Thyristor SCR instead of a relay?

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  • So, I seem to be having a lot more trouble than I expected getting my relay module to function properly at very low temperatures. I think I actually broke one - it's suddenly very loud when it closes and no longer works reliably at 6v DC. I'm currently waiting for a new one to arrive from overseas.

    When reading my Mimms book on intro electronics (highly recommended), I came across a thing called a Thyristor Silicon Controller Rectifier. It's this:­ta/semicond/thyristor/thyristor.php

    It looks like it's a lot smaller than a relay module, works with about the same voltages, is instant, and cheaper on ebay. It seems like it should work great (I'm implementing this project:­r).

    There has to be a good reason why it's not being used by everyone instead of a electromechanical relay. Any ideas?

  • What sort of heater are you using? What is powering it?

    With a thyristor, my understanding is that if using it with a DC source, you can't turn it off (with AC, it turns off every cycle - which is why these are often used to switch AC) without turning off the power supply to it, which means you'd need another switch...

  • The heater is a light bulb (12V, 20 watts), and the power source is a 6V battery pack of 4 AA batteries.

    Ah yes, after careful re-reading it does seem to "stay on" until you interrupt the power supply. That's not as useful as I thought.


  • The other issue here is one of Isolation. A relay gives you two electrically separate circuits. The relay coil, the low voltage (safe) side of the circuit and the contacts which can switch hi voltage hi current loads (depending on the relay spec).

    An SCR can switch hi voltage hi current loads but the input (gate) would need to be connected to the low voltage side of the circuit. If this was switching mains or hi voltages it would mean that the Espruino itself would be at dangerous voltages.

    The solution is to use an optical isolator. The Espruino drives a small LED and a Photo Transister provides the gate voltage to drice the SCR.

    There are several packages that contain both optical isolator and SCR that can operate as Solid State Relays.

    Search articals on SCR, Triac, Thyristors and Solid State Relays.

    A big advantage of a relay is the configuration of the contacts, often multiple sets of isolated contacts with normally open and normally closed switches. An SCR is a single normally open switch.

    The advantages of an SCR are the speed at which it can operate and that there so it can be uses for power control such as light dimmers and large motors.

    Hope this helps:)


  • He doesn't need isolation, though. He just needs current handling capacity.

  • You might want to consider using a power Mosfet to switch on your light bulb like the one in the link below. They even reference switching on a car headlight with a 5v micro controller.

    Sparkfun sells a couple of breakout boards to switch loads using Mosfets.

    p.s. I am not connected to this Sparkfun in any way other than being a customer.

    If you want to control AC loads solid state relays seem to be simpler to interface with a microcontroller

  • My god, the comments section, right at the top of that mosfet page is just... wow... this discussion about using an N-channel mosfet as a high-side driver (with drain connected to +5V and source connected to the positive side of the load), and how that didn't work when they tried it (no shit, it didn't, because as soon as they turned on the fet, the voltage on the source rose (since the voltage drop across the load is - hopefully - higher than that across the FET), so Vgs shrank and the fet turned part-way off - naturally nobody explained that in the discussion over there), and then someone suggests a charge pump to generate high voltage for the gate.... rather than moving the mosfet to the other side of the load or using an appropriate part.
    If you must switch the high side, you use a P-channel mosfet, a pullup from gate to source, and drive it low to open the gate (or, if the positive voltage is too high to expose your io pins to, drive it with an NPN transistor or N-channel mosfet) - or you switch the low side with an N-channel mosfet (generally, you want to switch the low side if you can, since n-channel mosfets are easier to drive and have better specs for the same price)

    As for that part ( - it looks like it would work kinda for loads of a few amps on a 3.3v microcontroller like the Espruino; the performance won't be great, but it may be fine for what lavamantis is doing (also, apparently sparkfun ships two different parts there, and they don't perform the same at low gate voltages) Driving it by putting a pullup on the gate, and driving with a 5v-tolerant pin in open-drain mode (and driving pin low to turn fet off) will make sure you get the mosfet completely on, and you'd want to do that if switching a larger load.

    Of course, there's a world of mosfets that have spectacular performance at very low drive voltages... in SOT-23 and other inconvenient (for most people here) surface mount packages.

    MOSFETs are simple to wire... if you're using a part that will do what you want it to at 3.3v, just put source to ground (for an N-channel), drain on the negative side of the load, a 10k resistor between gate and source (so it turns off if the pin is allowed to float), and connect the gate to an output pin. If you're using something that needs 5v on the gate, put a 10k resistor between gate and +5v instead, and make sure the pin you use is 5v tolerant and in open-drain mode. (this will be on if you aren't driving the pin low, though, so be aware of that)

    I'm going to search more aggressively for an acceptable MOSFET in throughhole package that'll provide decent performance with 3.3v on the gate.

    Edit: This looks like a good candidate: IRF3708PBF­ts/data/irf3708pbf.pdf

    Edit2: I'm working on a mosfets tutorial for the espruino docs.

  • @DrAzzy a tutorial would be awesome. I know you gave me some tips about driving a motor in another thread, but I just don't have enough background to feel confident about wiring it up and troubleshooting it when (inevitably) it doesn't work. I have a buddy who's an electrical engineer, I may enlist his help.

    Quick update, I got my new relay from overseas and it works fine. The problem now is that when the system runs for a few hours and the voltage in the battery pack drops a bit, as soon as the relay closes and the lamp comes on, the Espruino goes dark and everything shuts off. So, for now, I'm going to try to use two separate dedicated power sources - one for the Espruino and one for the lamp. That seems easiest.

    Thanks @Stuart.d.d and @user7143 for your tips as well.

  • The problem with the Espruino turning off when the light tries to come on as the batteries get low, the battery voltage is falling below 3.3v and resetting the Espruino. I'd argue that this is just an issue of the batteries running out - What voltage are you getting out of the batteries while they're running the light at that point?

    Here's the work in progress. It's waiting on images and diagrams (I've got schematics ready, but I I want a diagram of how to wire, and some pictures, before I put in a pull request)­ocs/blob/master/devices/

  • You could use 12vDC to power everything.

    Obviously you would then need a relay with a 12vDC relay coil if you are still using a relay.

  • @DrAzzy a FET tutorial would be fantastic - realistically just finding a good FET that's usable at 3.3v seems like the biggest hurdle though! The IRF3708 you found looks great though. I'll have to get some and have a play too.

  • My IRF3708's got here in the mail today. I'll try using them with the Espruino to drive something big-n-hungry - at least 20W, or 100 if I can find a supply that can take it (I'd rather not take apart my BFFL to get the power source out of that).

    As part of getting pictures for that tutorial. Does the tutorial look like it's going the direction you'd like it to?

  • Yes - it looks great... Only thing I'd say is a lot of people may also use it as a quick-reference, so I'd add the usual 'Buying' heading, and make sure that someone flicking to it and just looking at the pictures gets the right idea :)

  • @DrAzzy I am glad that I posted a link to the N mosfet tutorial even though it was incorrect! As it looks like you have provided great information to the Espruino user community as a result. I don't see the photos that are referenced in the tutorial. Am I doing something wrong when viewing Github or are they not posted yet?

    Thanks again for your informative post.

  • The images are there, but they don't display when viewing in github. They're in devices/mosfets or something like that.

  • Thanks to @DrAzzy, the mosfets tutorial is now on the Espruino website:

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Thyristor SCR instead of a relay?

Posted by Avatar for lavamantis @lavamantis