[brinkmann]

We have a system in which the outputs will drive solenoid valves (120V). Normally we drive a relay which closes the 120V circuit to energize the valves. Whether right or wrong we try to protect the PLC in the event the solenoid valve fails. On this panel we are limited on space and would like to drive the solenoids directly from the output card. I believe the module can handle .5 amps per point. Which allowing for the surge current rating should be ok. Any others precautions we should take?

Thanks!

[Mickey]

brinkmann, on Nov 6 2009, 02:12 PM, said:

We have a system in which the outputs will drive solenoid valves (120V). Normally we drive a relay which closes the 120V circuit to energize the valves. Whether right or wrong we try to protect the PLC in the event the solenoid valve fails. On this panel we are limited on space and would like to drive the solenoids directly from the output card. I believe the module can handle .5 amps per point. Which allowing for the surge current rating should be ok. Any others precautions we should take?

Thanks!

Surge suppression, see below from the 1769-OA16 manual

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[paulengr]

brinkmann, on Nov 6 2009, 05:12 PM, said:

We have a system in which the outputs will drive solenoid valves (120V). Normally we drive a relay which closes the 120V circuit to energize the valves. Whether right or wrong we try to protect the PLC in the event the solenoid valve fails. On this panel we are limited on space and would like to drive the solenoids directly from the output card. I believe the module can handle .5 amps per point. Which allowing for the surge current rating should be ok. Any others precautions we should take?

Thanks!

There are two different problems to overcome. First, there's the "surge" when the magnetic field of the coil first goes into saturation. This will draw quite a bit more power than normal. Look at the documentation on the coils because that's where you find it hiding. This is usually well documented.

The second post about surge suppression is partly off the mark. If you drive a solenoid from a triac output, it can only switch off when the voltage is at zero (at the zero crossings). Thus you don't get any ringing or spikes/surges when using triac outputs directly to drive coils. In the bad old days of true relay ladder logic, the ringing and electrical noise from panels full of relays was so bad that many engineers had every single relay protected by a surge suppressor simply so that the whole panel didn't constantly misfire from ringing effects.

So with triacs, surge suppression doesn't do anything because the device naturally suppresses the problem on it's own.

If you mix technologies, switch contacts or relays and triac outputs, look out! Let's say that for instance you wire in a "test" or "bypass" push button in parallel with your triac output. When the test button is closed, the coil energizes. BUT, when the test push button opens, you are virtually guaranteed that the coil is in an energized state. The magnetic field collapses, pushing a very small amount of current through the very high resistance of the triac device. The magnetic field has to go "somewhere", so the coil is effectively turned into a spark coil just like the one used in an engine...zap! Goodbye triac. The exact same thing happens in plants where electricians use "Wiggies" or "Wiggins" all the time. If you do have Wiggies in your plant, take them away and throw them in the trash can. You will probably notice an immediate uptick in the lifespan of your I/O cards. The reason is because simply testing a circuit with a Wiggie subjects it to inductive kick from the coil in the Wiggie itself.

That's what all the warnings about "surge suppression" are concerned with...zapping your solid state outputs via inductive coil flyback effects. If you do have such a circuit, it costs about a dollar per coil to protect it. Use a diode surge suppressor. These are rated mostly for voltage. You can buy them very cheaply from electronics sources such as Newark, Digikey, Mouser, or Allied (google them). These devices are two diodes wired back-to-back. Under normal conditions, since they are wired back-to-back, there is no conduction in either direction. However, diodes have an "avalanche" or "zener" knee in the negative direction. When the voltage exceeds the knee, the diode acts almost like a short circuit. The diodes are constructed with a very specific zener trigger point. For 120 VAC circuits, usually the surge suppressors are set to trigger at about 150 VAC. So a diode surge suppressor placed across the coil will short circuit the coil during the transient caused by opening mechanical contacts, draining the coil magnetic field away harmlessly by heating up the surge suppressor and the coil itself.