irrigation valve on DC instead of AC

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gskinner
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Posted by gskinner: Sat. Feb 28, 2009 - 04:43 AM
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I have a large irrigation system already wired, and now need to add a few zones (adding more wires is $$). My first idea is to power the valves using DC. Each valve could have a diode in series with it, and a 2nd valve next to it could have an opposite polarity coil.
Power the pair one polarity to get valve 1. Opposite polarity to get valve 2.
I measured the current of the valve solenoid at 24VAC at .2 amp. At DC the valve trips around 5V at the same current.
To accommodate long wires, I might use a current source. will also add a flyback diode across the coil.

Anyone see any issues?
wire corrosion due to DC?
solenoid turn into permanent magnet over time?
long term failure of solenoid?

thanks in advance.

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Learning and Information, General Electronics
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Kartman
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Posted by Kartman: Sat. Feb 28, 2009 - 11:07 AM
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The solenoid most likely won't like it. With DC the solenoid will saturate and start pulling too much current, thus it will overheat and melt. If you want to use DC, get DC solenoids - they'll most likely be larger.

If you wanted to get real smart - add a small AVR at each solenoid with a little bit of circuitry. You could impress high frequency tones on top of the AC and have the AVR decode the sequence (like X10) and switch each solonoid individually on the same run of wire. Of course, the size of the wire will limit the max number of solenoid on a run.

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Kleinstein
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Posted by Kleinstein: Sat. Feb 28, 2009 - 12:30 PM
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It should be possible to run the valves with DC.
The only real problem could be the remaining magnetization that could cause the valve to get stuck when turning it off. However this Problem will hardly get worse over time (except that the springs insige the valves may get old). To estimate how easy the valve switches off, slowly reduce the current to seen at which current its turning off. If this turn off current is very small (e.g. < 20 mA) there could be a problem. It then actually help not to have a freweeling diode, but a capacitor in paralel to the coil.

The AC valves have the nice feature that they take more current when open and they actually need to perform work. Once they are closed they take less current.
So it may be neccessary to use two valtages: e.g. 12 V to turn it on and 5 V later on just to hold. You allready noticed that the voltage is ususally lower with DC. At the same current there is even a little less heating with DC.

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stevech
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Posted by stevech: Sat. Feb 28, 2009 - 05:38 PM
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I have 1 in. valves that operate with a DC solenoid toggle actuator. A brief 9VDC pulse turns the valve on or off. You'd think that the valves might miss a pulse and get out of phase (on/off) but it's rare. The reason for this is that the controllers are 9VDC battery powered. No AC mains. We have many of these.

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gskinner
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Posted by gskinner: Sat. Feb 28, 2009 - 10:34 PM
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I am using Irritrol brand solenoids
Turns on around 4V 150 mA (at 7V it draws 260mA) [~ 27 ohms]
Turns off around .8V 30 mA

Kleinstein - what does a capacitor across the coil accomplish?

I just ran the coil overnight at 5V 200mA, it is barely warm, and seems to cycle OK.

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Kleinstein
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Posted by Kleinstein: Sat. Feb 28, 2009 - 11:27 PM
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A capacitor (e.g. 100nF) in parallel to the coil makes this a resonant circuit. So on turn off this limits the voltage and produces some oscillations when turning off. These oscillations can help to demagnetize the core of the magnet.

It looks like the valves works with DC as well. As you allready tested, it is not even be necessary to reduce the current for just hilding the state. Depending on the supply it may be still usefull to save energy.

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