Why 24vac won't actuate a solenoid

[KrayzKajun]

Originally Posted by Kiril

Too many numbers .... brain is boiling .... can't reme

Ughhhhh someone wipe the drool from my mouth.... Brain is overloaded from all the numbers
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[irrig8r]

Originally Posted by 1idejim

Quite a few times i see a post where the OP states that he has 24 volts but the solenoid won't work.

I always ask the same few questions, what is the resistance of the wire path and solenoid and what short to earth is.

I took a call from a friend in Texas this week, 24 vac, but he also had 180 ohms resistance from the controller.

Most troubleshooters will begin fooling with the splices because that's what we're taught.

Before I look at anything i will take a short to earth measurement to test the wire insulation. If the insulation is good i look at the splices, if the insulation is bad i grab the A-frame.

Most solenoids require 300mA - 500mA inrush to lift the plunger. That's 7 - 12vA.

Using Ohms law you can see why the valve won't open.

24vac/180ohms=0.133333mA.

There are 24vA in one Amp. 1A/24vac = 0.041667mA or 1vA.

0.133333/0.041667 = 3vA, less than half the current required to magnatize the coil and lift the plunger.

Electrical troubleshooting can be made a lot easier by doing a little more testing and a lot less guessing.

Hope this helps someone.
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I've never measured resistance "from the controller" itself... or do you mean he measured the resistance of the solenoid via the wire path to/ from the controller?

A reading of 180 Ω would make me suspect a partial connection or badly corroded wire connector (as you mentioned, checking the splices).

You know what they say about "when you hear hoofbeats, think of horses not zebras"?

Anyway, though it makes sense to suspect the insulation, please explain how you test the "short to earth" because it's outside of my limited knowledge and experience.

[Wet_Boots]

It is never a given that any part of the controller output is grounded. That can make resistance-to-ground measurements superfluous, unless all conductors are tested.

As noted, "I got 24 volts" says nothing about the actual circuit path quality.

Even resistance measurements may turn up nothing. Old-time mechanical controllers gave zero info about electrical faults. This is where the clamparound ammeter came in, as it could show a higher-than-usual current draw on a solenoid that refused to go to shorted status while the repairman was watching. They had to have voltage applied to show the fault.

The yellow clamparound ammeter was a Sperry model that had an extra range of 1.5 amps full-scale, good enough for the higher current draws of old-time solenoids.

[Sprinkus]

Originally Posted by 1idejim

I took a call from a friend in Texas this week, 24 vac, but he also had 180 ohms resistance from the controller.

It wasn't me but I have been asked to troubleshoot a 100+ zone two-wire system installed in phases by several of the crappiest installers around.
One part of me says:
The other part says:
And of course the non-part of me says:

[1idejim]

Originally Posted by irrig8r

I've never measured resistance "from the controller" itself... or do you mean he measured the resistance of the solenoid via the wire path to/ from the controller?

If i had meant to include the controller i would have said "through the controller", i could have said "at the controller" which is a location, but didn't mean that. Instead i said "from the controller" which implies direction.
In hindsight i could have chosen better words but i understood my Texas friend very well.

A reading of 180 would make me suspect a partial connection or badly corroded wire connector (as you mentioned, checking the splices).

As you should, that's what the books say and the teachers teach.

You know what they say about "when you hear hoofbeats, think of horses not zebras"?

Anyway, though it makes sense to suspect the insulation, please explain how you test the "short to earth"

Adjust a DMM to 2,000kohms and insert the black probe into the earth, touch the red probe to the conductor under test. >600,000ohms look at the splice. <600,000 look at the buried wire path first.

because it's outside of my limited knowledge and experience.

I have no idea why that would be, you spend as much or more time with LV lighting and should be more familiar with a DMM than most of the irrigation forum members.

2wire should be a cinch for you. Voltage drop, wattage and amp calc formulas should also be second nature by now.

All that i am trying to accomplish is to bring to light a few simple tools that have been in use for decades that may help make troubleshooting easier.
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[1idejim]

Originally Posted by Sprinkus

It wasn't me but I have been asked to troubleshoot a 100+ zone two-wire system installed in phases by several of the crappiest installers around.
One part of me says:
The other part says:
And of course the non-part of me says:

They picked the best of the best IMHO, you are my hero Sprink.

You get all the good jobs :
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[Wet_Boots]

repeat after me - "Time and Material"