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Old July 30th, 2021, 03:52 PM   #1
jetwax
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Drive 30W solenoid valve with 24V DC 1756-OB16 output card

Hi Experts!

I'm trying to control a solenoid valve (don't have part number, but I've been told its a 24V DC 30 watt coil) with a PLC output card.

Supplier says to use a SSR interposing relay but customer prefers not to. I don't really have room in the cabinet either. Not fond of going against the equipment suppliers recommendation, but while they may not be the most sophisticated (no part numbers), they clearly have experience.

I'm looking for your comments on the feasibility and reliability of not using an interposing relay, and instead connecting the solenoid (via a fuse) to the output card directly. I'm thinking of using a 1756-OB16D output card. It is rated for 2A continuous (subject to maximum card of 8A) and 4A surge.

It also indicates its Pilot Duty (DC-13SQ) of 2A. Can someone enlighten me on what DC-13SQ means? (would my 30W coil require a rating of (30/24 = 1.25A?)

From my limited understanding, the greatest load is at initial energization, 4A at 24V is 96W. This should accommodate a 30 watt coil? I don't know if 30 watts is the maximum, or the continuous power consumption...

The other very important consideration is the inductive nature of the load, and its behavior on de energizing the solenoid. Luckily the OB16 card has the diode from RTN to OUT to provide a source for the current and limit the voltage.

Thanks for any feedback you give!

Cheers,
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Old July 30th, 2021, 05:27 PM   #2
rupej
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How often is it going to be fired? My preference would be to use a SSR as suggested, and put another diode in parallel with the solenoid to protect that too. They also make DIN connectors with surge suppression built-in.

Surely you can squeeze in a terminal-block sized SSR somewhere? Add another DIN rail to the side of the enclosure or behind the door if need be...
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Old July 30th, 2021, 05:48 PM   #3
jetwax
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Thanks Rupej,

I expect it will be fired once a minute.

Any comments on the inrush current of a 30W coil? (DC).?

Any insight on the DC-13SQ raiting, and how to interpret?

Thanks for your response!
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Old July 30th, 2021, 08:26 PM   #4
danw
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Gotta check with the solenoid vendor.

ASCO is tricky, a global statement the DC draws no inrush current, and then in a separate section, explaining that the Next Generation coil (now 15 years old) does draw inrush current:






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Old July 30th, 2021, 09:22 PM   #5
JeremyM
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I tend to isolate all but the smallest inductive loads with interposing relays.
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Old July 31st, 2021, 05:41 AM   #6
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Just point out the cost and inconvenience of replacing an output card compared to a small plug in SSR, they will soon change their minds
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Old July 31st, 2021, 05:46 AM   #7
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The problem with DC coils is on disconnection, the energy is released with a reverse polarity voltage peak to that of the supply.

The peak suppressor diode is better to place it on the same coil, this way prevents it to circulate through the wiring and induce noise on other nearby cables.
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Old July 31st, 2021, 09:14 AM   #8
L33er
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ASCO statement is correct. What they are saying their 'Next Generation' coil included a Peak-Hold circuitry that peak higher to open the valve and holding it open at lower amp. Similar to fuel injectors in your car that use Peak-Hold driver.
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Old July 31st, 2021, 02:06 PM   #9
JesperMP
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I agree with Ife.
It is not the inrush current that is the problem.
It is the inductive load due to it being a coil that is the problem. This causes much more energy to be released in the switching device before the coil when it de-activates the coil.
One can mitigate this with surge suppressors, but better is to use an SSR or a contactor to switch the coil.
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Old July 31st, 2021, 02:28 PM   #10
pturmel
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Using a contactor or SSR doesn't solve the de-activation spike, it just moves it to another point.


I'd use the output card directly in this case, as long as there aren't other high-load consumers, too. (The card has per-point specs and per-group specs.)


No matter what you use, the coil needs spike suppression somewhere.
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Old August 1st, 2021, 01:56 PM   #11
rupej
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Quote:
Originally Posted by pturmel View Post
Using a contactor or SSR doesn't solve the de-activation spike, it just moves it to another point.
For sure- but it could be moved to a much more robust and cheap+easy to replace point. That's the argument for adding an additional SSR.
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Old August 1st, 2021, 05:15 PM   #12
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Quote:
Originally Posted by pturmel View Post
(The card has per-point specs and per-group specs.)

True.

Current per point max:
2 A @ 30 C (86 F) linear derating
1 A @ 60 C (140 F) linear derating

Current per module max:
8 A @ 30 C (86 F) linear derating
4 A @ 60 C (140 F) linear derating
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Old August 2nd, 2021, 01:48 PM   #13
Jim3846
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If your worried about blowing an IO point, why not just change the terminal strip TB to a fused terminal?

Select a fuse that will blow before the IO point on card. Its only a few cents more but will save you a card replacement+ time.
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Old August 2nd, 2021, 02:22 PM   #14
Tom Jenkins
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You can buy a lot of SSR's for the cost of an output card. Not to mention you can replace a plug-in-style relay without stopping the system. And if you pick the right style panel space isn't much of a factor.
https://www.phoenixcontact.com/onlin...7-80ac975b67c5
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Old August 3rd, 2021, 11:28 AM   #15
jetwax
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Thanks all for your replies!

The consensus seems to be to use a SSR as an interposing relay. And a Diode.
The problem with this approach is one also needs to add the diode between ground and the output, connected 'backwards' (on the output side of the SSR) to protect against the de energization current that occurs when turning off the solenoid with inductance causes high voltages in an attempt to limit the dI/dT
Does any one know of a SSR with the diode integrated? From my looking through phoenix, AB, and weidmuller I don't see one. Maybe I'm not looking closely enough.

Thanks Tom Jenkins for your suggestion, but the ones I see on phoenix contact that have the free wheeling diode are still only rated for 1A inductive, DC 13. The similar, but less than, the 1756-OB16D card...

I get the idea of it being simpler to just change a relay vs. changing the card, but the cost of the card vs. the cost of the relay is very small in comparison to the cost of a failure in the first place. I'd rather it not fail, and accomplish this with the fewest components.

Last edited by jetwax; August 3rd, 2021 at 11:39 AM.
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