Learn how to differ a thrystor, transistor and triac with a test circuit. - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 18, 2020, 09:25:21 am
content: i am looking for some help on how to differ these 3 components by practical means.From what I know a triac will conduct in both directions so if I turns an LED on from both directions then it is a triac. If not it is either a thrystor or transistor.But then... between thrystor and transistor. What test would differ them?I could use my BK precision curve tracer. I know what plot a usually get from a transistor. But not sure if I would get a different kind of plot from a thrystor...

author: pwlps
author_rank: Frequent Contributor
timestamp: October 18, 2020, 12:38:38 pm
content: A triac is basically two thyristors connected back-to-back and biased in opposite directions. It makes a sort of bidirectional thyristor.A transistor is a current amplifier, a thyristor is just an on-off switch.The on state of a thyristor is triggered by the grid voltage and will be latched (stay on even if when you remove the grid control voltage) as long as the current remains bigger than a critical value. To see it on your curve tracer you can set the grid voltage phase shifted with respect to the main voltage.Edit: "grid" should have read "gate" of course. I was reading another article about vacuum tubes at the same time

author: gkmaia
author_rank: Frequent Contributor
timestamp: October 18, 2020, 12:46:38 pm
content: Quote from: pwlps on October 18, 2020, 12:38:38 pmHow do I differ these two with a practical testing circuit?How this difference my appear on a curve tracer?

author: pwlps
author_rank: Frequent Contributor
timestamp: October 18, 2020, 12:50:16 pm
content: Quote from: gkmaia on October 18, 2020, 12:46:38 pmI was reediting my answer at the same time, see above.

author: Zero999
author_rank: Super Contributor
timestamp: October 18, 2020, 01:45:11 pm
content: Quote from: pwlps on October 18, 2020, 12:38:38 pmYes, a thyristor will only turn on, once the gate current exceeds a certain threshold and it will remain on, until the anode current drops below a certain value.Just to be pedantic, a TRIAC isn't quite two back-to-back SCRs, because it can be triggered when the gate current is the opposite sign, to the anode current. Two back-to-back SCRs will only trigger when the anode current is the same sign, as the gate current.

author: ebclr
author_rank: Super Contributor
timestamp: October 18, 2020, 02:15:26 pm
content: Those very cheap transistor tester do this very easily ,https://www.aliexpress.com/item/1005001344530542.html?spm=a2g0o.productlist.0.0.c959aabbs4avF1&algo_pvid=38b3a1df-9235-4c85-94e1-6d6c08a596bc&algo_expid=38b3a1df-9235-4c85-94e1-6d6c08a596bc-6&btsid=0bb0624616030304772936763e21a4&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

author: pwlps
author_rank: Frequent Contributor
timestamp: October 18, 2020, 03:16:38 pm
content: This may be digressing from the actual OP question but here is a related question I was asking myself when I first learned about these devices: thyristors and triacs are told to be reserved for high power applications but why ? In principle you can make an on-off switch using a mosfet or a BJT (in saturated regime). Actually it isn't easy to find this sort of question answered in tutorials. As I understand the difference is due to the current path geometry in the planar structure of these devices. In FET transistors the current is in-plane, in BJT the emitter-collector current is perpendicular to the plane but the base-emmitter current is in-plane and is thus limited by the power dissipation in the base. In a thyristor the only in-plane current is the short gate-triggering current pulse, therefore there is no dissipation by in-plane currents and the current density can be thousands times bigger than in a transistor. I'm not an expert in this field though, consider it rather as a new question rather than a reply.

author: Alti
author_rank: Frequent Contributor
timestamp: October 18, 2020, 05:33:31 pm
content: Quote from: pwlps on October 18, 2020, 03:16:38 pmFrom applications point of view it seems it is a matter of BOM cost.Lets consider mains 230V AC application at 10A nominal, 100A inrush currents, 600V 8/20us transients, BOM in 1k volume:A) As reference, lets take triac, ~1V drop. Triggered by 1us 2V 50mA pulse, 100Hz. I'd say BOM around €0.5, 10W losses.B) two parallel BJTs with two supplies that deliver 1/50th of Ic to Ib get to 1V drop in both directions. Possible, complicated. So the peak base current Ib= 2A at 1V and then it drops to 200mA at 0.7V, rms. You need some DC/DC here. Additionally BJTs for 600V arenot particularly cheap. BOM 2xBJT + 2xSMPS, at least €10.C) With IGBTs this is not possible to get to 1V drop because even if you connect two in series you end up with body diode drop + IGBT drop in one direction and IGBT drop + body diode in the other direction. So you need some crazy expensive MOS controlled mains rectifier with close to 0V drop first. BOM = €30? With two IGBTs in series you end up with 1V on a bdiode and 1V on IGBT, best case. Bigger heat sink, thermal problems. Fortunately IGBT tolerates inrush currents well.D) With MOS you can get to 1V drop for two transistors in series if you control both transistors in both directions (overriding body diode). Quite complicated drive but doable. However, MOSes in condution are resistors so on inrush 10x current means 100x power dissipated, not 10x! If this device is supposed to sustain even short 100A inrush, you end up with either super low RDSon or massive package capable of inhibiting this initial inrush. Quite hard to estimate but 'd say €15 for pair of transistors alone.The advantage of B, C and D over A is that you can include dead-short protection, if needed.Also, it might be more economical for C and D to use 400V transistors and sophisticated transient protection but forget about using TVS or MOV as with ordinary triacs capable of 600V.QuoteReference please.EDIT: Version B can work only with diodes in series with BJTs because reverse breakdown voltage of BJT is quite small. So in terms of voltage drop B and C are quite similar.