Maker Pro
Maker Pro

transformer use question

corvairbob

Jun 14, 2022
12
Joined
Jun 14, 2022
Messages
12
i have a lawn sprinkler system and my pump makes more water than one zone uses, so to get the most out of the power bill i want to turn on more zones. my controller will not turn on more than one zone, i know the pump will handle it as i can do it manually but i would like to set this up using a bank of double throw switches. now i have the setup done but i need to find a transformer that will power 1 or 2 water control valves. i checked the coils and they have .5 amp inrush and .2 holding but i do not know if a 50va transformer will be enough to keep 1 or 2 coils working and not overheating and burn out.

thanks
 

Bluejets

Oct 5, 2014
5,989
Joined
Oct 5, 2014
Messages
5,989
Va is volt amp so you need to supply whatever voltage the solenoids are rated at and the wattage.
As well you need to know what mains voltage.
 

corvairbob

Jun 14, 2022
12
Joined
Jun 14, 2022
Messages
12
ok the transformer is 120 vac input 24 vac output with a 50va rating, the solenoids are .5 inrush amps and .2 amps holding

i think the transformer can handle this but would like to get another opinion just in case. the switches are double throw with center open so i will run the sprinkler controller to 1 switch left terminat and the transformer to the right switch terminal and the coil to the center terminal for an output to the coils. does that help a bit more? if not let me know thanks
 

AnalogKid

Jun 10, 2015
2,671
Joined
Jun 10, 2015
Messages
2,671
VA means volts times amps. The transformer is rated for 50 VA, so at 24 V that is a current capability of just over 2 A. At 0.5 A per valve, that is 4 valves. You can turn on up to 4 valves simultaneously.

ak
 

corvairbob

Jun 14, 2022
12
Joined
Jun 14, 2022
Messages
12
ok thanks i kind of figured that but i have a 2 va dc.transformer and it got hot running 2 valves on the bench for testing and even real warm on 1 valve. but that was a dc transformer and that might have been why it got hot. thanks
 

hevans1944

Hop - AC8NS
Jun 21, 2012
4,761
Joined
Jun 21, 2012
Messages
4,761
ok thanks i kind of figured that but i have a 2 va dc.transformer and it got hot running 2 valves on the bench for testing and even real warm on 1 valve. but that was a dc transformer and that might have been why it got hot. thanks
Firstly, there is no such thing as a "dc transformer". Transformers operate on ac. You can create dc by connecting the transformer secondary to a diode rectifier. Secondly, solenoid-controlled valves come in two types: ac solenoid coils. and dc solenoid coils. The main difference between the two types is how the magnetic circuit for the moving solenoid plunger is constructed.

An ac solenoid coil will be specified to operate at a certain ac rms voltage, anywhere from mains voltage down to some lower voltage, such as 24 vac, derived from a step-down transformer connected between the mains and the solenoid load. A dc solenoid coil requires direct current excitation. So it is important to know which type of solenoid you are trying to actuate.

An ac solenoid has an ac impedance that is different than its dc coil resistance. This impedance varies from some minimum value before the solenoid is energized to a maximum value when the solenoid plunger is "pulled in". This happens because of physical changes in the magnetic circuit. A dc solenoid has a dc coil resistance that is constant, whether the solenoid is energized or not.

The fact that your solenoid valves specify an in-rush current that is considerably larger than the holding current indicates these are ac solenoid valves. DC solenoid valves do not typically exhibit this effect. A dc solenoid valve will have an operating voltage (or current) that is larger than its minimum holding voltage (or current), but this is attributed to physical changes on the magnetic circuit: as the plunger is pulled into the solenoid, the magnetic force moving the plunger increases until it reaches a maximum value when the plunger is mechanically stopped. It then requires a reduction in the voltage (or current) applied to the solenoid before the solenoid spring can return the plunger to the de-energized position.

It is most likely that you are using 24 volt ac valves that use 0.2 A per valve when the valve is actuated. Your valve uses up to 0.5 A per valve when first energized (the inrush current specification). To be on the "safe side" your transformer should be able to provide a continuous output of 0.5 A per valve, as @AnalogKid in his post #4 above described. So, 24 vac transformer providing 2 A continuous current to four valves will require a 48 VA transformer. A larger transformer with a larger 50 VA rating is okay.

More than you probably ever wanted to know about ac versus dc solenoid valves can be found at this link.
 

corvairbob

Jun 14, 2022
12
Joined
Jun 14, 2022
Messages
12
say thanks i think i will get one of those 50va transformers and give it a go then. will save me a bit to trouble going to teh zone boxes and working the valve manually.
 

AnalogKid

Jun 10, 2015
2,671
Joined
Jun 10, 2015
Messages
2,671
ok thanks i kind of figured that but i have a 2 va dc.transformer and it got hot running 2 valves on the bench

Your valves are 12 VA peak and 4.8 VA continuous. AND they are designed for an AC input only. The current draw with a DC source will be different, and probably higher.

ak
 

corvairbob

Jun 14, 2022
12
Joined
Jun 14, 2022
Messages
12
i can't say what the volt amp is on them all i have on the valve body is 0.4 inrush amps and 0.2 holding amps. thanks
 

AnalogKid

Jun 10, 2015
2,671
Joined
Jun 10, 2015
Messages
2,671
i can't say what the volt amp is on them all i have on the valve body is 0.4 inrush amps and 0.2 holding amps.
All you have is all there is. In AC power calculations, VA is just that - volts times amps. 24 V times 0.5 amps (post #3) is 12 VA. This is the apparent power.

AC power is way more complex (!) that DC power. Phase angles, power factor, real and imaginary vectors . . . I'm not trying to overload you, but here is what Wiki has to say:

https://en.wikipedia.org/wiki/Volt-ampere

https://en.wikipedia.org/wiki/AC_power#Apparent_power

ak
 
Top