"Cutting" 4-20mA with a relay contact

theripley · Aug 31, 2016

Is it safe to "cut" a 4-20mA line with a relay contact?

I have connected a motorized valve on a temperature controller. The valve moves proportionately with the temperature from the controller. However, on some cases, I would like to totally close the valve regardless of the temperature reading. I would think cutting the 4-20mA line may do it.

Thanks

Aabeck · Aug 31, 2016

Run the signal to a SPDT relay COMMON terminal.

For interrupting when the relay is ON wire the NC contact to your valve input and the NO contact to the other side of your valve, this way the loop is still complete - it just bypasses the valve.

For interrupting the when the relay is OFF simply swap the NC & NO contacts above.

danw · Aug 31, 2016

The 4-20mA source might detect an open circuit and throw an error flag, hence the idea of completing its circuit by using the alternate contact on the relay to connect the switched (+) back to the (-). A short circuit (across a relay contact) will not damage a 4-20mA output because 4-20mA is a current regulating circuit.

You'll need to see whether the motor has fail-safe detection on its control input signal and whether it can be configured to drive in the direction you want (closed) upon loss of the control input signal.

osmanjdt · Aug 31, 2016

what about your main safety valve,is working?

danw · Aug 31, 2016

osmanjdt has a good point.

You're not trying to substitute the slow closing of a modulating control valve for a pair of real honest-to-goodness dual block and bleed safety shutoff valves are you?

Tharon · Aug 31, 2016

This seemed to be pretty common practice in the hard wired gas furnace mod motors I've seen.

A 4-20ma signal came from a Temperature controller to a modulating motor. But a contact closure was used to force the mod motor 100% open during purge, and 100% closed after purge. Cam switches proving open before starting the purge timer, and back closed before starting the fire cycle.

Redundant valves controlled and monitored by a flame sensing controller were elsewhere in the supply line for safety purposes. The contacts on the mod motor's control circuit were just the operational side of things.

Tom Jenkins · Aug 31, 2016

I have done this. A couple of cautions: Use bifurcated gold contacts on a DPDT relay. Make sure your transmitter won't be damaged by the open circuit - most won't, but check. Make sure your valve is set to close on signal fail. Some can also open on fail or hold last position on fail.

Check the operator characteristics. Many have a Local/Remote or Man/Auto input that will allow you to take control of the valve and force closure with a contact closure.

theripley · Aug 31, 2016

Tharon said:
But a contact closure was used to force the mod motor 100% open during purge, and 100% closed after purge.

Can I force 100% opening? How can I do it?

theripley · Aug 31, 2016

There is no shut off valve installed, unfortunately. Hence, the need to force closing of the motorized valve to hinder steam entrance when time is over.

theripley · Aug 31, 2016

Tom Jenkins said:
Make sure your transmitter won't be damaged by the open circuit - most won't, but check.

Did you use an SPST contact?

theripley · Aug 31, 2016

Aabeck said:
Run the signal to a SPDT relay COMMON terminal.

For interrupting when the relay is ON wire the NC contact to your valve input and the NO contact to the other side of your valve, this way the loop is still complete - it just bypasses the valve.

For interrupting the when the relay is OFF simply swap the NC & NO contacts above.

Since I am a "visual" person, I made a hand-written diagram just to make sure. Please see (pardon the handwriting) the attached photo. Is this correct?

Note: Red line indicates flow when my relay is ON, the black line indicates flow when my relay is OFF.

Will it be safe if I ,instead, connect only the normally open contact?

Tharon · Aug 31, 2016

theripley said:
Can I force 100% opening? How can I do it?

That depends on your valve. Ones in my example had a terminal I could close to force it open. I would just use a relay contact to remove the 4-20ma signal from the input and another relay contact to operate the force open terminal.

Edit:

Don't short the output with the normally closed contact. Just put the NO contact in series with your wiring. As mentioned earlier the only thing you have to look at is what your device does with 0ma. If it has open circuit detection.

jrwb4gbm · Aug 31, 2016

theripley said:
Since I am a "visual" person, I made a hand-written diagram just to make sure. Please see (pardon the handwriting) the attached photo. Is this correct?

Note: Red line indicates flow when my relay is ON, the black line indicates flow when my relay is OFF.

Will it be safe if I ,instead, connect only the normally open contact?

Others may have a different answer/opinion, but if it were me, I would not interrupt the current loop with the N.O. contact, only let the N.C. contacts "short" out the the valve side of the current loop. Put another way, don't put a contact where you show the N.O. contact, just leave that wire intact. Wire the N.C. contact as you have shown. That would always maintain the loop continuity.

Edit:
As you can see, Tharon and others have a different view/opinion. Please don't use my post unless you are sure that an open loop will cause problems. Both methods have merit.

theripley · Sep 1, 2016

Tharon said:
That depends on your valve. Ones in my example had a terminal I could close to force it open.

I see. Mine don't have the force open terminal.

Tharon said:
Edit:

Don't short the output with the normally closed contact. Just put the NO contact in series with your wiring. As mentioned earlier the only thing you have to look at is what your device does with 0ma. If it has open circuit detection.

We tried disconnecting the (+) terminal on our motorized valve input, it fully closed.

I was just worried if the "activation" of the NO contact may cause some problem on either the controller or the valve.

rupej · Sep 1, 2016

jrwb4gbm said:
Others may have a different answer/opinion, but if it were me, I would not interrupt the current loop with the N.O. contact, only let the N.C. contacts "short" out the the valve side of the current loop.

Thats actually an interesting idea- probably easier on the transmitter. The only downside I see is that it isn't fail-safe. If a wire falls off the relay or the contact fails, the valve will not close.

What about an early make / late break relay? Best of both worlds?

"Cutting" 4-20mA with a relay contact

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