# 24V AC to 12V and 3.3V DC - using a tapped transformer?

#### oranja

Joined Jun 18, 2021
4
I have a 24V AC power source (doorbell transformer).

Around that I'm hoping to build a circuit with two voltages:
1) 3.3V DC for an ESP8266 (for which I have a bridge rectifier and a D24V25F3 buck-down convertor)
and
2) 12V DC for a simple illuminated button (could also works with 5V or so, I don't need a bright light).

Idea:
I was looking for parts in an old DVD power supply board that I had around, and saw that it had multiple output voltages (25.5, 10.5, 6.3, 3.3). The transformer has 9 pins on the low voltage size and I realized that the voltage is probably split this way before regulating to those different desired voltages.
So it got me thinking, that instead of driving down ~34V DC one time to 12V and another to 3.3V, I could start by splitting the input into two 12V AC inputs, and that it would be simpler to get 12V DC from ~17V DC (maybe with a Zener diode?) instead of from ~34V.

Where I'm stuck:
So I looked for a center tapped 1:1 transformer and thought I could find a low voltage one that's the about the same size of an medium sized inductor, but my local place for electronics didn't really offer any, and looking online I mostly see 240/110V to 24V, etc. step down transformers. Rarely
I could find low voltage 1:1 transformers in some places, but they were described as pulse, RF or audio transformers and the prices did not make sense for me ($6 for the part,$30 for shipping) and I couldn't figure out if any of those would even fit my use case.

I'm curious why are those low-voltage 1:1 transformers aren't more common? What is the catch? What is the better alternative?
What would be a simple, yet efficient way to get the 12V and 3.3V DC supplies for this kind of small project?

p.s.
While writing the question and looking at Zener diodes, I got a sense that maybe it's not such a great idea and that instead it would be best to drop the 17V to 12V with a buck converter anyway. In which case I can just as well get a converter that's rated for 50V or so, and skip the magical tapped transformer that is much harder to find.
But thought I'll ask anyway, and learn as much as I can from the answers.

Joined Jul 18, 2013
23,950
What I have done many times in the past is to 'modify' an existing transformer, you need to remove the outer winding covering paper, in your case it would require removing some of the 24v winding to reduce to ~9vac and then add a few turn secondary winding turns for the 3v version.
It is a little fiddly but with patience it is usually quite easy.

#### Ian0

Joined Aug 7, 2020
3,785
I'm curious why are those low-voltage 1:1 transformers aren't more common?
Because what you seem to want is a mains to 12-0-12V transformer, and they are common!

#### ronsimpson

Joined Oct 7, 2019
1,679
Now I use cell phone chargers go get 5V. From there to 3.3V is easy. I don't use transformers any more.

Joined Jul 18, 2013
23,950
Depends somewhat on the application and power required, e.g. my last modification was a supply for Electric Fence controller, the tranformer had a copper screen between pri & sec windings.
Also the Va was in within the required size.

#### oranja

Joined Jun 18, 2021
4
What I have done many times in the past is to 'modify' an existing transformer, you need to remove the outer winding covering paper, in your case it would require removing some of the 24v winding to reduce to ~9vac and then add a few turn secondary winding turns for the 3v version.
It is a little fiddly but with patience it is usually quite easy.
It's a rented apartment with an old doorbell. It hasn't been working as a doorbell since day 1, but I tested its transformer and it still works. So I'm trying to see if I can build something without modifying anything that's already there - unless it's fully reversible.

The second transformer I can salvage already has 8 pins on each side, so I could split the voltage in all kinds of ways, but it's also a mains-to-some-low-voltage transformer so starting with 24V on primary, I'm going to end up with way less than 12V on the secondary. Reversing it is also not a good idea IMHO, as I'll just end up with mains voltage again.

Because what you seem to want is a mains to 12-0-12V transformer, and they are common!
Thanks! I saw those actually, but I already have a mains to 24V transformer, and I really like the idea of not interfacing with mains, if possible. That's way I stated that my starting point is 24V AC.
And again, if my starting point is 24V, a mains-to-lower-voltage transformer seems to overshoot my goal. Unless I'm missing something?

#### oranja

Joined Jun 18, 2021
4
Another idea that came to mind:
Get 3.3V to power up the ESP8266 first, then use it to generate a PWM signal (variable resistor to fine tune the duty cycle, I suppose) and use the PWM signal to switch an ad-hoc buck converter.
Would that work or buck converters are much harder to built than it looks like?
The load is just one button light...

Third idea: take apart the 12V button and just replace the built-in resistor. No reason it couldn't work with much lower voltages, no?

#### Ian0

Joined Aug 7, 2020
3,785
Unless I'm missing something?
Most people's starting point is mains voltage. Not many people start with 24V AC. Everyone has mains, very few people have a 24V AC supply. That's why almost every transformer you see has a mains voltage primary.