24VAC -> 24VDC, 5VDC, 3.3VDC

Thread Starter


Joined Dec 8, 2018

for a circuit I need 1x 24VDC and 1x 3.3VDC and 1x 5.5VDC. As input voltage I have 24VAC from a transformer. The 24VDC powers solenoid valves, sensors and contactors with in total approx. 1A. The 3.3VDC and 5.5VDC are for low power electronics like a microcontroller.

How would you realize the power supply? Two step-down converters for 24VDC and the low voltages, which are then stabilized with LDO's? Is there an alternative with less components? Maybe a transformer with multiple outputs? I do not need galvanic isolation.


Joined Jul 18, 2013
I assume the solenoids and contactors are DC?
Solenoid's, sensors and contactors, Only 1amp?
You could use a dual secondary transformer with 24vac secondaries, the solenoids and contactor would only need a bridge rectifier, with reverse diodes across the loads.
The other secondary for the 5.5v and 3.3v DC supply using 5.5 and 3.3v regulator IC's.
If you obtain a toroidal transformer, it is very easy to modify or add any winding if needed.


Joined Sep 9, 2010
Can your transformer meet the current requirement, in other words ≥1A?

I'd use a full-wave rectifier to give ~32V followed by a filter capacitor (eg 50V, 1,000µF or more) and a 7824 voltage regulator. The minimum voltage for that IC is listed as 33V but I bet it would be fine for the loads you named. Maybe not a precise 24V but good enough.

I'd knock the voltage down to 5.5 and 3.3 VDC starting with the 24VDC supply. There's probably a simple buck converter that can do this. I'm not familiar with them. I'm sure someone else here can advise.

Thread Starter


Joined Dec 8, 2018
I wanted to use a smart high-side switches to monitor the current consumption during switching and other failures. These need 24VDC.


Joined Aug 7, 2020
If you could use +/-12V instead of 0-24V for your solenoids, then you could use a 12-0-12V transformer. Then the low current 5V and 3.3V supplies could just be 7805 and 7833 linear regulators. Not very efficient, but low current so the losses remain small.

Thread Starter


Joined Dec 8, 2018
I thought about a down converter with transformer. But I do not know where to get the right transformer. Apart from the fact that this generally seems much more complicated than using a simple buck converter.


Joined Mar 14, 2008
An interesting option is to use a center-tapped 24Vac transformer with a bridge rectifier to generate two DC voltage (LTspice simulation below).
The two diodes to ground in the bridge do double-duty, acting as half of the full-wave bridge rectifier, and also generating the 1/2 DC voltage at the center-tapped output (removing the top two diodes will show how this works).
You can then use linear regulators to generate the desired voltages, 24Vdc from the top output, and 5Vdc/3.3Vdc from the bottom output.

Note that the transformer should have about double the AC current rating of the total DC load current from all the outputs.

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