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I have a power supply that says it is 24VAC output, but in reality is is giving a steady 29VAC of output. Is this normal? I am powering sprinkler solenoids that say 24VAC. So my question is how can I drop the voltage so I can supply 24VAC. I found some buck converters, but they seem to be DC only or AC input and output DC. I am assuming resistors might work, but an unsure of how to calculate for AC, and wondering if there is maybe a specific device that is made for this purpose?

I have done some work in DC, but AC is kinda new territory, so any help/explaination is much appreciated!

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    \$\begingroup\$ Thanks to both of you for answering! I hooked everything up and under load was fluctuating somewhere between 23.5 and 24.5 Volts. \$\endgroup\$
    – Elmer
    May 5 '15 at 4:28
  • \$\begingroup\$ Well, you can't ask for much closer than that with an unregulated supply. \$\endgroup\$ May 5 '15 at 4:52
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For a "power supply" that consists of a merely stepdown transformer, and with only a light load attached, that's pretty normal (16% high).

When a load is attached (when it is driving the solenoids) the voltage will drop a bit. Even if it was actually 16% high under load, it should not be a problem for the solenoids unless the ambient temperature is extremely high- close to the maximum for the solenoids. It will cause about 1/3 more heating, so if the coils heated to 50°C at room temperature they would heat to closer to 58°C.

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It's likely an "unregulated" power-supply (if it doesn't say "regulated", then it's a fair bet). Under a decent load that 29Vac will drop down significantly (in other words, as your load approaches the max specified it's capable of, Vout will get close to 24Vac). For your purposes - 24Vac solenoids - I woulnd't worry that it's 29 instead of 24.

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If you are 100% sure the solenoid is rated for AC input, it will be ok if you apply 29VAC instead of 24VAC. To make sure about that just monitor the solenoid (while powered) for half an hour. If it doesn't get hot (not able to tolerate the heat by your finger-touch), It will be ok.

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