24VAC Transformer from Air Handler, 7805

[b1arrk5]

HVAC control transformers are notorious for over voltage when lightly loaded. We used to place an MOV (metal oxide varistor,) rated at 30 volts on our electronic thermostats for protection. These thermostats were designed almost twenty years ago. Place a heatsink on your regulator, place the regulator near the top of your circuit board, and place your temperature sensor near the bottom, with the board mounted vertically the heat rising from your regulator will pull fresh room air over the temperature sensor.

Melanie's idea about centertapped transformers would make your project easier, but most residential HVAC equipment sold in the US won't have a center tap. However, you can buy one and replace the transformer in your air handler and take 12 volts off the center tap. The newer switcher regulators don't produce the heat that an LM7805 will, making them a better choice, I wish we had them all those years ago.

Jerry.

[Pic_User]

You could use a half-wave rectifier with a large filter capacitor.

-Adam-

[Melanie]

This isn't a difficult problem, so step back and think...

At the start you said you need 5v about 350mA... Firstly question - do you?

I've designed shed-loads of HVAC stuff, and generally find that the 5v part rarely consumes anything over 50mA... You don't need to switch Relays at 5v - they can hang on the 24v line, dump any LCD that needs a Power Station to drive the Backlight, etc etc...

If you do need that kind of current, easiest to do it in two steps... Drop the (near 40vDC) down to around 15-24vDC with one Regulator, and then use your 5v Regulator thereafter.

A switcher is best and most efficient.

Or why not drop the 24-30vAC down with a Transformer. Now you're going to say there's no such thing as a 24v Input, 12v Output Transformer... but a Transformer is just an AC ratio stepper... so if you're in the States, look for a 220/110v device (of suitable Wattage) or any Transformer with a 2:1 or 3:1 Input/Output ratio that will give you around 12vAC out for 30vAC in.

[nemesis]

Thanks for everyones reply. I found a switcher that will take 50V and output 5V@1A. I need to pick up a few more components like an inductor.

The transformer sounds like a really nice idea, instead of putting together all those additional little components.

Thanks again everyone