Does the TRIAC+OPTOCOUPLER make sense ? - Page 1
Forum section: electronics / projects

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 12, 2019, 08:20:39 am
content: Hi there,I am working in a circuit similar to below one .....couple_match.PNG

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 12, 2019, 11:44:59 am
content: Quote from: Md Mubdiul Hasan on December 12, 2019, 08:20:39 amYour AC load isn't even close to resistive if it has a PF of -0.4 (which is exceptionally poor, btw). In fact, the link you posted shows someone wanting to use triacs to control reactive current flow in a delta-connectedcapacitorbank, so not even slightly resistive, really. This kind of circuit is typically used to actively correct the PF of large induction motor loads because it's slow response speed is not a hindrance and the cost savings vs. a high-frequency PFC circuit is tremendous. It does, however, require back-to-back thyristors, not triacs, and exceptionally good triggering and forced-commutation circuits to work reliably; you aren't going to pull this off with some cheap-o consumer-grade triacs like the BTA100 series.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 13, 2019, 04:16:46 am
content: Nice to get your feedback.QuoteI do agree with you. If you calculate apperent power(S) its 30KW for single phase, the load is going to be too low. As its a lagging power factor, then dont you think I and V has a phase difference that does make sense with "zero-crossing MOC3081? Even TRIAC conducts full cycle of current!My system is kind of 2 leg SCR type power control take a lookQuoteYes, I saw its kind of Delta-C (grounded) and Y-R or may be altered. Y-DELTA.[ Specified attachment is not available ]For the inductive load the driver circuit could change.Need to explain more theoritical why this kind of TRIAC could fail !

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 13, 2019, 12:41:35 pm
content: Quote from: Md Mubdiul Hasan on December 13, 2019, 04:16:46 amSo far you haven't posted any two circuits that do the same thing, really. Do you want to switch a 3-ph. resistive-inductive load on/off, or do phase angle control of a reactance to counteract an opposing reactance, or do phase-angle control of a resistive-reactive load?Quote from: Md Mubdiul Hasan on December 13, 2019, 04:16:46 amOkay, that question, at least, can be answered. A triac/SCR (aka - a thyristor) automatically turns off when the current falls below the minimum holding value and remains so for a minimum time (rarely specified in triac datasheets, unfortunately). In a purely resistive load the current and voltage waveforms are in phase so the thyristor turns off at close to 0 dV/dt and dI/dt, but in a R-L load the current lags the voltage, so at the moment when current falls below the holding value the voltage has already begun increasing again (in the worst case, the voltage will be at the peak value of the sine wave for a purely inductive load - that is, displaced by 90 deg). In other words, dV/dt will be quite high during turn-off of a R-L load, and if it exceeds a certain value then the thyristor won't, in fact, turn off. This is a type of commutation failure and it is a terrible thing for a thyristor to endure.

author: Md Mubdiul Hasan
author_rank: Regular Contributor
timestamp: December 14, 2019, 01:04:41 am
content: Very impressive feedback for sure.I like what you have said and all application note said the same. I am living in a country where people don't want to care about theories and limitations. May be BTA 100 saves money because, 2 back to back SCR for single phase takes more space on PCB.Group leader said me make a SSR without phase control or zerocrossing detection. I suggest MCU application to make things more accurate, but he doesn't want.Last 2 leg circuit is reasonable, just consider a AC power source ( 380V, 40A or 440V,40A), then a SSR( what I am developing ) then a circuit breaker, a TR after then delta -Y or Y- delta R-C load.For R-C current leads the voltage pf goes below zero, even its not purely resistive but he wants to test!As you said dv/dt raises at a level that SCR/thyristor could not "OFF". So people use RC snubber to suppress high dv/dt. May be it woks with transients, having a dampling frequency for LC what AN-4003 explained well.In my case if its R-L load then snubber R=872 ohm and C= 320uF. The capacitor charging time is 279mili second.The parasitic capacitance/intrinsic capacitance plays a role for OFF dv/dt. Normally CAP of snubber should be 10 times of parasitic.Without snubber BTA100's dv/dt is 10V/us.When its gate is open,(may be at zero current/ holding time)its 500V/us.Whats your in mind for safer commutation? What cause damage for R-C load now?