Driving solenoid with PLC

Hi,

I'm trying to drive a water solenoid valve (24V, 11W) using an on-board (sinking) output on an Automation Direct D0-06DD1 PLC. The PLC has AC power, and +24V from an external power supply connected to "+V", and C0,C1,C2,C3 connected to 0V on the external power supply. The solenoid is wired with one end on the +24V from the external power supply, and one end to output Y1 on the solenoid. The attached image shows a sketch of the circuit.

The first time I turned on Y1, the solenoid activated as expected. However, when I turned off Y1, the solenoid did not close until I turned off the PLC power. Now, when I turn the PLC back on the solenoid is activated independent of the state of Y1. Additionally, I smelt a faint burning plastic scent, so I'm guessing I burned out the output transistor on the PLC.

Is it possible to drive a solenoid directly from the DL06 DC output, or do I need to do a relay? My application has a lot of on-off cycles for the solenoids, and is deployed off-site, so I'd really rather not use relays due to the limited cycle lifetime. Do I need to put in a shunting diode to account for the spikes of the inductive load of a solenoid?

Thanks,
Justin


View attachment 21225

While a shunting diode is always recommended for an inductive DC output and lack of which could possibly have led to this failure double check you work. While we typically use sourcing type circuitry (DD2) this looks to be within the driving range of this unit. I would have expected failure due to the turn-off spike would occur only after multiple actuations.

Y0 and Y1 are the 'high-speed' outputs, and are only rated for 0.5A. 11W solenoid / 24VDC = 0.458A. Awfully close to that 1/2 amp rating. The inrush of the solenoid was probably a LOT higher than that. If you had used one of the other outputs, it 'may' have worked.

Don't feel bad... Most of us have had the same experience...

A relay isn't your only option. You can drive a solid-state relay instead. For example, a Crouzet DR-ODC24 is rated for 3A, with a 5A inrush. That should handle your 11W solenoid. Do you know what the inrush current is on your solenoid?



-Eric

Oops, as usual, Bernie is right... I missed the asterisk on the specifications page. Y0 and Y1 are only derated to 0.5A when used as PTO outputs...



-Eric

Thanks for the replies. Glad to know it's not out of the question to drive a solenoid directly from a DL06 unit. Would there be any particular difference between using a unit with sinking outputs and one with sourcing outputs, other than how the load gets wired?

To double-check my wiring, I moved to Y3 (so now concern about the special-purpose Y0,Y1 pins) and instead of a solenoid I put in a 1k resistor. Everything behaved exactly as expected, with a voltage differential and current flow only showing up when Y3 was enabled.

Next, I turned the PLC off, replaced the resistor with my solenoid, and turned everything back on. The solenoid was closed until I enabled pin Y3, at which point it actuated. Disabling the pin didn't turn the solenoid off / close the valve, and I again smelled some faint burning.

Turned the PLC off again, put the resistor back in as my load, and now I see a voltage differential and current flow independent of the state of the transistor. Any suggestions on what to try next?

Luckily the PLC has a lot more outputs than I need, so I can still salvage the unit for my application. Any help is of course greatly appreciated.

Thanks,
Justin

Are you absolutely sure this is an 11 watt unit? Use a meter set on current inline to the solenoid and connect directly to 24 VDC.

I have to ask... Is the solenoid valve assembled? Without an armature in the coil, current draw will stay high...

2 down, 14 to go...

I would not - I would use an external relay with a flywheel diode in it for sure. Much easier to replace than a transistor on a PLC card.

The easy way to handle this problem is use a relay (solid state the best) rated around 5 amps to turn on this (24V 11W) solenoid. The relay coil would be connected between Y1 and +V. But, you will still need a shunting diode across the relay coil as Bernie suggested.

Here's advantage and disadvantage of using a relay to turn on this (24V 11W) solenoid. The advantage is you don't kill the output on the PLC with something that is over the rated for the PLC output. And, you are not limited on what solenoid you want to use, as long as the relay is rated for the you what solenoid you want to use. The small disadvantage is the relay, is you will need to make some room in the control cabinet.

The internal transistor is not capable of having backvoltage and no peaks so you must use some protection like a diode
You can not use high current and high switching inside PLC as the tor will get too hot.
use a SSR on the outside.
and yes it is easy to repair the PLC as only the tors are burned.
you have alkso not to connect the 24 Volt internal as it can only supply 300 mA .
the coil is way too much for the PLC,
The solenoid is also not good in many cycles as the small plastic ring will get damaged in time.

I didn't have a multimeter or scope handy to test what the current draw of the solenoid actually is, but I did have a different 24V 9W solenoid to play with. I put that in place of the 11W solenoid, and everything seems to work just fine, even without a shunting diode. I wrote a small program to actuate the solenoid 1000x and both PLC and solenoid seem fine. It's possible something was wrong with the initial solenoid, or that it's just over the specs of the PLC. 11W should be less than 0.5A, within the 1A rated output.

In general, I thought PLC transistor outputs were designed to drive both resistive and inductive loads without additional protective circuitry. The DL06 manual specifically recommends the use of a shunting diode when driving inductive loads with a PLC relay output, but not (that I've seen) for the transistor outputs. The transistor output schematic (2-32 for the D0-06DD1) has a shunting diode in there already. Is there anything to be gained adding an additional one? Putting an EM relay between the solenoid and the transistor output seems rather circular, as the relay itself is an inductive load.

Thanks for all the help,
Justin

I REPEAT - I NEVER DRIVE A SOLENOID FROM A TRANSISTOR!!!
Inrush, spikes, solenoid s**ts iteslf and the transistor is gone. Replace PLC, replace card - damn site easier to replace an external relay at 3AM - been there many times before.
If you do not heed this advice I am sure somewhere along the way you will wish you did - took me about 12 years to find out - 3 day plane trip to get there - another 4 days to get some gear - the system was under warranty - cost me heaps - middle of Australia - no beach - no tourist attractions - no bars - 3 days to get back home - accomodation, food and alcohol costs from a supermarket - nothing else to do - no internet - hardly even a phone - had no books either. Sat in the sun and got sunburnt - no chemist for some cream either!

Hi Justin,

How many cyles per hour are you expecting? we run a number of similar 12V DC coil solenoids at sewage pump stations for well washers, and at many we have at least 10 starts per hour. We drive all our solenoids with 12V DC Omron relays and have very few issues - don't think i've replaced a relay yet actually. mechanically, they are rated at 20M cycles, however the contacts are only 100k at rated load. Your load is significantly less, so I'd expect much better milage out of the contacts than that. the ones we use are rated at 1800 closures an hour, so set up a test rig on Friday arvo and by Monday morning you'll have reached "rated" closures!

have had many more problems with some of the earlier solid state relays, but newer ones might be better?

Lastly, Make sure you at least mark those burnt outputs as damaged if you don't replace the PLC, otherwise some poor bugger will be cursing you in years to come when he tries to use one!

Just to be contrary....
I have been using some PLCs lately that are specifically designed to switch solenoid loads directly, through transistor outputs. These have built in free wheeling diodes and are rated at up to 4A inductive (depending on which IO I am using).
However, I agree that if I was using one of these Automation direct PLCs, I would prefer to use a relay, with diodes attached at the appropriate places.
Check the relays carefully as well, I have found that some of the new compact 5mm style relays will only do about 3000 or so load switches.

As a rule of thumb with an inductive load such as a solenoid you should allow for a current rating of at least 5 times the steady state current which for your solenoid would be about 2.5 amps. You should also have a diode connected in reverse bias mode across your coil to protect your transistor from the coil back emf. If you cannot meet this requirement then you should add some type of interposing relay.