24VAC Transformer from Air Handler, 7805

[Archangel]

> Maybe a voltage divider before the regulator . . . ?

Single-handedly Joe you've just created a ready-market for all those WW-II surplus dropper Resistors that nobody knows what to do with... brilliant! Even better, this is for a Heat-Pump application... first job - remove the heat from your own PSU...

This is a furnace, yes? Or buy a transformer . . .

[nemesis]

Right now I am powering my circuit off of a 12V@1A wall-wort. The regulator is really hot to touch. The overall draw of the circuit is never over 350mA and typically around 225mA.

1. Why is it so hot to touch or why am I dissapating so much heat?

2. Why would someone use a 7805, when you could make a voltage divider? Is it because the voltage divider is not as constant voltage of different current draws as 7805?

The heat in the project box is a problem, because I am using this as a termostat and it is raising my temp sensor apx 3 degrees.

Thanks!

[mister_e]

Power dissipation is something like (Vin-Vo)*I = (12-5) * 225mA = 1.575 watt assuming all parameters are right, usually wall wart provide higher voltage than specified (depending at least of current load).

You have few choices, move on a switching regulator, reduce Vin, compensate the temperature in software...

[nemesis]

Thanks for the reply. If I lower Vin with a voltage divider is there anything I need to worry about? And will this just burn off heat off the resistors and I might as well have moved over to the switcher? I was thinking something like two 100 Ohm 1/4W resistors.

[mister_e]

You don't want to use a voltage resistor, voltage will never be stable, this will screw-up almost everything.

[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-