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Looking for some advice, I am trying to create an onboard SSR to control the switching of a 120VAC load (heater/lighting), with an operating power of around 1500W max. I chose the following parts and am looking to find out if there is some fatal error I am not seeing (my first time building with Triacs).

The SSR will be turned on and off by an ATTiny85, which produces 40mA at its output. The MOC3020 (optocoupler triac) chosen requires If = 50mA max. The Q4025R5 (Power Triac) is rated for 400V, 25A, 50-50-50mA. Will this circuit work?

Do I require a zero-crossing triac for this load? Is this Q4025R5 a zero-crossing? And if not, can I simply just trigger the circuit on a rising-edge using an ISR for example by the ATTiny85?

The triac will be operating in quadrant 1 and 3 (for AC switching) in this circuit I believe.

The resistor (180, 1.2k) and capacitor(0.2uF) values chosen were given by a circuit I found online, so if anyone has any recommendations about how to perform circuit analysis with Triacs it would be greatly appreciated.

enter image description here

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  • \$\begingroup\$ I have answered a very similar question here. \$\endgroup\$
    – jay
    Nov 4, 2021 at 19:00
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    \$\begingroup\$ 40 mA is a lot for a microcontroller output. The MOC devices should trigger on 10 mA or so. The 50 mA is probably the maximum it can tolerate and you shouldn't operate near that value. Check the datasheets for both. \$\endgroup\$
    – Transistor
    Nov 4, 2021 at 19:05
  • \$\begingroup\$ Tristen, is there a reason you aren't considering an AC-coil relay? For example, the JQX-40F is a 40 A capable relay that powers directly from your 120 VAC line. The MOC3063 (5 mA) might be a good choice here. It can drive the relay directly just fine. (Been there, done that.) Your need is at least 15 A for the relay, but the excess spec is probably the way to go. Relays are cheap, bulky, but you can find them requiring less than 1 W of active dissipation while engaged. The SSR will need heatsinking, which greatly adds to the bulk, and more open environment to dissipate that added heat, too. \$\endgroup\$
    – jonk
    Nov 4, 2021 at 21:02
  • \$\begingroup\$ Transistor -- Thank you for the input, I will bear that in mind and put a series resistor in then to limit the current. Jonk -- The reason why I am not considering an AC-coil relay is that I am trying to produce a product which will hopefully go into production. I need to be able to keep costs down as low as possible, additionally I need about 4 of them per device. \$\endgroup\$
    – Tristen Michaud-Laughton
    Nov 4, 2021 at 22:47

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"Zero crossing" is a function of the opto-triac circuit used (or it could be controlled via the MCU but that requires more complexity). The MOC3020 does not have the zero-crossing circuit built in, parts such as the MOC3041 do have it built in.

To control a 1500W 120VAC load means the triac will be dissipating about 12W, which means you need a large heat sink and/or fan, depending on the maximum ambient temperature and how reliable you want it to be. Keep in mind that triacs typically fail 'on' so you may need to provide some fail-safe protection such as a thermal fuse for when that happens. The tab on the Q4025R5 triac is tied to MT2, so you either have to insulate the device from the heatsink (which has a penalty in terms of conducting heat out of the triac) or run the heatsink electrically 'hot' and prevent any possibility of contact with the heatsink.

You may find it a better idea to simply purchase a safety-agency approved SSR that has electrical isolation from the heat sink interface, some heat spreading (it will still need a large heatsink and/or fan) and the opto-isolation built in.

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  • \$\begingroup\$ Thank you for your detailed explanation. Since I am designing something that will ultimately be brought to market, would I be able to run the heatsink 'hot'? What is the downside? Also any idea what amount of wattage I would expect to be flowing through the resistors and cap? \$\endgroup\$
    – Tristen Michaud-Laughton
    Nov 4, 2021 at 22:50
  • \$\begingroup\$ If you have to ask about isolation, it's unwise to take advice from the internet. The resistors and capacitor should not get appreciably warm unless the triac does not trigger for some reason, in which case the resistors will tend to go up in flames. The capacitor may fail short and that would have a similar effect. \$\endgroup\$
    – Spehro Pefhany
    Nov 4, 2021 at 23:08

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