Lawn Sprinkler Solenoid Valve Question

[bobledoux]

My lawn sprinkler controller switches on and off 24 volt AC solenoid valves. I want to know what type of circuit would be appropriate to perform this function.

I broke open my controller and the switch units consist of TO-92 transistor-type units marked "9786." I find no cross reference so its probably a house number.

Has anyone a schematic for such a unit? I'm not interested in the microcontroller guts, just the hardware that's driven by logic level voltages to turn the valves on and off.

 

[ronsimpson]

To switch AC you need a triac.
Something like this below.

 

[Sceadwian]

Ron, the solenoid could be driven from pulsed DC. Easily created with a 555 timer and some transistor bypass for the higher voltage.

 

[RCinFLA]

Most sprinkler controllers use 24 vac to run solenoid, using a low power triac to switch them.

I am assuming you would rather repair your controller then invent a new one.

 

[crutschow]

Ron, the solenoid could be driven from pulsed DC. Easily created with a 555 timer and some transistor bypass for the higher voltage.

To use DC on the solenoid valve you will need to measure the current of the value on AC, measure the solenoid resistance, and then adjust the DC voltage (either with a linear regulator or PWM) to give the same current.

 

[Sceadwian]

pulsed DC

Pulsed DC from a square wave can easily substitute for the low voltage AC this is describing, with only a small correction factor for RMS of a square wave vs a sine wave. The harmonics of a typical pulsed DC source are gobbled up in the core/coil itself and are irrelevant.

 

[ronsimpson]

Ron, the solenoid could be driven from pulsed DC. Easily created with a 555 timer and some transistor bypass for the higher voltage.

Could be but its not.
Sprinkler controllers use 24 vac to run solenoid, using a low power triac to switch them. Some times relays.

 

[Sceadwian]

You're forgetting the 5 to 1 or 10 to one transformer that the use of the 24V triac requires from mains.

 

[bobledoux]

Thanks for the data sheet for the MAC97 Triac.

I want to build a simple custom controller to replace my current Toro unit. My well can only supply about 45 minutes of irrigation water at a time. So I need to stage sessions during the day, separated by well recovery periods.

I'll program a PIC with start times, duration and zones that meet my requirements. My Toro controller won't do it.

The valves pull less than 0.2amps in operation. The most simple technique is to use a reed relay rated for 0.5 amps and operating on 5 volts to switch the valves. Any of the small "power" relays pull too much current to be fed directly from a PIC pin. If the reed relay works I can eliminate switching transistors between the PIC and the power relays.

I'm also considering using the AC line as the oscillator for my PIC. I discussed this in a separate thread under Microcontrollers.

I hadn't given any thought to using a square wave power source. I'll think about it.

 

[ronsimpson]

A while I did a lawn controller project. In the process I purchased many products to look at. They all had a external transformer or an internal step down transformer.

 

[crutschow]

Pulsed DC from a square wave can easily substitute for the low voltage AC this is describing, with only a small correction factor for RMS of a square wave vs a sine wave. The harmonics of a typical pulsed DC source are gobbled up in the core/coil itself and are irrelevant.

You need to have an equal plus and minus voltage square-wave for that to work properly, otherwise the solenoid will saturate and the current will be excessive.

You can force the plus and minus to be equal by running the square-wave through a large series capacitor.