What is the downside to using a 240v primary/ 24 secondary transformer for a 120v primary/ 12v secondary application? I know it works but is there a reason I should not do it if I size it for the correct kva? I need to use it to power 12volt automotive lights.
Thanks

Hi Joe,
I don't think there is a down side as long as it's rated properly. The rest is turns-ratio - i.e. they are the same.
Don

Remember that you need to halve the transformer VA, too. Current remains constant.

Are you planning to use a Transformer with single windings on both sides [Primary and Secondary]? And this Transformer is rated only as 240 Pri / 24 Sec.?

If the Volt-Amp rating [VA or KVA] was 250 VA for this Transformer, and you plan to drive it at 120 VAC, only load it up to 125 VA. Otherwise it will burn out.

125 VA @ 12 VAC would be around 10.3 amps.

Falling under the 125 VA rating assures the smoke remains inside the windings

If the transformer has "Split Windings", then connect the windings as needed to be driven from 120 VAC and output of 12 VAC.

If only one side has split coils, leave the arrangements as 240 VAC Pri / 24 VAC sec.

I used a very large rating for a Control Transformer [250 VA]. They are usually around 25 VA min. to 100 VA max., with 50 VA being a normal rating.

If you plan to drive an Automotive Headlamp, you should consider a larger Transformer.

Scott SET