run AC solenoid valve on DC?

check to see if the "rod" (the part the electromagnet pulls) is becoming magnetized. try reversing polarity and see what happens.

This happens with the 3-port valves commonly used in the UK on heating systems. In one mode, the coil is energised with DC (to lock the armature). When the voltage is removed, the valve is supposed to return under spring tension, but normally it stays locked in place due to residual magnetism induced. The way this is solved is to provide a very tiny AC current through the coil; a resistor is used to allow a milliamp or two of AC to flow in the coil whenever it isn't energised, and this instantly clears the residual magnetism and allows the armature to move under spring tension.

When the slug of the solenoid is out of the AC coil (the position its in when the power is off), the AC coil has no iron core and thus has low inductance. So when power is first applied, the only impedance is the resistance of the coil. So, a higher than normal current flows through the coil.

This higher current generates enough 'pull' to pull in the slug. But as soon as that happens, the inductance goes up and the current goes down. The lower current will generate enough 'pull' to *hold* the slug in place. And that is the current that the windings are designed to handle for long periods of time.

By using a lower DC voltage, you are getting the same current as the windings are normally designed to handle to *hold* the slug in place. But this isn't enough current to *pickup* the slug.

Some large DC contactors/relays have a resistor in series with the coil, and a normally closed contact in parallel with the resistor. So when power is first applied, the resistor is shorted by the contacts and a large current flows through the coil. Once the coil 'picks-up' the armature of the relay, the NC contact opens and inserts the resistor in series with the coil. This reduces the current to a lower value that the winding can take.

AC coils get that same effect for 'free' by using the variation in the inductance of the coil as the slug moves into the coil.

Bottom line, solenoids designed specifically for AC, often cannot work on DC.

daestrom

I have often operated ac solenoids from dc photovoltaic power systems.

My solution is to measure the ac current, then add a series resistance in the DC circuit to reduce the current to the AC value. Then, place an electrolytic capacitor across the resistor, usually 250 MF at the DC voltage or higher. The capacitor produces an initial surge into the coil until the capacitor charges up. 24-VAC solenoids work well on 12 volts DC with a resistor about equal to the DC resistance of the coil. My experience is with 1/4" solenoids, you may need a larger capacitor.

Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@kaszeta-removethis.org

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What about using a PTC resistor? When turned on, it allows a large current through which will let the solenoid activate, then as the PTC heats up, current is reduced. If the solenoid is decativated and activated again before PTC is cooled completely, it might not activate however.

Now, why didn't I think of that!!! Of course, need to be sure the surge is enough to pick it up, but even if it doesn't, it won't burnout because the resistor is always in the circuit.

daestrom

That's called a "Blond high five!" :-)

What about using a PTC resistor? When turned on, it allows a large current through which will let the solenoid activate, then as the PTC heats up, current is reduced. If the solenoid is decativated and activated again before PTC is cooled completely, it might not activate however.

This is the very thing that makes the BUzzzzzzz sound when you turn on your TV.

Ken

replying to Bill Kaszeta / Photovoltaic Resources, rickyrockrat wrote: Hello, any need for the diode in parallel with the coil for the back current when switching off when operating with DC?

replying to Bill Kaszeta / Photovoltaic Resources, rickyrockrat wrote: Hello, any need for the diode in parallel with the coil for the back current when switching off when operating with DC?

I always do, saves relay points and silicon drivers. They are a nickel, why not? You can get a pretty good idea of what voltage to use with an ohm meter. DC is easy.