Purpose of electrolytic capacitor across an opto coupler? - Page 1
Forum section: electronics / beginners

author: permal
author_rank: Regular Contributor
timestamp: May 04, 2017, 06:31:42 pm
content: Hi,Could someone please explain to me the purpose of the electrolytic capacitor C26 (lower left corner) inthis schematic?I'm guessing it is meant as some sort of protection, but aren't regular capacitors used for that? Also, when U13 closes to ground, C26 will be shorted; that can't be good?I might be totally wrong, but as I see it all that capacitor accomplishes is to increase the rise and fall times of the input!?

author: Andy Watson
author_rank: Super Contributor
timestamp: May 04, 2017, 06:46:00 pm
content: Quote from: permal on May 04, 2017, 06:31:42 pmWhat are the optos monitoring?I think the capacitors are there to form a low-pass filter, for example, to hold the output stable if the input to the optos is A.C. (mains ?), or perhaps to eliminate button-bounce on switch contact inputs.

author: Benta
author_rank: Super Contributor
timestamp: May 04, 2017, 06:59:03 pm
content: The time constant is relatively long (~100 ms). It's for filtering 50/60 Hz AC inputs to provide a stable signal for the logic inputs.

author: permal
author_rank: Regular Contributor
timestamp: May 04, 2017, 07:12:35 pm
content: Quote from: Andy Watson on May 04, 2017, 06:46:00 pmCould be anything afaik. Is is the schematic of a general purposeI/O card.Quote from: Benta on May 04, 2017, 06:59:03 pmYes, it makes sense that if AC is applied, the optos would turn off at each zero-crossing. Having an electrolytic capacitor to keep it steady makes sense.So what you're saying, Benta, is that due to the long time constant, the capacitor isn't discharged fast enough to damage it?

author: Benta
author_rank: Super Contributor
timestamp: May 04, 2017, 07:21:27 pm
content: It's not a nice design, but probably within the peak collector current specs of the optocoupler.

author: permal
author_rank: Regular Contributor
timestamp: May 04, 2017, 07:24:21 pm
content: Quote from: Benta on May 04, 2017, 07:21:27 pmYeah, it feels rather brutal, even for a newbie like me. If you have a suggestion on a better solution, I'd love to hear it!

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2017, 07:36:24 pm
content: You could omit the capacitors, ensure the software knows that the inputs will be 2×mains frequency waveform and accept that the LEDs will flicker very slightly.

author: Benta
author_rank: Super Contributor
timestamp: May 04, 2017, 08:23:30 pm
content: Looking at Fig.6 in the LTV354 datasheet, the design is borderline and relies on "typical" VCEvs. ICbehaviour.A simple fix would be a 100 ohm resistor in series with each 10 uF cap. That will not interfere with operation, but limit collector current through the coupler.

author: permal
author_rank: Regular Contributor
timestamp: May 04, 2017, 09:10:17 pm
content: Quote from: Benta on May 04, 2017, 08:23:30 pmAwesome, thank you for your insights.

author: BrianHG
author_rank: Super Contributor
timestamp: May 06, 2017, 03:49:17 am
content: Quote from: Hero999 on May 04, 2017, 07:36:24 pmYou are assuming that the AC pulse is turning on the opto long enough to always detect that pulse in software.Agreed that the 10uf caps are a cheap functional solution around this problem.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: May 06, 2017, 05:09:12 am
content: transistor output optoisolators are inherently limited in collector current by the CTR - you can't turn it on hard enough to fail the transistor Ic_max spec without blowing the LEDwhich is different from saying you can't exceed the Pdiss_max with high enough Vcoll

author: permal
author_rank: Regular Contributor
timestamp: May 07, 2017, 07:48:18 am
content: Quote from: f5r5e5d on May 06, 2017, 05:09:12 amMy question is about inputs, but your statement is still of interest. Are you saying that the LED on theinsideof an opto coupler will blow before the transistor on theoutputside is fully open? what does CTR stand for?

author: Cervisia
author_rank: Regular Contributor
timestamp: May 07, 2017, 09:06:38 am
content: QuoteWhat do you mean with "fully open"? In every transistor, the output current is limited by the input current.QuoteCurrent Transfer Ratio. See any optocoupler datasheet; it's essentially the same as a transistor's hFE, but measured in percent because the number would be ridculously small otherwise. For many common optocouplers, the guaranteed minimum CTR is less than 100 %.You can blow a 'normal' transistor by sending too much current either through the base, or through the collector. With an optocoupler, it's unlikely that the second case happens because you will not be able to get enough photocurrent into the base.

author: permal
author_rank: Regular Contributor
timestamp: May 07, 2017, 12:43:02 pm
content: Quote from: Cervisia on May 07, 2017, 09:06:38 amI don't know what I was thinkingThanks for the explanation re CTR.