Hi - I'm working on a model railway project using pico boards, but have spotted something that ought to be improved.The power to each section of track is scheduled to be switched by a relay. I've a bank of 8 traditional relays, but have just realized the total relay current will be up to a half amp or so worst case. I'm looking to a solid state solution.The relays are switching DC at 12V, but the polarity is determined before the on/off relay. Most SS relays I've seen are triac-based for AC only; one type would do DC, but not with the polarity changing (and at 10x the cost anyway!).Does anyone know anything solid state that might do this without breaking the bank?Thanks.

You could use a H-bridge, rather than build it from discrete transistors/FETs they come as a single or dual bridge in an IC, possibly with built-in snubber network.

solid state relay??

Aug 01, 2025 Last reply: 10 months ago 28 Replies

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Theo 11 months ago

I don't think you are. Regular AC SSRs use triac switching. The thing about triacs is that once they're on they don't turn off until the current drops to (near) zero. For regular AC that's the zero crossing point and happens every 20ms on 50Hz mains. But for variable-polarity DC it's a constant current and so the triac doesn't turn off while the current is there. It will next time the current goes to zero, but that may never happen.

GTO (gate turnoff) thyristors are an old way to handle that, but they're really for electric locomotive scale applications, not small scale stuff. That's what got me thinking about IGBTs.

Some of them have 3-24V inputs. I think the Crydom ones I have do that.

Those are fakes and lie about their ratings, FYI. OK if you don't take them anywhere near rated current. Also the baseplate needs heatsinking as it's just a single triac with the markings filed off.

Do small GTO SSRs even exist as a product? The only GTOs I can find are tens/hundreds/thousands of pounds?

+1. Or bistable relays if the coil current is a nonstarter.

There is that.

Theo

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Mike Scott 11 months ago

Yes, sound advice. Thanks. I was working out late last night this will be outside a normal usb current limit. I do have a suitable bank of relays, and I'm pretty sure there must be a spare 5V wall wart around somewhere.

Thanks to all for comments offered. I think I'll have to stick to the old-fashioned way here.

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Mike Scott 11 months ago

"Sorry, this item's currently unavailable in your location.".

That looks a good possibility, thanks. I do see that ali offer these on a module as well, with added components. I'll have to check the datasheet, but I imagine the pico won't drive the relay coils directly. It's also £4 a pop on a module. x8.

So I imagine. I bought a board that charges a big capacitor, then uses that to run the point motor, automatically cutting off the charging power when current is drawn. Then you can't burn them out. The currents are really quite large: a SEEP motor seems to be about 2 ohms for the coil, and I'm running from a 19V psu, so we're talking around 10amps. For that, I'm using some overly beefy mosfet modules. A much simpler problem than steering power to the tracks. I have a pico-based control board to control 6 points with USB to a desktop pc. Works well in testing.

I'll give the relay modules a bit more thought. Thanks.

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The Natural Philosopher 11 months ago

I've got bare pcbs with a 5V psu and buffered 4 buffered relay coils that can interface to a Pi if you want

Supply the voltage and have transistors and LEDS to drive and show relay status

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Its interfacing to a zero there, but a Pico can run off 5V as well

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Mike Scott 11 months ago

Thanks for the offer, but as I already have the 8-way bank, I'll settle for that.

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The Natural Philosopher 11 months ago

Great.

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Jeff Jonas 10 months ago

MOSFET based solid state relays handle DC or AC and shut off immediately whereas SCR or TRIAC turn off AC only at the zero crossing.

see:

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Mike Scott 10 months ago

Part of the issue is that I want the load and its psu electrically isolated from the control logic. Could probably be done with mosfets and optoisolators, but I've settled for an old-fashioned mechanical relay, at least for now. Thanks for the thought though.

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The Natural Philosopher 10 months ago

Can do. Most MOSFETS are unipolar

Look up GTO thyristors

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