The Correct Method? (power supply)

Thread Starter


Joined Feb 9, 2008
Hi there,
Just looking for some opinions,

I was given a project to build over the next year or so, (nothing to do with school/college) just something to be working on.

At the moment I am researching to see if its possible to acheive, and also trying to avoid it becoming extremely difficult and complicated half way through the build.

This machine/device will be taking power from a 400v 3 phase industrial power supply.

At the heart of this machine will be a micro-controller.
So the two main control voltages I will need for control are 5v for logic and 24v for operating contactors and other relays.

So could someone let me know if the following procedure is good practice/the correct way to implement it.

From the 400v 3 phase, I will take two of the phases and connect to the primary coil of a suitable transformer.

400v Primary coil
24v AC output/secondary coil

Convert 24v AC to 23v DC allowing for V drop in bridge rectifier diodes (Not yet sure about the correct voltage drop).
23v is then regulated to 5v to operate control circuits.

Is this method correct?
Is regulating 23v to 5v normally done? Seems like a big drop, would it be advisable to look for a multiple output transformer?

Thanks for your help in advance.
Regards NM


Joined Apr 5, 2008

When you rectify and buffer the 24 Volts AC to DC,
you will get about 24 * 1.41 - (2 * 0.7) = 32.5 Volts DC.
When you want to regulate this to 5 Volts with a linear regulator you will lose a lot of power.
You also will need some regulation for the 24 Volts relays.
Better use a switching regulator. This will have a efficiency upto 95 %.