Problem Supplying Power to my Photon from a 24v AC source

I’m planning to add a photon to my lawn sprinkler system to shut off the sprinkler system based on recent rainfall or freezing weather. I’s like to power my photon from the the same power source that the sprinkler system uses, but I’m having trouble getting good 5v dc. I’m made bridge rectifier to obtain a DC output and an LM7805 to drop and regulate the output voltage. I seem to be getting an output that fluctuates between 4.95 and 4.94 volts, but it it will not power up my photon. Here is a fritzing of my power circuit. Any help would be appreciated.

Oh yes, I’m using N4004 diodes.

Either I’m too far off the track or your bridge rectifier is wired wrong.

The DC side should always come off the side where the two anodes (GND) and the two cathodes (V+) meet, but your diagram shows it differently.


How frustrating! I fritzed my setup incorrectly. I’m actually wired as you correctly show. Because of the slight voltage fluctuation, I think I may have a capacitance issue, but I’m not knowable enough to understand the math needed to size my capacitors and I don’t have a scope, but I would guess it would show I haven’t smoothed the DC output sufficiently.

Sorry about my inaccurate drawing, that wastes everybody’s time.

Hi @ray.hill

I would start by replacing the 100uF with 470uF or larger and move it closer to the diodes. You also need a few more caps near the regulator: the recommended circuit has a 0.33uF from input to ground and a 0.1uF from output to ground as close as possible to the regulator.

You are asking a lot of that little regulator! You are very near the upper end of the DC input voltage for the part and the power dissipated in the regulator is going to be around 29-5 V times 100mA for just the Photon or 2.4 W. You will need a large heatsink on the regulator in order to make it work for very long. Otherwise it will burn itself up. If it is very hot now (you can’t touch it) you might be hitting the thermal shutdown.


@ray.hill, stepping back a bit, the LM7805 may get quite hot given the large difference between the input voltage and output voltage. The 24VAC is a peak value with an RMS voltage of 17V after the full bridge rectifier. At 5VDC out, the 7805 will drop roughly 17v - (5v + 2v) = 10v. On average, the Photon will consume about 150ma without any peripherals. That is 10v * 150ma = 1500mw or 1.5watts of power. You will need a heatsink.

Please make sure your 1uF input capacitor is rated for at least 25VDC, ideally 50V.

You may want to consider a DC to DC converter instead of the 7805 like this one at Adafruit:

Or this one:


Thanks guys. When I get back home I’m going to play around with bko’s suggestions, just so I can get a little better understanding of working with capacitors. But, bko and peekay123 both point out that I need to get away from the 7805. I’ll take a look at Adafruit’s converter.
Thanks again!

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For what its worth, I just wanted to second the idea of using the modules suggested from Adafruit and Pololu. I’ve used dozens of the Adafruit 1065 modules in very rough environments and they are tough.
If you put your package in a closed up waterproof box you want to minimize any unnecessary heat. These efficient converters do that well.
I use the Pololu stuff too but the encapsulated Traco package is hard to beat even if a bit more expensive.

Good day @boley thank you for the tips. I am looking to use a 24v Dc input and bring it down to 5v. Is this the unit you would recommend from TRACO?

TDN 1-2411WI 9 - 36 VDC 5 VDC / 200 mA

Link here:

Yes. That is the one I would choose for use with a 24VDC power supply for the Photon.

Thank you so much for the quick reply. I love the support of this bored. I am very new so I am sure I will have lots of questions. Trying to make a 4-20ma data logger from my particle. Ordered more parts so now I just need to learn how to code… lol.