Relay outputs for solenoid valves

Biker

Member
Join Date
Jul 2003
Posts
2
Is it advisable to use a plc with relay outputs to energize solenoid valves. The relay is rated a 1.2A make and 2A continuous. The solenoid is a 24vdc 1.3A.
Thanks,
Biker
 
No, in a few day's, weeks (dep. on the brand) you will fry the contacts.
A safer way (only way) to do something like this is to use an external relay witch alows more current.
Perhaps a solid state relay is needed. Only when you switch a lot.

DON'T forget the diode over the solenoid. A solinoid like this gives a big surge when you switch of the solinoid.

Also keep in mind that when the normal current is 1.3 A, the initial curent can eassely reach 10 times higher.

Sorry for my bad englisch. It's not my mother langauge.
 
JvdV
Don't confuse the characteristics of AC and DC solenoids.

AC Solenoids - High inrush current and lower holding current
DC Solenoids - Current builds up slowly to continuous level, no inrush

Biker
The solenoid is within the specifications of the relay contact rating due to the slowly rising current when the contact is closed. However it is near the limit. I would not recommend this relay if the solenoid is turned on and off frequently. Use a 2 amp solid state output or an interposing solid state relay or a mechanical relay with a higher current rating.

Whatever way you go, be sure to put a diode across the solenoid to provide a path for the inductive current in the solenoid when it is switched off. Without the diode, a very high voltage will develop across the opening switch, which will quickly destroy it. The diode should be rated for the solenoid operating voltage (use at least 200 volts) and the solenoid operating current (1.5 amp). The diode cathode (negative side with bar) must be connected to the positive terminal of the solenoid.
 
Thank you for your comment on my post VIC.

For sure there are two kinds of voltages.
We had some big problem with 24 Vdc solenoids. The power was supplied from a short cicuit protected power suply. When the power suply "saw" a short cicuit the voltage droped to 1,5 Vdc.
The problem where the solenoids. When we switch them in one by one with a small delay there wher no problems. When we switched them all (8 pieces) at the same time the power suply switch off. Only for a second, but that was more then enough for the emergency stop relay to drop OFF. The prower suply came back to 24 Vdc within milli seconds. But the whole system was in stop and you had to start over again.

The solenoids (and emergency stop relay and some led's) should pull around 11 Amps. nominale. The power supply was 20 A. More then enough you should think. We had to alter the whole programm for that part to make them switch in sequens.

Biker, may by another think to keep in mind. A separate relay/solinoid is easier to change when something breaks down. When you fry a relay contact in your PLC unit it cost you a lot more time to change it. And in some cases you have to have a lot of experience to get the job done. Especially when you have the SMD electronics.

But one thing is for sure VIC and I agree on one part. The induction voltage. Don't underastimate these. You can very easy damage stuff these voltages.
I usually put a diode over the solenoid. Normally a 1N4001 it can be used up to 1000 V when I'm not mistaken. And even these get blown sometimes. Normally not under normal circumstances but they can be damaged.
 
If I remember correctly, 1N4001 was 1A/60V diode,
1N4002 was also 1A but for higher voltage etc.
The one for 1A and 1000V was 1N4007.
If you need 3A diodes use the 5400 series
(1N5400 is good for 3A/60V, while 1N5407 is
a 1000V model).

panic mode
 
You remember correctly Panic, the 1N400x series are all 1A diodes, and the last number indicates the max. voltage rating...

1N4001 - 50V
1N4002 - 100V
1N4003 - 200V
1N4004 - 400V
1N4005 - 600V
1N4006 - 800V
1N4007 - 1000V

beerchug

-Eric
 
Biker
On a few projects I worked on it was specified that we use interposing relays and break both sides of the solenoid. Served the same purpose as using a diode but I always used interposing relays whether breaking both sides or not. Using the interposing relay made troubleshooting easier for the repairmen and $aved on output cards.
Note ;
I used Allen-Bradley or Potter & Brumfield square base relays with arc suppression and LED indication.
Roger
 
Being new and at the risk of sounding stupid, aren’t a lot of things controlled by a PLC solenoids/coils? If so, do you use the relay output of the PLC to energize the coil of a ice cube relay and then use these contacts to control the other solenoid or am I missing something?

Thanks,
Robert
 
Being new and at the risk of sounding stupid
Doesn't sound stupid to me.

aren’t a lot of things controlled by a PLC solenoids/coils?
Yes, this specific discussion is refering to a type of PLC output card that contains a small relay for each output point. As with any devices, the pieces of the system you wire together have to be compatable. In this case, the current rating of the contact contained within the output card is very close to the current draw of the device being controlled.

For the sake of argument say that the current draw is twice the rating of the relay in the output card. Obviously, you will fry the card (or at least that point) if you connect the devices directly. So, you use the plc output to "drive" an external relay (sometimes called an interposing relay). The relay chosen will have a coil compatable with the output point on the plc and a contact rating compatable with the device you are ultimately trying to control.

This is also necessary when you need to control a device with a different voltage than you can get an output card for. A real common example is starting a 3-phase 480 VAC motor. You won't find a plc output card that can directly switch 480 VAC so you use a 24 VDC or 120 VAC output to control a 3 phase "motor starter" which is just a relay with three parallel contacts designed to handle the switching on (and more importantly switching off) of power to a motor.

Hope this helps. I think you have the idea though
 
Is a diode recommended for 24v IEC starters too? When would i use a snubber? or is that the same animal.
 
SLaubach

Yes, a diode should be used on the coil of a 24 VDC starter also. It is a good idea to use a diode on any DC inductive load. The only disadvantage to using a diode is that it lengthens the drop out time by 2-3 times. If the drop out time is critical, use another type of surge suppressor.

A snubber is a resistor and capacitor connected in series. They are used to reduce the rate of change of voltage across a device and are commonly found across AC coils. The amount of inductive energy stored in an AC coil is much lower than in a DC coil and a snubber does a good job of reducing the voltage spike when the coil is switched off. They will somewhat reduce the voltage spike on a small DC coil such as a relay, but have less effect on larger coils such as solenoids and starters.
 
I'm not trying to cause trouble, but...

I have to respectfully disagree with Vic's statement about DC coils and inrush current. DC coils DO have an inrush current value, and for the same reason as AC coils.
I don't profess to be a magnetics expert but here goes. The coil is storing energy in the form of a magnetic field. When it is first energized the current doesn't need to travel through the whole coil before it establishes the magnetic field. As soon as it hits the coils, the current starts forming the field. While the rate of field formation is self-limiting, this rate definitely allows currents higher than the DC resistance of the coil would indicate.
Short story is don't discount the effects of inrush currents, especially in big DC coils which seem to have a larger inductance to resistance ratio. The inrush current to holding current ratio is higher with these coils.

Keith
 
Keith

It is not my intention to start an argument but I stand behind my statement that there is no inrush current in a DC coil.

You stated
When it is first energized the current doesn't need to travel through the whole coil before it establishes the magnetic field.

This would indicate that the current can be different in different parts of a single conductor. This is not correct.

While the rate of field formation is self-limiting, this rate definitely allows currents higher than the DC resistance of the coil would indicate.

In fact, just the opposite is true.

The following is summarized from a Physics Textbook

When a coil is connected to a DC source, a current starts to flow. This current causes a magnetic field to begin to build up. This changing magnetic field induces an emf (voltage) in the same coil whose current causes the field in the first place. By Lenz's law, the self-induced emf is such as to oppose the change in flux that is responsible for it. The source must therefore push electrons through the coil against the self-induced emf due to the increasing magnetic field. When the current in the coil reaches its final value, there is no more self-induced emf.

The text goes on using mathematics to show how the current reaches its ultimate value in a gradual manner. If you need more proof, look at any college physics or circuit analysis textbook.

Vic
 

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