Sourcing Current to SSR

Upon attempting my first Electric Imp project (woo!) I realized the imps pin couldn’t source more than 4mA.
Turns out, that’s plenty to turn the LED on inside the SSR, but not enough to flip it - rendering it unusable.

My question is - is this 4mA per pin, or total? I haven’t dug into the SSR datasheets to determine the current needed, but if this is per pin I may be able to attach multiple output pins to the control signal to source enough current to flip the SSR.

Otherwise… I’ll have to come up with a plan B. I know the +5 from the USB wall wart (cell charger) sources plenty of current to turn it on, as I have tried with a cut USB cable. Perhaps I can take advantage of this will maintaining control via the imp.

Thanks for any input!

It’s per pin. If you’re not driving much else, you will find that the imp can source/sink a lot more than this but we spec it for all pins.

What’s the spec on the SSR? It may have an internal resistor which means the current is limited by the 3.3v voltage of the imp.

It looks like the trigger current is 7.5 mA. I’m using a Fotek SSR-25DA, cheap off of eBay. I’m not using the imp for anything besides the one relay.

In theory, if I were to strap two digital outs together, I should be able to reach the required trigger current. I’ll give it a shot tomorrow. Better yet, I’m using the April board so I could tie the 3.3V and a single digital out together to achieve the necessary current. Although I suspect that’s a huge waste of energy over time.

Will report back tomorrow.

Hmm, it says 7.5mA / 12v. Though it also says minimum trigger voltage is 2.4v, most of these are optocoupled so there will be an LED and a series resistor in line - lower voltages will result in less current and no switching.

The imp can supply 8mA easily from one pin (just don’t try that from all of them concurrently). To see if it really is the voltage, connect the SSR to the April board’s 3.3v/GND pins?

I built a number of A/C switches with Crydom SSR’s last year. They are expensive, but you can drive them with a single pin. I used one to switch my Christmas tree lights on and off with an Imp during the 2013 holiday season. Things were packed just inside the Bud Industries enclosure that I used, and I did notice that something was getting hot enough to slightly brown/melt part of the enclosure near the SSR. I didn’t investigate why, so It could have been a bad solder joint or any number of issues, but it did work the entire time. The Crydom SSRs have versions for resistive and inductive loads, so build with that in mind.

I’m not an EE, so those are just the notes of a tinkerer. :slight_smile:

Ahh… good to know. Crydom makes SSRs that only require 3mA to trigger.

I used Recom AC/DC convertors.

To power the imp in-line (in-package) I presume? I had looked at those but went a cheaper route.

I’m using a Samsung Wall Cube since they’re cheap. Pig Tailing the AC line and using female spade terminals to connect it directly to the terminals. I break it out of the mini-cube to make it even more compact. Then, cut the usb cord that comes with it to length and solder it to the +/- battery contacts on the April board.

I love your form-factor, very compact. Great job.

Thanks. Yes I wanted to power the Imp inline… and your right, it isn’t the cheap route. I’ll be rebuilding this with P3V3 and something to handle the SSR heat.

For now, I’ve solved my issue by tapping into the +5v from the USB power and am using a NPN transistor, with the output from my imp, to control the SSR. Essentially the imp flips the NPN on, which in turn flips the SSR on. It isn’t perfect, but will work for this relay for the time being.

Yeah, seems like it’s not a current issue but a minimum voltage, which would figure. Seems their datasheet is slightly fictitious!

The relay itself even says 3v minimum, go figure it wouldn’t work. Thanks for your input Hugo.

You could solve the SSR heat issue by using a latching relay instead. I see some on eBay that take 3v drive and can handle 16 amps of mains, which is enough for all normal purposes in the uk. Sure they won’t have nice zero crossing detection so might give some switch on spikes, but it could be a more appropriate solution.

The Imp works like a charm when directly linked to the Sainsmart 2A solid state relay (available on ebay) board (2x2A), that uses the Omron G3mb-202p relay). The heavier 2x 5A module wasn’t working, but that could be a minimum draw issue rather than a LED switching issue, I am not sure about that.

This SSR worked from an IMP pin. SSR’s are great but I’ve found one downside; they are quite never fully on or fully off.