0
\$\begingroup\$

I'm using a JQC-FC3VDC relay. I control it from a microscontroller using a 3.3V signal through an opto-link. And the business end of the relay controls a 24VAC output which then activates the irrigation valve. All pretty obvious.

The relay output as three pins, COM, NC, and NO. I've been connecting 24VAC to the COM and using the NO pin as the output to the valve. The NC pins is disconnected (floating) in my setup. So, when I turn-off the relay, the solenoid suddenly has floating lead and a large di/dt, which causes a sizeable voltage spike. A couple of things I realized:

  1. The COM and NO pins are connected via the mechanical relay, so it does not matter which has the 24VAC supply and which is the output to the irrigation valve. Its essentially a switch, there is no directionality there.

  2. Therefore, if I connect the 24VAC supply to NO and run the the COM pin to the irrigation valve it should operate the same. However, I can then connect ground to the NO pin, so that when the irrigation valve shuts off instead of having a floating input to the solenoid, its held to ground. This should squash the voltage spike cleanly.

I guess what I'm asking is, does this make sense? I've been using the COM pin of the relay as the supply input and the NC pin as the output to the solenoid, but it does not have to be that way.

\$\endgroup\$

1 Answer 1

Reset to default
0
\$\begingroup\$

Most of your thinking is correct, but -

There still will be an open circuit as the armature breaks the NO contact and travels to the NC contact. If you want to really suppress the arc, leave the wiring the way it is now and install a bidirectional tranzorb across the two wires going to the valve; that is, in parallel with the valve solenoid coil. This will clamp the inductive kick to something comfortably above the AC peak voltage, like 50 V, no matter what the switching arrangement is.

To determine the diode clamp value, measure your AC supply open circuit voltage, multiply that by 1.414 (square root of 2), and add 10%.

The same thing can be done with two back-to-back zener diodes, but a tranzorb is specially constructed to handle very large current spikes in a relatively small package.

Where are you located?

\$\endgroup\$
5
  • \$\begingroup\$ I'm in California. I'm not familiar with the term tranzorb. I have TVSs which are rated for 56V which I can place across the solenoid. They are beefy enough to allow the 24VAC to work normally, but clamps the spikes pretty well. The issue I have is the TVS will be outside at the solenoid to have best effect which exposes it to weather, heat, water. So, it has a limited lifetime. \$\endgroup\$
    – Jim M
    Commented Jul 19 at 20:49
  • \$\begingroup\$ To add a little ore background. What's happening (as best I can figure) is my controller is getting a large ground bounce when the relay is turned off which is causing the reset pin on the controller to drop and causes the part to reset. Kind of funny really. If my re-wire does not work, then I will try the TVS \$\endgroup\$
    – Jim M
    Commented Jul 19 at 20:53
  • \$\begingroup\$ Tranzorb is a brand name of a line of TVS (Transient Voltage Suppression) diodes. Make sure your devices are not MOV's (Metal Oxide Varistor). Weather, heat, and water should not be any problem as long as the connections to the wiring are secure. Locating the TVS at the controller rather than out at the valve protects the controller from lightning-induced spikes in the wiring. \$\endgroup\$
    – AnalogKid
    Commented Jul 19 at 21:32
  • \$\begingroup\$ Ah! Makes sense. I'll put one at the controller side and see how that works! Thanks for the help! \$\endgroup\$
    – Jim M
    Commented Jul 19 at 22:19
  • \$\begingroup\$ make sure your part is the DC type, not the AC type. Often, the only difference is a single letter in the part number. \$\endgroup\$
    – AnalogKid
    Commented Jul 20 at 0:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.