Wonky relay on Barix

[James Sentman]

On the AC side I use SSR’s Solid State Relays which work great. Low voltage low current input can switch on a 20 amp 240v load or more. I also used them to just switch the 24vac that controls my furnace with an arduino based thermostat I built so many years ago that I can’t even remember but it’s been running perfectly and reporting to XTension for at least 7 years without my having to tweak anything ;) 

For DC a simple mosfet is really so simple that it feels like cheating. There are “logic level” mosfets like the ones that adafruit sells here: https://www.adafruit.com/product/355 that can switch 15 amps of current without a heat sync and can be turned on and off by as little as 2.5v. The problem with driving larger mosfets is that you have to provide a much higher voltage to make sure they are all the way on and therefore not adding resistance to the power and heating up. With these you don’t need anything but about a 330 ohm resister between the logic level output from the microcontroller and the gate of the mosfet. And even that is not strictly necessary (like it is with a regular transistor) it will work fine without that most of the time, but the gate does have some capacitance and in order to limit current through the arduino to below the max thresholds on them you should add a tint bit of resistance like that. They are also very static sensitive so make sure to treat them carefully. They are the same things as on those dimmer boards. 

I”ll bet there are DC SSR’s as well that would have optisolators and everything. I have never looked into that however so I can’t post an actual link. I like SSR’s as they are self contained often with screw terminals and din rail mounts on the larger ones and you don’t have to do any development at all, just connect the logic output to them and turn it on.

The digital outputs on a Barix 100 should have no trouble turning on a 3 to 5 volt input SSR or a mosfet.



> On Jan 12, 2019, at 9:19 AM, Charles Pendleton <charlie at pendletonweb.com> wrote:
> 
> What do you use for 24VDC relay switching?
> On the 24VAC side (HVAC), I have been using just standard mechanical relays in order to get a digital input in my Barix’s with no issues but I would like them too to be of the SSR variety although quite cost prohibitive from what I have found out there.
> 

Thanks,
 James


James Sentman                       http://www.PlanetaryGear.org		http://MacHomeAutomation.com




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