View Full Version : 24VAC Transformer from Air Handler, 7805

- 8th February 2009, 14:06
Hi All,
I recently put together my own thermostat for my home running a heat pump. I bread boarded the whole device and tested with a 9VAC transformer and everything works as expected. Upon doing a test hook up to the 24VAC transformer (the air handler uses for the control relays) the voltage after my 7805 is roughly over 1 volt instead of 5 volts. I am attaching a schematic.

Basically I take the 24VAC output from the transformer and put it throught a full wave bridge rectifier. On the output of the rectifier I have a 1000uf and a 100uf cap. Right before the 7805 regulator on the input I have a 0.33uf ceramic cap. On the output pin I have a 0.1uf ceramic cap followed by a 470uf cap.

I can't seem to get my regulator to put out 5V off of the 24VAC Transformer.

Any suggestions or troubleshooting tips will be greatly appreciated.


- 8th February 2009, 14:51
So you read the Datasheet for your 7805?

By the time you take 24vAC, rectify it and smooth it, you may have a DC voltage which exceeds the input specification for your Regulator. If this is the case, you've just blown up your Regulator.

Some 7805's require a load on the output (regardless how small - even 1K should be sufficient), otherwise they just shut down.

You may also find that 470uF on the Output (which isn't in your schematic) is excessive... the 7805 may see this as a dead-short initially and shut-down. The shut-down is around 1v on these devices, so I'd look at that first.

Reading manufacturers Datasheets saves a lot of headaches...

- 8th February 2009, 15:25
Thank you very much for your post. I am a newbie at electronics but love it. This is actually my first project other than my wonderful blinky led.

Here is what the voltages are...

My 24VAC Transformer actually outputs 27.6VAC my service to my home runs a little hot (roughly 112VAC) which might account for the higher voltage.

Right before the 0.33uf cap before the regulator the voltage is 37.9V. So you are exactly right on the over-voltage. 7805 that I have is rated up to 35 volts. I did read the datasheets pretty throughly, but I wasn't expecting the voltage to rise from the caps. I only expected the voltage to raise from the rectifier, which it rectifies and the voltage is roughtly 33 volts. Looks like the caps push it up 4 almost 5 volts.

What is the best suggested way to drop the voltage within range? Should I try to drop it on the AC side or the DC side?

Would a choke inline before the regulator be the best way to go about this?

If a choke is the best solution, how do I determine the henry value? My circuit won't draw more than 350ma.

Thanks Again,

- 8th February 2009, 20:39
Take your AC voltage, when full-wave rectifying it you'll have VACx1.414 as an off-load DC across your primary smoothing Capacitor. In your case that could be 27.6 x 1.414 = 39v. An inductor isn't going to do much for you... you need to loose 10-15 volts somewhere or use a different AC source. Your Transformer might be 24v... but it might also have additional tappings (ie 12-0-12) or similar.

- 8th February 2009, 22:48
Some readings

Linear and Switching Voltage Regulator Fundamentals

Vout(Min)(V) 1.25
Vout(Max)(V) 125
Iout(Max)(A) 0.7
Vdo(Max)(V) 20
Vin(Min)(V) 21.5
Vin(Max)(V) 150
Pin/Package 3DDPAK/TO-263,3TO-220

Example Transistor Circuits

Serie voltage regulator

- 9th February 2009, 05:10
Thanks Melanie and Mister_E. Those are some great links for me to read up on. I ended up running a seperate feed from my basement from another DC source. I still have the power section of this circuit on the bread board and I'm still going to figure out how to get it to run off that AC feed! I think those links will help greatly.

Thanks Again,

- 9th February 2009, 06:18
Maybe a voltage divider before the regulator . . . ?

- 9th February 2009, 06:55
> 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...

- 9th February 2009, 07:54
> 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 . . .

- 24th February 2009, 18:22
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.


- 24th February 2009, 18:39
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...

- 24th February 2009, 20:13
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.

- 24th February 2009, 20:45
You don't want to use a voltage resistor, voltage will never be stable, this will screw-up almost everything.

- 25th February 2009, 00:08
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.


- 25th February 2009, 02:08
You could use a half-wave rectifier with a large filter capacitor.


- 25th February 2009, 08:35
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.

- 25th February 2009, 19:23
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