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Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 1:17 PM

Trying not to dig up my entire yard:

My 24VAC rainbird sprinkler system keeps blowing my controllers 1 amp fuse. I know that this is caused by one zone solenoid valve (buried seperately somewhere in my yard) that has gone bad. I know from experience that if I turn off the water supply, that I would be able to hear/locate a good valve being cycled as my helper intermittently breaks valve power at the controller. I also know that I can sometimes force a stuck valve open by using a stronger power supply, which I have in the form of a DC supply. So, my plan is to force this valve back to a working condition, long enough to hear/locate and replace it.

My simple question: If my 24 VAC solenoid draws .7 amps, what will it draw if I use my stronger 12 volt DC supply. I'll use that value to help determine how much low duty cycle DC power to use to "kick start". Is this a matter of calculating the resistive component of the impedance?

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Guru

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#1

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 2:10 PM

Your solenoid has a resistance of approximately 35 ohms. At 12 volts it will draw about 1.4 amps. If it is blowing a 1 amp fuse in your controller, it is likely shorted out. Applying 12 volts to it will likely cause whatever circuit protection your 12 volt power supply has (fuse, circuit breaker) to trip as well. You, my friend are probably looking at a shovel instead of a meter or power supply to troubleshoot this problem. The only other option is to rent an underground cable locator. One of these would show you exactly where the wire is routed. Remember, the problem could also be a break/short in the cable. Good luck.

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#2
In reply to #1

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 2:20 PM

I think you missed, current will be 0.35 Amps at 12volts thats is half of 0.70 Amps at 24 Volts.

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#3
In reply to #2

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 2:56 PM

Wow, I must be having a bad math day!

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#5
In reply to #2

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 4:03 PM

Thanks for the response guys - I'm affraid that you are right about the short which means that any math based on the assumption of a good solenoid would be wrong. But, if it was a good solenoid, wouldn't the resistance "seen" by the DC supply be lower than 35 ohms because I would not "see" the inductive reactance that comes from using 60 Hz power?

Anyway, it just ocurred to me that I should just measure the resistance with my DMM and then do the math...... and then get my shovel out if all else fails ...... my neighbor (a sprinkler guy) with his detector was unable to follow the cable.

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#4

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 2:58 PM

Your 12DC supply may not be able to effectively do anything, but then again it just maybe fine. I'm not trying to just be confusing, I just cannot give you any usable numbers for the following reason. Your 24VAC solenoids have a reactive impedance that is the vector sum of your DC resistance (R) and your phasor impedance from the inductance (jωL) of the solenoid. AKA XL=R+jωL Now ω is the radian frequency of your AC power that will likely be 2*pi*(50 or 60) Hz depending on where you are. Regardless of your power's frequency the magnitude of your impedance is 24VAC/0.7A(RMS?) ≈ 34 ohms. If you measure the DC resistance of a spare solenoid with a Volt Ohm Meter you will instead be measuring only the value of R in the above equation. As you may guess, this should be less than 34 ohms. This is the impedance that you need to know to see if a 12VDC supply can drive enough current to activate the solenoid at rated current. If you are lucky enough to have a 17 ohm DC resistance in your solenoid then your 12VDC supply will drive precisely the rated current for each solenoid connected. I suspect that you'll actually find that the DC resistance though will measure closer to 30 ohms than 17 ohms. This will mean that less current will flow.

But all may not be lost. You're not trying to permanently replace the 24VAC supply with this 12VDC supply. You are just trying to diagnose a problem with this beefier wattage supply. Give a try, but keep in mind that it just might not work.

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#6
In reply to #4

Re: Testing a 24 VAC Solenoid Using a DC Supply

07/21/2011 4:10 PM

Thanks RedFred - I didn't see your response until after my last post. Makes sense to me.

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