I have a set of 8 ea. 24 vac sprinkler valves that are controlled by a microcomputer through a bank of eight small DC relays. Every thing works well when the micro-controller is controlling just the bank of DC relays. But when the power to the 24 VAC sprinkler valves applied and then removed, intermittent problems arise with the micro-controller.

What type of device should I place across the sprinkler valves to kill the back emf?

would it be unreasonable to expect a small RC snubber circuit , e.g. 1mf & 120 ohm, to work. Or perhaps some sort of TVS diode or varistor?

Thank you.

 

You could try RC snubbers.
Across the relay contacts first before trying the coil.
Max.

 

Try using one valve at a time,then add another and build them up,see if the problem starts when several valves are used at once.

You can't use diodes as it's AC.

 

You could even try back to back zener diodes (say 40V) across each coil, each provided with a blocking diode to stop conduction of the Zener in the forward direction.

 

All great ideas. Thank you!

I'm thinking of ordering some .1 mfd caps and some 100 to 200 ohm resistors. Might also buy some varistors to put across the RC circuit. Seems you can do that for about $1.00 per circuit.

@MaxHeadRoom - Would like to know which way would be more effective insofar as across the relay contacts or the coil itself. It would seem the object would be to dampen the spikes across the coil as opposed to protecting the relay contacts, but both seem important. If the rc circuit was across the relay contacts, a small current would flow continually @ 60 Hz.

@Dodgydave - typically, two valves are on at the same time - the main valve and a zone valve. But it seems even when one valve is shut off, the effects of the voltage spikes are noticed - but not always.

Before connecting weather related sensors (wind direction and speed, rain gauge, temperature probe) to the micro-controller board, which added 200 foot of wire to the arrangement, the back emf was not an issue. It had worked fine for several years. So it seems the back emf may be induced into the extended wiring as opposed to being fed back through the power leads. Does that make sense? I don't have a scope capable of displaying the surges.

 

@MaxHeadRoom - Would like to know which way would be more effective insofar as across the relay contacts or the coil itself. It would seem the object would be to dampen the spikes across the coil as opposed to protecting the relay contacts, but both seem important. If the rc circuit was across the relay contacts, a small current would flow continually @ 60 Hz.
.

In some cases either may work.
One manuf. design sheet.
Max.

 

Make sure your 24 VAC AC circuit is 100% isolated from your DC circuit.
I think, each "snubber" should be across the two wires going to each Sprinkler Valve - the source of the BEMF.
Can you post a schematic of the DC Supply, Micro, Relay Board, AC Supply and 24 VAC Sprinkler ?
Part #'s & Models #'s ?

 

Good information. It seems placing the RC circuit across the switch attenuates the switch arcing effects. Whereas placing it across the coil attenuates the back emf.

A storage scope would be nice equipment to have.

 

@mvas I have a schematic for quite a bit of it. It isn't detailed, but it will give you a good idea of what I've done. I will find it.

As far as isolation is concerned... I was using the same 24 VAC transformer to power the electronics and the valves. That worked fine for several years. Then I added the weather sensors and began having trouble with the microcomputer locking up. So I connected a second 24 VAC transformer into the mix just to power the valves. So there was nothing in common electrically beyond the 110 VAC supply. Doing this appeared to make no difference. That made me wonder whether the back emf was radiating into the additional sensor wiring and entering the microcontroller that way.

 

Have you seen the articles on converting to DC valves, particularly the impulse latch/unlatch, someone has done an interface for the Arduino I believe.
Max.

 

You can't use diodes as it's AC.

Yes, you can.

Back-to-back zener diodes will clamp the power lines at the zener voltage plus about 1 V. 24 Vac is 34 Vpeak, so 40-50 V zeners should clip the spikes without conducting during normal operation.

A zener specially built to be a transient suppressor is called a TransZorb. They come in an AC version with the two zeners in one package. They are great because unlike MOV's, the clamp voltage and response time do not degrade with each transient.

Measure your 24 Vac value with no load to determine a safe diode voltage.

ak

 

There is a simple way to use a diode to suppress the inductive spike from an AC circuit, and I have used it a few times. Two diodes, with adequate voltage and current ratings, back-to-back, with the inductive load across one of the diodes. It is also an effective way to reduce the mechanical buzz of AC solenoids. The one down-side is that it does slow the release of the solenoid a bit, by quite a few milliseconds. Probably that is not a problem with a sprinkler system. But it is vital to have the diodes back to back, rather than in series. It does not provide contact protection, just spike reduction.

 

@mvas Here is the wiring diagram I am using. The "Webcontrol server" is a CAI networks board wc8.

The diagram works with LTspice IV, so a few "connections" are there so that LTspice will work.

The transformer is a Hunter sprinkler control transformer. And, as explained above, I disconnected the transformer from the valves and I am using a second Hunter transformer to power just the valves for a test.

If you have questions, please ask.

And all advice is appreciated.

Frank

 

@AnalogKid . the TransZorb diode seems to be a brand name for the TVS diode. Seems like it would well. I had not heard of TVS diodes before.

 

with the inductive load across one of the diodes.

I don't understand putting one of the pair of diodes across the coil. Wouldn't that result in the diode conducting for half the cycle?

 

I don't understand putting one of the pair of diodes across the coil. Wouldn't that result in the diode conducting for half the cycle?

Exactly what happens. For half the cycle the ac current flows, generating the magnetic field. Then for the other half cycle the collapsing magnetic field generates a current in the other diode, which maintains the magnetic field. No capacitor needed and no mechanical buzz, and no transient spikes because current can keep flowing in the coil. Simple but effective.

 

With a second transformer powering the valve coils there were still the relay coils connected to the control board, and probably those were the ones causing the problems. Relay coils have a reputation for doing that sort of thing.And it is not clear what sort of arrangement there is for that part of the relay circuit board. So there is a real possibility that is where the problem lies. So my suggestion is to look at the relay board and see how that circuit is set up. By eliminating one problem source you have revealed another one. Good technique!

 

Thank you for the advice MB2, the "relay board" is one those $12 relay boards with the 8 relays inline. Each relay has an opto isolator between the relay coil and the input pin connected to the webcontrol board. Also, if I remove the power to the valve coils -- to just allow the relays to pick up and to release normally, there is no problem. As soon as I apply the 24 VAC to allow the valves to pickup, the webcontrol board intermittently goes nuts.

And prior to connecting the weather stuff to the webcontrol board, I had no problems. That was using just the one transformer to power the whole thing. While I could very well be overlooking something, it seems the collapsing fields are radiating noise into the wiring to the webcontrol boards inputs. While I have a hard time believing that, I don't know of another explanation.

I need to disconnect all the weather stuff for a test to ensure all is still well with it disconnected. Although I have changed nothing else in the last several months.

 

Also MB2, I still can't picture the two diodes back to back with one of the two across the coil. I know I am misunderstanding the explanation. I don't know where each diode is wired into the circuit.

 

@AnalogKid . the TransZorb diode seems to be a brand name for the TVS diode. Seems like it would well. I had not heard of TVS diodes before.

Like Kleenex, it is a brand name that has become the general name for the category.

ak