I'm currently developing a PID controller for my espresso machine. In an effort to get as consistent a temperature as possible throughout brewing I've decided to let my arduino control more than just the boiler heater.
What i've decided to do is turn the brew switch into a software switch and have the arduino control the pump. This will a) let me know when brewing is happening so I can bypass the PID algorithm and turn the heater directly to 100%, and b) will let me add other useful features like shot timing
The problem I'm having is with the pump control. The pump is a 55W 120V vibratory pump and I'm trying to control it with a 25A zero crossing relay. When I apply a control voltage to the SSR, it turns on and conducts no problem, but the pump doesn't turn on.
The weird part is that if I leave the relay turned on and cycle the power switch, the pump turns on. So, i'm essentially getting the following behaviour:
Machine powered up, turn SSR on and the pump doesn't turn on
Machine powered off, SSR already on, the pump comes on when power is turned on
I've attached a schematic that isn't identical to the machine I'm working on, but it is fairly close. All I have done with my arduino is basically swap out the "espresso" switch with an SSR
The SSR is rather oversized (25A) for the load it needs to switch (0.46A). There would also be a minimum load requirement (could be 0.2A).
On the control side of the relay, its probably 3-32VDC, and you're probably using 3.3V or 5V control signal. The combination of low load, low drive level, and possible low quality SSR or replica, would very easily cause the problem you're having.
Test it with a 12VDC (or higher) control signal and I'll bet it works. I think you'll need to source a better quality relay with 2-5A rating and purchased from a reputable supplier.
I've read that there can be issues with inductive loads with a zero crossing SSR.
I have one that I am getting ready to test to start and stop a router motor, but have not tested it yet. I hope it will work. I've used plenty in the past, but that was to fire heater strips, not a motor.
Looking at prices on Amazon, the price increase from a zero crossing SSR to a random turn on SSR is at least a 10 fold increase ($8 to $80 or higher).
I'm hoping my zero crossing SSR will work, but if not, I may fall back to a relay.
Hopefully I will get a chance tomorrow to test it.
