24VAC to 18VDC Regulator Circuit

Thread Starter


Joined May 21, 2008
EDIT: Forum doesn't like ascii diagrams

I am relatively new to power electronics, I have a 24VAC line from a 120-24 isolation transformer. I need to convert it to 12V and 5V DC. (I assume this is somewhat common). I initially tried to rectify the 24VAC line to 18V by using a capacitor, resistor transistor, diode, zener diode circuit. From there I was going to use two small buck regulators to get 12V and 5V. Unfortunately the transistor runs too hot. I assume there is a much better way of doing this (maybe a package you can purchase) that everyone knows except me, so thought I'd throw it out here for suggestions.

24VAC feeds into a diode, rectifying circuit, then into collector of transistor
transistor base connected to 18V Zener, which is connected to ground (transistor closes when zener breaksdown)
10KOhm resistor connected between collector and base
Capacitors on both collector and emitter to hold charge.

Sorry for the crappy circuit. Thanks.
Last edited:


Joined Mar 6, 2008
What amount of current do you need?
Have you considered the 7805 and 7812 fixed-voltage regulators? Or the LM317? (here you can select the output voltage with 2 resistors).

But of course, those are linear regulators, they aren't the most efficient thing in the world, but for small currents, can work.


Joined Apr 15, 2008
From what I understand of your description of your circuit, you were basically making a discrete linear regulator. So you will get the same result with any linear regulator.
You don't really need to regulate the power to feed into the switching power regulators. You just need to rectify the 24VAC to a capacitor for storage, and use that unregulated voltage to input into your switching regulators.
If your switching regulators cannot handle the voltage (about 34VDC), then perhaps you should use a different transformer to get the proper voltage range out or different buck regulators.

Thread Starter


Joined May 21, 2008
Thank you both for the info. I will need about 100mA of current from what I can gather. Unfortunately the buck regulators I chose can only handle up to 24VDC. The transformer cannot change. I will see what I can do to find different regulators. I'm trying to find a solution with the lowest possible heat output to do this. If my diode circuit is essentially just a linear regulator, then I'm just sort of chasing my tail (and it then makes sense as to why so much heat is dissipated on the transistor). Just rectifying into some other switching regulators may be the way to go, I just need to find some appropriate ones.


Joined Dec 20, 2007
The 24VAC makes an unregulated supply voltage of abot 32VDC which is way too high. An ordinary 12V 7812 linear regulator needs an input voltage that is only 15V then it will be much cooler.


Joined Apr 20, 2004
Is the transformer secondary center tapped? If so, you can do a 2 diode full wave rectifier and have about 16 volts on the filters - close to ideal.