# Electrical – Circuit to control 24VAC OR 24VDC solenoids

ach-bridgesolenoidsolenoid-valve

Is there a circuit out there that could control both 24VDC or 24VAC solenoids from a 24VDC power supply? I am working on a sprinkler control board that I would like to be able to use both on. The first idea that came to mind was using an H-bridge to either run the DC valve, or chop up the DC current into a square wave pattern to run the AC solenoid. Any better ideas?

Your user profile gives no clue as to your level of electrical knowledge (and that's why you should put some info in there) so this may be of no use to you. Some ideas to consider:

• AC solenoids will have low inductance when first energised. This means that initial current is quite high and pulls the solenoid actuator into the coil.
• With the solenoid actuator in the coil the inductance will increase resulting in a lower current and lower operating temperature while running.

If you measure the AC holding current on the solenoid and measure its DC resistance using an ohmmeter you can calculate the safe DC voltage that can be applied without burning out the coil. $$\ V_{DC} = I_{AC}R_{COIL} \$$. You will then need to test this using a variable voltage PSU or by adding series resistors to get the required voltage.

Your next problem is to see if the solenoid will pull in with the DC voltage. If not you'll need to give it a higher voltage pulse to pull in.

simulate this circuit – Schematic created using CircuitLab

Figure 1. C1 charges up to +24 V while RLY1 is open circuit. On closing RLY1 the solenoid gets a 24 V pulse and then the voltage drops to the level set by the ratio of R1 and RL.

There is a risk that running the solenoid on DC will permanently magnetise something. I've never tried it so I don't know but someone will point it out in the comments.