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Topic: Power from 24VAC (Read 4 times) previous topic - next topic

dc42

PS: another possibility is to use a 115V to 36V transformer, also available from Digikey. Feed it with 24V and you will get about 7.8V out of it. Rectify that with a bridge rectifier and smoothing capacitor, that should give you about 9.5V to feed directly to the Vin pin of the Arduino.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

growerdick

Thanks everyone for the input.  At this point it looks like the best solution is a DC to DC converter and a bridge and cap.  There aren't many and they are a little pricey but this one looks like it will do the job:  http://www.digikey.com/product-search/en?x=15&y=9&lang=en&site=us&KeyWords=102-1839-ND
It has an 18v to 72v range and is $19.95 q1.

rayshobby

Depends on how much current your circuit draws. If it's small, like tens of milliamps, a zener diode is good enough and won't waste too much power. If it's something like hundreds of milliamps, a switching regulator is better since it's much more efficient. A half-wave rectifier (i.e. single diode) plus big cap is sometimes good enough for AC-DC conversion.

I used a MC34063 switching regulator for my sprinkler controller circuit:
https://raw.github.com/rayshobby/opensprinkler/master/OpenSprinkler%20Controller/hardware/v1.4/OpenSprinkler_v14.png
look at the "power supply" section on the top-left corner.
DIY electronics project at http://rayshobby.net

holmes4

Have I lost the plot here?

The OP wanted to start with 24VAC feed it to a bridge rectifier to get 12 VDC (ish) clean it up with smoothing caps.

All of which is simple and cheap. Then use a LM7805 voltage regulator which gives a nominal 5v (+/- 0.2) as its output and is happy on a 12v input.

The total cost in the UK (inc 20% vat (sales tax)) about 2 pounds.

Mark

rayshobby

If power consumption is not an issue, there are a variety of simple ways to drop 24V to 12V. Zener diode, 7812 or other linear regulators... But if the circuit is drawing sizable amount current (like mine draws about 180mA current), then (33-12)*0.18 = 3.78 Watt power will be wasted. Note that the 33VDC comes from rectifying 24VAC. In fact, there will be more because the 12V to 5V conversion will waste another (12-5)*0.18 = 1.26 Watt. That's a total of 5 Watt. So in this case switching regulator is more efficient.
DIY electronics project at http://rayshobby.net

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