I keep blowing up irrigation solenoids

Question

I built my own Wi-Fi connected irrigation controller using an Arduino Nano and some mechanical relays. The Arduino is opto-isolated from the business end of the relays which switch 24 V AC on/off to standard irrigation valve solenoids.

I seem to be "blowing up" the solenoid in the irrigation valve occasionally (too occasionally). When it fails, the solenoid in the irrigation valve is seen to be open-circuit (infinite ohms) versus the ~50 ohms I would usually see and the valve no longer operates.

I'm guessing that when I'm turning off the relay and cutting the current to the valve solenoid the inductive load is causing a big voltage spike and killing the solenoid. This is happening only on one set of valves which is located about 100 feet (30 m) from the controller which is probably adding additional inductive load through the long run of wires all bundled together.

Switching to a DC solenoid is a non-starter as 99% of all irrigation valves use AC. I'd rather not have to design my own irrigation valves. I also don't think a soft turn-on/turn-off is practical as this would require a new relay design.

I've been looking at a diode or snubber circuit to go across the solenoid to absorb voltage spike, but I am not too familiar with that. How can I proceed with this?

Answer 1

Let me guess: the solenoids that keep burning out are the ones furthest from the controller out of all the others?

It's not voltage spikes, or lightning, or inductance on the lines, or anything like that.

Unlike DC solenoids where resistance limits the coil current, AC solenoids' current is limited by both the resistance and inductive reactance of the coil. However, the initial inductance is low because there is an air gap in the coil's center until the plunger/magnetic core is pulled down all the way into the coil.

For this reason, the inrush current for AC solenoids is typically several times (5-10 times) the holding current. But the length of this current spike is so brief that it doesn't have time to cause an appreciable amount of heating.

Except when your drive voltage is too low.

An AC solenoid is relying on that plunger getting into position quickly so the inrush current spike can be quick as well.

However, if your drive voltage is a little bit too low, then the plunger won't move into position fast enough and the inrush current spike is prolonged enough to blow the coil. Even if it causes a short, a short in the coil will very quickly give way to an open circuit failure from the wire acting like a fuse.

The rule of thumb is to not go more than 10% below the specified voltage for an AC solenoid for this reason. Your controller is probably losing a bit more voltage at the relay compared to the commercial controller, and when combined with the voltage drop from the 100 feet of wire between it and the solenoids, the voltage is probably just not quite high enough to reliably drive the solenoid. All it could take is slightly more force required to pull the plunger one time, or maybe it's a little too hot that day (increasing copper resistance) etc, and the plunger doesn't get into place in the coil fast enough and the inrush current kills the coil.

Answer 2

If the solenoid were being killed by a voltage spike, it's far more likely there would be a short between turns, or to ground.

Applying 24 VAC for too long, dissipating ~12 W in the coil until it burns out might be the cause. Check that the coil is rated for 24 VAC continuous duty. Also check the the steady-state voltage across the coil is no more than 24 V.

Another possibility is electrolytic corrosion. This is not unlikely in a sprinkler system. If any water gets into the solenoid, the very fine wire in a coil will be "eaten" through.

Answer 3

Mechanical armature jamming and inductance

One thing that might be relevant:

When an AC solenoid is de-energised the armature is pushed out of the coil by a spring and pulled in by the energised coil.

In the deenergised position the solenoid's inductance (and its AC impedance or "resistance") is reduced. This has the beneficial effect that on turn-on a high current flows until the armature is pulled in. At that time the inductance increases and the current is reduced to a value sufficient to hold it (but not enough to pull it in the first place).

Perform a mechanical inspection on your failed solenoids. Look for torn or leaking diaphragms, water in the armature / coil, etc., and foreign material, mineral deposits that might gum up the movement.

If the armature can't move it is likely that the prolonged high current is overheating the coils. This can also be a problem with AC powered contactors used in industry. It would be very unusual for a coil to burn out due to a switching transient.

Related (but for a DC relay): https://electronics.stackexchange.com/a/487353/73158

Answer 4

In normal operation the valves will see the peak AC voltage on each half-cycle, this will be around 1.4 times the RMS so around 33-34V. When the relay opens the inductance of the coil and wiring will generate a large voltage spike, as you’ve identified. If you place two zener diodes in series with one reversed then this will limit the voltage. I’d start with about 40V devices, and with a current rating to match whatever the solenoid draws. Enhancements to this basic circuit could be a small series resistor to limit the current and a parallel capacitor to absorb high-frequency transients. Just using the two zeners should do the job well, the other components may be desirable if there are any EMC issues.

Answer 5

when I'm turning off the relay and cutting the current to the valve solenoid the inductive load is causing a big voltage spike and killing the solenoid

The solenoid is a bunch of wire wound in a coil. It is about as hard to electrically destroy as that coil is. The primary ways to kill it are to:

• Overheat it - it will then fail open,
• Apply a transient overvoltage - the insulation on the coil wire will likely break down, and the coil will appear partially or fully shorted.

Switching to a DC solenoid is a non-starter

All solenoids are DC solenoids, more-or-less. They may need to run from a much lower voltage, though, since their AC reactance does not limit the DC current like it did for the AC current.

But why did you think you have to change to a DC solenoid?

Anyway, there are a couple of reasons why irrigation uses AC solenoid design:

• the controller doesn't need a rectifier bridge nor a huge DC smoothing capacitor,

• galvanic corrosion may be slower with AC than with DC

I've been looking at a diode or snubber circuit to go across the solenoid to absorb voltage spike.

You need to protect both the relay contacts from arcing, and the solenoid coil from overvoltage.

Thus, you need back-to-back Zener diodes or a TVS, either rated at 48VDC or so, both at the relay contact, and at the power entry to the solenoid.

Answer 6

It's (barely) possible that the spike is causing a short in the coil and your power supply is skookum enough to fry the coil to the point where it opens up. You can kill the spike with a bipolar TVS eg. P6KE56CA or 1.5KE68CA across the AC coil, and your relay will last longer. MOVs are possible too, but they do wear out and fail short with some degree of fireworks.

If this is the case you should be able to see that the coil has overheated. A similar effect would occur if your supply voltage was as bit on the high side, since the heating effect responds to the square of the voltage.

Or some kind of mechanical or electrolytic damage caused by design, manufacture, or installation.

Voltage spikes by themselves do not cause coils of wire to fail 'open'. If there is no evidence of overheating, I suggest looking elsewhere.