24VAC to 12VDC Power Supply

51Magnum · Joined: 03 Feb 2006 Posts: 75 Location: Boise, Id

Can someone please help?

I have a transformer that takes 120VAC to 24VAC. I need 12VDC at about 500mA. My problem is if I use a bridge rectifier and some smoothing caps I can get a good DC voltage of 38VDC. this is right on the edge for most voltage regs. How can I reduce this voltage? I know I can get a 60v max voltage reg but the problem is the heat. It dissipates so much heat. Is there a was to reduce the DC voltage out of the voltage reg?

Thanks

Estelles · Joined: 14 Dec 2005 Posts: 208 Location: Scotland

Hi

It all depends on the power you need. You can always use a switch voltage regulator. Most of them come with schematic examples:

http://www.onsemi.com/pub/Collateral/MC34063A-D.PDF

You also can use a MOSFET to chop off part of the AC you don't want.

What are your requirements?

Ed

51Magnum · Joined: 03 Feb 2006 Posts: 75 Location: Boise, Id

I am using a switching reg right now and it works well. Just kind of seems like overkill.

I need 12vdc at 500-800mA.

I am interested in the MOSFET idea. Do you have an example?

Estelles · Joined: 14 Dec 2005 Posts: 208 Location: Scotland

HI

Have a look at the attached file. The Mosfet disconnects whenever the voltage goes above the value set by the zener. You may need to change resistor values as this desing is for a high input voltage. Also the MOSFET is quite overkill, maybe something smaller would do it.

Let me know if you need any more info on how the chopper works?

Regards

Ed

Estelles · Joined: 14 Dec 2005 Posts: 208 Location: Scotland

Just a quick note, if you set the zener value too low, lets say 12 Vdc, you may find the the're is not enough current. I would advaise you to set it just below the voltage you need and use maybe 2 x 220uF caps at the output.
You may need a blocking diode if you are not using a voltage regulator at the output.

You could regulate it with a simple zener at the output but it all depends on how stable you want the PSU.

Hope it helps