Transformer power supplies, which one would have a 24VAC secondary when the DC output is 9vdc > 12vdc > 15vdc > 32vdc

I need 24VAC 300ma for a solenoid and have these old transformer power packs with outputs of: 9vdc, 12vdc, 15vdc, 32vdc. I'm willing to crack open one and use the transformer but I only want to open the one that would have or be close to having the 24VAC secondary.

Which one's the lucky one.

The solenoid specs: 24Vac 50Hz 8VA > 8VA / 24 = 333ma should work fine?

Unfortunately it was a successful failure. I got the 24VAC from the Power pack but on connecting a distinct unhealthy hum was present from the solenoid with 24VAC at 370ma. It did not sound right.

I then tried DC power packs I have and all quiet, no humming just V/V opening and closing sound:

12VDC at 250ma

18VDC at 490ma

24VDC at 570ma

30VDC at 770ma

Solenoid says 'ac' but works better with DC, I have absolutely no idea!

• Open them neatly and measure... Commented Mar 27, 2021 at 22:45

24 * sqrt(2) = 34.

Most likely the 32 VDC is the lucky one.

• The 32VDC is what I also was thinking but I kept finding on the net people whom supposedly use 9VDC and 12VDC power packs to power their 24VAC solenoids. Which doesn't make sense to me. Commented Mar 27, 2021 at 23:14
• It may be that the solenoids are somewhat flexible in what voltage will trigger them. I would open the 32V one first also. But there is no guarantee it will work. Commented Mar 27, 2021 at 23:17

First check that they're not switched-mode power supplies. If they are marked for universal inputs (typically 100 - 240 V AC) then they are SMPS and no good.

A good clue is the weight of the PSU. Regular transformers have steel cores which are heavier than the small transformers used in SMPS.

A 24 V transformer, rectifier and smoothing capacitors supply will have an output voltage approximately equal to $$\ 24\sqrt 2 \$$ = 34 V DC. That means the 32 V one is the most likely to have a 24 V transformer.

• Yep, the ones in question are all transformer and not switch mode. Looks like the 32VDC is the candidate. Commented Mar 27, 2021 at 23:25

Possibly none of the above.

Voltage rating is not enough. It must also supply the power you need.

If the solenoid is 24Vac 28W then you need at least 28W (1.2A) supply but most likely more than 32 VA due to inductance at 24Vac and transformer derating required for Watts.

A more efficient solenoid would be (to buy a solenoid rated@) 12Vdc.

• The power side should be fine, they're all above 500ma and it's only a momentary operation. I only have what I have for it's only a quick solution to a cat problem. Motion activated hose made from parts I have. Commented Mar 27, 2021 at 23:23
• Ok then solenoid is not 28W but rather much less, in any case DC is more efficient so why convert? Commented Mar 28, 2021 at 0:06
• @TonyStewart The AC relays are shaded. You can easily spot the shader ring on them. Probably better to run them AC. (The shading creates a timing lag.) Probably will work with DC, but lower-rated magnitude I'd imagine. Some testing would be needed.
– jonk
Commented Mar 28, 2021 at 0:31
• I meant to say buy a DC solenoid. @jonk TY Commented Mar 28, 2021 at 1:06