Trigger SCR with high to low pulse? - Page 1
Forum section: electronics / beginners

author: Stan21
author_rank: Contributor
timestamp: August 16, 2023, 08:59:15 am
content: So I have an SCR that discharges a capacitor when triggered.I need to trigger this SCR from photo camera hot shoe terminal.When camera button is pressed, hot shoe wires go from 3.5v to zero for 10 ms.So I suppose I should probably use some PNP transistor, something in this style, is this correct?

author: Manul
author_rank: Super Contributor
timestamp: August 16, 2023, 03:48:02 pm
content: Yes, you can do it with a PNP, but your circuit is not correct. Mainly, the emitter should go to the positive side, so the current can flow through emitter-base junction when the base is pulled low (transistor symbol has an arrow to indicate current direction) and turn on the transistor. So something like below.Note that such circuit is not universal and depends on power supply voltage being close to camera's output high level. If you would power your circuit with significantly higher voltage than camera's output, like 5V, it would trigger even from "high" level.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2023, 06:26:16 pm
content: Yup, that covers it.To be fair, the upside-down PNP will still work at this voltage and current, probably well enough you wouldn't notice a thing. The main thing is performance will be much less than expected: in particular, hFE in this "inverted" configuration is typically quite low. The SCR probably doesn't need much gate current to trigger, hence it might go unnoticed.Low gate current can mean slower turn-on, which can damage the SCR (it's generally wise to turn them on strongly), but whether any of this happens, depends on the device and surrounding circuit.Tim

author: Manul
author_rank: Super Contributor
timestamp: August 16, 2023, 07:27:38 pm
content: Fair observation. Although there is also an emitter... errr... collector resistor in original schematic. Values are not present, but that resistor reduces the gain to even less, we don't know by how much.

author: Stan21
author_rank: Contributor
timestamp: August 16, 2023, 08:02:48 pm
content: Thank you for responses, my schematic was just a draft, and I am sorry I messed up with polarity.Besides, I will connect camera with ECR through the optocoupler.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2023, 10:29:56 pm
content: Yes -- an emitter current-limiting resistor is also effective, but not if base can be pulled to 0V; gate trigger voltage will generally be more than the ~0.6V leftover in that condition. So a base resistor divider, and either emitter or collector resistor to limit current, will do; and it doesn't really matter which.Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: August 17, 2023, 07:21:10 am
content: Like this:You don't need to pull down the gate, its current driven not voltage driven.Once the transistor turns off, so does the gate drive.

author: Manul
author_rank: Super Contributor
timestamp: August 17, 2023, 12:18:40 pm
content: I still think it is useful to pull down SCR gates. SCR trigger current can range from 10s of uA to maybe 100s of mA. There can be some leakage, both inside SRC and it's driver, so the resistor drains the leakage. Furthermore and more importantly, there are capacitances, both inside SCR junctions and the driver. These capacitances and dividers formed by them affect how sensitive the circuit is to external noise and dv/dt effects (on both SCR anode voltage and driver supply voltage). RC snubber circuits are sometimes used with SCRs to limit dv/dt. If the gate is kept at 0V by pull-down resistor, so say 0.6V below it's conduction, it gives additional margin compared to floating the gate very near it's junction conduction. For sensitive SCRs it might help avoid spurious triggering. Same with BJTs, their gate is current controlled, but leaving the base floating near conduction makes it more sensitive to noise and C-B capacitance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2023, 04:52:48 pm
content: Quote from: Terry Bites on August 17, 2023, 07:21:10 amThis may fail to operate: consider what happens when Vgc > Veb. All the emitter current goes through the base.Tim