Op-Amp has 0Ohm output impedance. Would I be able to discharge a capacitor through the output of an op-amp? In my design, im actually using a comparator, which I assume has an op-amp at it's output. Im using the LT1017.I simulated this in LTSpice, and it works, but i'd appreciate any feedback from the analog veterans on here. Thanks!

author: ledtester
author_rank: Super Contributor
timestamp: August 03, 2021, 03:20:47 am
content: I'm no analog veteran, but the datasheet should answer this.The output sink current has a typical value of 65 mA.Note the asymmetry in source and sink current capability. Op-amps typically can source and sink the same amount of current, but since some comparators have open-collector outputs you really need to check the datasheet.Datasheet link:https://www.analog.com/media/en/technical-documentation/data-sheets/10178ff.pdf

author: Terry Bites
author_rank: Super Contributor
timestamp: August 03, 2021, 07:52:00 am
content: The 0R resistance is created by the negative feedback, the equivalent output impedance referred to the driving capabilty not an internal resistance as such. Discharching a high value cap will just melt the output stage of the opamp.Why would you want to do that?

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2021, 08:11:27 am
content: Op-amps normally have a current limited output and won't be damaged. Some comparators have a current limited output, others not. As long as the capacitor isn't too big, it can be discharged into an op-amp or comparator, without damaging it. Either way, read the data sheet.

author: luissizzle
author_rank: Newbie
timestamp: August 03, 2021, 03:15:08 pm
content: I've never noticed that spec on a datasheet before. Thank you! I was looking for discharge characteristics, but couldn't find much.Quote from: ledtester on August 03, 2021, 03:20:47 am

author: luissizzle
author_rank: Newbie
timestamp: August 03, 2021, 03:15:43 pm
content: It's not a huge cap, and the discharge rate is designed to be very slow. There is no risk of burning the output stage of the comparator, especially after looking at the output sink current spec noted above. I just needed to verify that there was a path to ground. Thanks for your input!Quote from: Terry Bites on August 03, 2021, 07:52:00 am

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2021, 04:17:39 pm
content: Quote from: luissizzle on August 03, 2021, 03:15:43 pmDefine not huge? What voltage will it be charge to? And how often will it occur? You need to think in terms of energy and the frequency of the discharge.An op-amp will only have a very low resistance, with negative feedback, up to a certain current. Beyond that, the output resistance will rise dramatically. The output of an op-amp, with negative feedback, is also inductive, which can cause oscillation, when a capacitor is connected to it.

author: luissizzle
author_rank: Newbie
timestamp: August 04, 2021, 12:39:07 am
content: There is a 22uF capacitor that discharges through a series 402K resistor at the output of the comparator. The maximum discharge current is 12uA. This doesn't happen often at all. It's part of an undervoltage detection circuit. When the comparator goes low for around 8s, given by the RC circuit, a secondary circuit gets enabled.Quote from: Zero999 on August 03, 2021, 04:17:39 pm