Electronic – Converting 24VAC to 5VDC – Need transformer dethestification

power supplytransformervoltage-regulator

I've got an automatic litterbox that 'needs' to be hacked for better control. As it ships, it comes with a 120VAC to 24VAC (300 mA) wall wart that drives the two motors in it. I'm trying not to change out the original power supply, as I don't want to change the motors.

What I'm looking to do is get 5VDC for a microcontroller. My first thought was to simply toss a rectifier circuit and a 7805 and I'd be done. However, it seems that the 7805 wants a max Vin of 20VDC. Next thought was to find a small transformer to step the 24VAC down to around 12VAC and then pass that to the rectifier and 7805.

To that end, I started looking around for a transformer that I could use. Sensibly enough, everything I can find seems designed for a 120VAC primary. I came across this small tranformer, which I'm guessing is an audio transformer.

Am I right in presuming that the resistance of each coil is directly proportional to the number of windings? Thus, if I were feed 24VAC into that transformer with a primary of 115 Ohms and a secondary of 69 Ohms, I think I would get 14.4VAC out. Is that correct? Also, can I get away with using what is probably an audio transformer for this purpose?

Best Answer

You can't rely on using resistance measurements to get turn ratios, because they almost always use different gauge wire for the two windings. The primary winding is usually several hundred turns of some fine gauge stuff, while the secondary is a few dozen turns of something heavier, to carry the necessary current.

Probably the easiest solution is to get a 2nd wall-wart that supplies a few hundred mA of regulated 5V, and you're done.

However, if you really want to DIY it, you may be able to find transformers with 24V secondaries that are center-tapped, in which case you may be able to use one leg to the c.t. to get 12V AC for your diode-and-7805 approach.