I am trying to drive three Asco solenoid valves, I can't figure out the exact electrical characteristics on the data sheet, but from field tests I know the three valves operate at 127VAC (Mexico mains voltage) and the steady state currents are 250mA, 495mA and 495mA respectively. Assuming the inrush current is 10x the steady state current, I am looking at 2.5A, 5A and 5A inrush currents respectively.
I want to use a solid state relay board like this SSR board, though mine runs on 12VDC. Should I use a varistor on each solenoid valve like on this diagram?
or will the chinese board have adequate isolation of both the semiconductor internals and the microcontroller?
The Omron relays you point do are rated at 240 VAC. HOWEVER, they are limited to 2 AMPERS!, So not enough for your project. The OMRON data tells you that.
But the steady currents are all below 2 AMPERS, will it not work due to the inrush current? I thought that since it last only a few ms it would be able to work
I think the 10X inrush is more for motors than solenoids. I see the subject of inrush current for AC solenoids is mixed according to a quick google search.
What I know:
Inductance limits the current when the solenoid is fully ON.
When the solenoid is OFF the inductance is somewhat lower. I'm guessing it is not 10x lower.
As the solenoid is moving there is some back emf lowering the current.
I think the SSR board from your link will be fine. It had built in snubbers and it is zero crossing both on and off. It is also spec'd for a non repetitive peak current of 30A. While I wouldn't suggest a design with 30A regularly I think the 5A you suggest (which I think is way overstated) will not be an issue.
You should also feel better to know how Triac's work (the switching element in these SSR's)
Once turned on at the beginning of the voltage cycle they stay on until the current through them drops to a very low number. This means any inductive current is zero or very close making off transients almost non existant.
Fluid valves do not usually have a complete magnetic circuit, but with the sort of solenoid where the armature is a laminated "T" and pulls in to complete the magnetic circuit, it could easily be a 10 times variation with a corresponding difference in current draw.
There is no inrush current as such with an inductor. Capacitors have inrush current.
The free-wheel diode used with a DC inductor suppresses the inductive kick-back voltage spike when it is turned off.
For AC a diode cannot be used for this so an RC snubber circuit is typically used to reduce (but not eliminate) the voltage spike at switch-off.
Only the resistance limits the current for DC. With AC the inductance (and perhaps the resistance too) limit the current when on, which dominates depends on the frequency and ratio of inductance to resistance.
If resistance dominates the DC and AC currents would be similar, if the inductance dominates the AC current is less than the DC current (never use an AC-only rated solenoid with DC for this reason). If the inductance can vary then the AC current may vary, but this will be only for a few milliseconds for a solenoid. The initial current (when the contacts close) will be zero and build up on a timescale of milliseconds too, so there's little reason to worry about arcing then - the problem is always at switch-off.
Your example is not a motor. The current is not fully controlled by back EMF. The coil inductance when the solenoid is "open" is lower than when "closed" due to the change in magnetic structure. However the delta inductance is no where near 10:1. The below suggests it might be in the order of 3:1
This being the case, the OP's situation the SSR is more than capable of handling the solenoid proposed.
Yes, for an open solenoid, that sounds very reasonable.
It could be anything up to 10:1 for the actuator I pictured though, which may (will) easily burn out if it is prevented from actually pulling in - as it did a couple of months ago on my washing machine. When I ventilated the place sufficiently to work on it and find what had happened, I simply disabled it as it was only the unused "suds return" valve.
Yeah. I now have spare machines - some with "problems" from the estates of my parents and my aunt. Just today I notice we appear to have three machines in storage - I can't recall why the third one.
When I grand parents passed away we were removing their old refrigerator. Easily weighted double more modern refrigerator weighted. My dad moved it to his summer home where it ran until he passed away. We sold the summer home with it still working.
Guess was about 60 Years old !
Yes, appliances are getting lighter. Insofar as this represents advances in engineering and technology, such as BLDC direct-drive motors with increased efficiency this is a good thing, just as long as it is properly engineered.
many thanks
