I have a MAC 45A-AA1-DACC-1BA solenoid valve rated for continous duty at 24VAC 7.7VA holding / 10.9VA inrush. Measured current at rated AC voltage is about 340mA and coil resistance is 24.3ohms. I want to control the valve using a relay. On the oscilloscope large spikes were seen when the relay contacts open. Adding an RC snubber in parallel with the solenoid reduced the observed spikes on the oscilloscope. I then tried a transparent relay to see if there was a visual reduction however I unfortunately still noticed sparking when the contacts open. After trying various combinations of RC values, a bidirectional TVS by itself, TVS together with the RC snubber, and the TVS on the relay contacts, sparks on opening still occured.

I read mixed reviews about running AC relays/solenoids with DC. Some on this forum have referenced this article where it is discouraged. Other articles such as this seem to hold more favorable views:

Will there be any benefits associated with the DC operation? Yes, you can expect coil life to be extended three-fold due" to the [+ or -]70% power reduction. In addition, you'll have clean, solid make-and- break operation with no contact chatter, hum, or buzz.

I tried running my solenoid valve and at 9V DC and its drawing 335mA which decreases to about 300mA after one hour. The valve body temperature after one hour is around 40c compared to about 60c when run at rated AC. I haven't observed any magnetic issues with the coil sticking. Finally using a simple diode I no longer visually see any sparking when experimenting with a transparent relay.

For hobbiest grade use what are your opinions of running AC solenoid valves on DC (assuming one already has an AC valve on hand)? It looks like it can be beneficial in terms of power dissipation/heat and controling relay sparking.

 

If your solenoid operates without problem on DC, with reduced temperature rise and contact sparking, then I see no other issues with doing that.

What is the source for the 9Vdc?

 

If your solenoid operates without problem on DC, with reduced temperature rise and contact sparking, then I see no other issues with doing that.

What is the source for the 9Vdc?

Thank you. I am currently using a bench power supply to test though I plan to switch to a 12V regulated adapter and drop the voltage to 9V.

 

Thank you. I am currently using a bench power supply to test though I plan to switch to a 12V regulated adapter and drop the voltage to 9V.

How will you drop the voltage?
Are you concerned about efficiency?

 

How will you drop the voltage?
Are you concerned about efficiency?

When I was first running it on AC I had an AC adapter rated at 24V 750mA so the output was higher with less load so I used a 15ohm 25w resistor in series. I was going to use a buck converter module with this DC setup I had laying around however your post had me think ... why don't I just use the resistor as I am not really worried too much about efficiency. I tried the same 15ohm resistor and it drops 4v with 7.72v accross the solenoid and 269mA current. Still working well! Thank you! I will use a series resistor to drop the voltage.

 

If everything works as expected, I think DC is generally better.

If You expect to have long Wire-Runs,
use a Current-Regulator Circuit instead of a Voltage-Regulator-Circuit,
this will keep the Current stable regardless of the length of Wire-Run.

Also, use a MOSFET instead of a Relay, and use a Freewheeling-Diode on every Solenoid,
a 100nF Ceramic-Capacitor on every Solenoid would also help avoid strange behaviors.

If You like the idea of eliminating the Relays, let us know.
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In the industrial (CNC) machines I have put together over time, I have made it a practice to use DC for solenoids and relays etc.
When I first came to N.A. , this was not a common practice, and I found the result was that most major manuf. had a large stock of spare coils in the maintenance dept.
Also, I experienced some push back from some MTB's "WE have always done it this way" !
I pointed out to one Manuf. Co. that the German made M/C they were using never had a solenoid failure, the M/C used 24VDC solenoids!!

 

I tried the same 15ohm resistor and it drops 4v with 7.72v accross the solenoid and 269mA current.

The resistor only dissipates about 1W, so a 2W resistor would work with 50% margin.

 

use a Current-Regulator Circuit instead of a Voltage-Regulator-Circuit,
this will keep the Current stable regardless of the length of Wire-Run.

True.
But it will also increase the solenoid dissipation as it warms up and the coil resistance increases , which normally decreases the current some.

 

You did the right thing to lower the supply voltage when operating at DC. Without the benefit of the AC reactance, at 24 VDC the coil would have drawn excessive current.

The only drawback I can think of is the selenoid will take a few tens of milliseconds longer to open, due to the circulating currents in the freewheeling diode. But there are ways to ameliorate that, if required.

 

If anything - I'd say find the lowest DC voltage that will pull your solenoid in. Whatever that voltage is multiply it by 1.33 and use that as your standard low DC voltage source. Doesn't have to be exact, just use that as a minimum. If you want greater confidence then whatever that voltage is - multiply it by 1.5. That way you can be confident the solenoid will operate under normal conditions.

 

ameliorate

New word for me. Ameliorate is to make something bad - better. or Bad - not so bad.
Thanks for the vocabulary expansion. My first guess was that you were not from the US. Texas, last time I looked, is still part of the US.

 

The first time I encountered this word, was during my first day at a job, where the new employees received the job’s regulations and guidelines.
The HR guy said exactly this phrase: “If we find that you have used mood-ameliorating substances, your employment will be terminated.”
Corporate speak for “use drugs and you’ll be fired.”
40+ years ago and seems like yesterday.

 

Why does the relay contact sparking (on opening) go away so easily with a diode on DC vs all the attempts I made (listed in first post) when operating on AC failed?

 

Why does the relay contact sparking (on opening) go away so easily with a diode on DC vs all the attempts I made (listed in first post) when operating on AC failed?

One diode is easily connected to clamp the inductive transient to just one diode-drop above a single polarity DC supply voltage, whereas it's more difficult to do that with an AC voltage which goes both plus and minus.

If the TVS diode were selected to clamp at just above the peak ±AC voltage then it should work to suppress the transient and minimize sparking.
An RC circuit should also work, but the R and C values need to be selected to critically damp the value of solenoid inductance (so you need to know that inductance value).

Of course the peak voltage for the DC is 12V here, whereas the peak AC voltage is 24*1.4 = 33.6V so that will create more AC sparking, even with both clamped to their respective peak voltages.

 

My question is based on my guess that the relay contacts are operating within their specifications. The reality is that relay contacts switching AC power will spark. That is just how it works. And if the contacts are applied to operate within their design limits then it is OK for that to happen.
Relays have a predicted life span number of operations at their rated load, and they are not generally claimed to last forever, although it sometimes seems that they do. So a bit of spark when relay contacts open is not a problem, except in locations where there are vapors present that may be ignited by that spark.
Relays switching DC loads should be rated for the DC operation, which is often much less than for their AC ratings. THAT is because DC arcs have less tendency to self extinguish.

 

As a matter of fact, a little sparkling is healthy for contacts, as it burns out any contamination films which can and will deposit on their surfaces.
Low currents which don’t cause sparking are called dry currents and eventually cause the relay to eventually stop conducting even when the contacts are closed.
This is not an imaginary scenario. I have personally witnessed this effect.

 

Why does the relay contact sparking (on opening) go away so easily with a diode on DC vs all the attempts I made (listed in first post) when operating on AC failed?

contacts switching AC power will spark.

Was going to say that. Sparking upon opening is normal.

a bit of spark when relay contacts open is not a problem

Was going to say that.

As a matter of fact, a little sparkling is healthy for contacts, as it burns out any contamination films which can and will deposit on their surfaces.
Low currents which don’t cause sparking are called dry currents and eventually cause the relay to eventually stop conducting even when the contacts are closed.
This is not an imaginary scenario. I have personally witnessed this effect.

Again, was going to say that. Yes, using a high power capacity relay to switch very low voltage signals WILL render a relay contact useless much faster than if you used the high power the relay was designed to handle. If you are using a relay to close low voltages you want to use a relay whose contacts are rated for that load.

 

seems like yesterday.

My mind thinks like that. Sadly, my body doesn't.

 

Why does the relay contact sparking (on opening) go away so easily with a diode on DC vs all the attempts I made (listed in first post) when operating on AC failed?

Not sure of your application in this instance. A diode generally goes across the coil and suppresses the Back EMF (BEMF).

I have a MAC 45A-AA1-DACC-1BA solenoid valve rated for continous duty at 24VAC 7.7VA holding / 10.9VA inrush.

A diode across the solenoid will not work. You're shorting half of the AC sine wave, which will destroy your power supply. A snubber circuit is advised.

I want to control the valve using a relay.

Since the solenoid draws as much current as it does - you should expect arcing. Each time the relay opens the arc will be different, based on the exact moment the contacts break. IF the AC voltage is crossing zero when the contacts open you won't see any spark. IF the AC voltage is peak (±) it will arc. A snubber on the solenoid will minimize that. Even still, your relay coil should, if DC powered, have a diode to block the BEMF. IF AC powered, a smaller snubber should be used to suppress BEMF on the relay coil as well as on the solenoid.