What is the point of capacitor in this cirucit? - Page 1
Forum section: electronics / beginners

author: Electric flower
author_rank: Regular Contributor
timestamp: May 31, 2014, 04:09:50 pm
content: On right there is optics cable with laser diode whose light points at photothyristor,, and when light comes at photothyristor it turns on.This is all information i got from book on this picture. So what is the point of capacitor? It is AC circuit so capacitor allows it to be closed circuit and then why is there photothyristor if you have capacitor?

author: TerminalJack505
author_rank: Super Contributor
timestamp: May 31, 2014, 05:08:28 pm
content: From what I can tell, the SCR will rectify the AC voltage and the capacitor will be allowed to store a charge. This charge will then be dumped through the SCR when it is triggered. This is probably done so that the negative-going pulse that is generated (at the SCR's anode) has a minimum width.

author: Dave
author_rank: Super Contributor
timestamp: May 31, 2014, 05:30:28 pm
content: My best guess would be that the cap is there to limit dv/dt over the thyristor, which may otherwise cause false triggering.

author: Electric flower
author_rank: Regular Contributor
timestamp: June 01, 2014, 06:58:52 pm
content: I still don't understand a thing, maybe i don't know basis but isn't AC flowing through capacitor all the time?Edit: If this was DC circuit i would be 100% clear about it

author: TerminalJack505
author_rank: Super Contributor
timestamp: June 01, 2014, 07:22:18 pm
content: Yep, you're right. My guess isn't correct.Dave might be on to something.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 01, 2014, 07:30:05 pm
content: Another vote for preventing high dv/dt from causing false triggering. Yes the capacitor will pass some A.C. all the time but presumably very little relative to the active load state - depends on the size of the capacitor.

author: John Coloccia
author_rank: Super Contributor
timestamp: June 01, 2014, 07:52:13 pm
content: Ditto. Looks like a simple bypass cap to bleed off high frequency noise. You might imagine that might get aliased on the other end into an actual signal.

author: Electric flower
author_rank: Regular Contributor
timestamp: June 01, 2014, 08:31:56 pm
content: Well here is what i got on my mind.Photothyristors are generally used in power electronics (conrtoling 3 phase sistems and motors) so frequencies are 50, 55 or 60 Hz, depends on your country...Capacitor doesn't have to have big capacitance becouse even small capacitor can limit dv/dt of thyristor.So Xc=1 / (2*pi*f*C) = very small resistance due to "low" frequency and small capacity, it would be 10K ohm minimum.And one reason for using photothyristor instead of regular thyristor is becouse gate of photothyristor is not sensitive to electromagnetic waves that get emited by high voltage stuff.Sorry for bad English.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 01, 2014, 08:43:23 pm
content: Quote from: Electric flower on June 01, 2014, 08:31:56 pmThe usual use for photothyristors - especially when optical fibre is used - is to provide a good amount of galvanic isolation between thyristor an trigger source. Typically, it is used in high-voltage. power applications where many thyristors are stack on top of each other.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2014, 10:25:43 pm
content: Are you sure it is not for modelling purposes - capacitor symbol added just to account for parasitic capacitance of device in SPICE model? Perhaps text accompanying the schematics explains it?

author: Electric flower
author_rank: Regular Contributor
timestamp: June 01, 2014, 11:33:16 pm
content: Quote from: Alexei.Polkhanov on June 01, 2014, 10:25:43 pmOn right there is optics cable with laser diode whose light points at photothyristor,, and when light comes at photothyristor it turns on.Literally this is all i have written in the book.

author: Marco
author_rank: Super Contributor
timestamp: June 02, 2014, 12:18:11 am
content: Quote from: Electric flower on June 01, 2014, 06:58:52 pmThe capacitor is likely very small, so it's impedance for 60 Hz AC will be essentially open circuit.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 02, 2014, 03:10:11 am
content: Then it is likely some sort of "CR absorber" - see here:http://documentation.renesas.com/doc/products/transistor/rej27g0020_thyristors.pdf.

author: John Coloccia
author_rank: Super Contributor
timestamp: June 02, 2014, 12:46:07 pm
content: Quote from: Alexei.Polkhanov on June 02, 2014, 03:10:11 amI didn't notice it was a thyristor at first glance. I think you typically want to put a resistor in series with it, though, to limit it.

author: nsayer
author_rank: Regular Contributor
timestamp: June 02, 2014, 10:15:07 pm
content: I've done a lot of triac circuits of late, and when the load is inductive, you use a resistor and cap in series on the output, as Dave said earlier, to get rid of the dv/dt caused by the inductive load. The pair is typically called a "snubber." It serves the exact same purpose as a reverse-biased diode across a (DC) relay coil when it's controlled by a transistor.Recall that inductors resist change in current by "supplying" voltage. If the voltage gets high enough - as it does when the circuit is abruptly turned off instantaneously zeroing the current - then it will falsely trigger the SCR (or triac, which is not much more than two back-to-back SCRs with a common gate).Now, whether the cap in this case is acting as a snubber or not... that I'm not sure.