24VAC to 5VDC

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jeffjohnvol

Joined Oct 15, 2008
37
I want to make my own thermostat with a small micro.

I'm trying to figure the most efficient way to go from 24VAC to 5VDC to drive a microcontroller.

I understand the basics of the bridge rectifier with a leveling cap and a regulator like a 7805 or so, but that is a major amount of power to regulate down, even at low current.

I searched the forum and only found this:
http://forum.allaboutcircuits.com/showthread.php?t=17963&highlight=24vac+5vdc&page=1

which was reading a value, not driving a voltage input.

I saw this, but wasn't sure if it would be a good fit: http://www.mouser.com/new/texasinstruments/nsclm257x/

Since size is an issue (don't want a huge thing on the wall), I want to avoid a step down transformer, unless there's a common one that is small enough.

Any suggestions?

Some research talks of a "buck" regulator, but I don't know enough about them.
 
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tracecom

Joined Apr 16, 2010
3,944
I want to make my own thermostat with a small micro.

I'm trying to figure the most efficient way to go from 24VAC to 5VDC to drive a microcontroller.

I understand the basics of the bridge rectifier with a leveling cap and a regulator like a 7805 or so, but that is a major amount of power to regulate down, even at low current.

I searched the forum and only found this:
http://forum.allaboutcircuits.com/showthread.php?t=17963&highlight=24vac+5vdc&page=1

which was reading a value, not driving a voltage input.

I saw this, but wasn't sure if it would be a good fit: http://www.mouser.com/new/texasinstruments/nsclm257x/

Since size is an issue (don't want a huge thing on the wall), I want to avoid a step down transformer, unless there's a common one that is small enough.

Any suggestions?

Some research talks of a "buck" regulator, but I don't know enough about them.
The most efficient way would be with a switched mode regulator, but before you rule out a linear regulator, how much current at 5V do you need?
 

MrChips

Joined Oct 2, 2009
27,159
If you design your microcontroller system to operate on very little current you can get away with a simple half-wave rectifier circuit and a 5V linear regulator.
Edit: You posted before me. 100mA is way too much for a microcontroller. MCUs are available that will run with less than 1mA.
 
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Thread Starter

jeffjohnvol

Joined Oct 15, 2008
37
FWIW, I'm hoping for something small and inexpensive (aren't we all!) with something that doesn't need a lot of heat dissipation.
 

Thread Starter

jeffjohnvol

Joined Oct 15, 2008
37
If you design your microcontroller system to operate on very little current you can get away with a simple half-wave rectifier circuit and a 5V linear regulator.
Okay, thats good to know. A lot of googling always has people commenting on the delta of the regulator.

If the rectified voltage averages to 28 volts (unregulated) the 23 volt delta would be at the far end of the limits of a 7805, wouldn't it?
 

timescope

Joined Dec 14, 2011
298
In other words, you want to use the 24v ac to provide the 5v dc power supply for the microcontroller. The LM2574HV is a buck regulator and the circuit shown only needs a rectifier and capacitor to convert the 24v ac to dc, connected to the unregulated DC input pin. Use the 5v version.

Timescope
 

Thread Starter

jeffjohnvol

Joined Oct 15, 2008
37
In other words, you want to use the 24v ac to provide the 5v dc power supply for the microcontroller. The LM2574HV is a buck regulator and the circuit shown only needs a rectifier and capacitor to convert the 24v ac to dc, connected to the unregulated DC input pin. Use the 5v version.

Timescope
Timescope, so you think the 2575 (5 volt version) would be a good option with the way you describe? I can get those pretty inexpensively: http://www.ebay.com/itm/10PCS-IC-NS...859?pt=LH_DefaultDomain_0&hash=item3a748dcb5b

Thanks.
Jeff
 

Thread Starter

jeffjohnvol

Joined Oct 15, 2008
37

timescope

Joined Dec 14, 2011
298
Yes, if 500mA will be enough for your application. These regulators are very easy to use if you follow the layout instructions on the datasheet.

Timescope
 

Thread Starter

jeffjohnvol

Joined Oct 15, 2008
37
Yes, if 500mA will be enough for your application. These regulators are very easy to use if you follow the layout instructions on the datasheet.

Timescope
Thanks Timescope and all that replied. this forum is a great resource. I really appreciate the time.

Jeff
 
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