I recently purchased and installed a new home alarm system which is supposed to be powered by a USB cable plugged into an adapter rated at AC100-240V input and DC5V 1500mA output.


The problem is that I don't have any power outlets anywhere near that closet and the only electricity I have there are wires from the old ADP alarm system which is AC29V according to multimeter readings.

Any ideas how to hard wire the USB cable to one of these AC29V wires and get the needed DC5V output?

I've searched for hard wire kits but everything I found so far is DC to DC hard wire kits... do AC to DC hard wire kits even exist?

  • 1
    You have a picture of an AC to DC kit right there in your post. Of couse, its input voltage range does not go low enough for the weird thing you want to power it from. Trivial to build if you know how, or learn what you'd need to learn, - probably impossible to buy off the shelf for that input voltage. If the old alarm system is disused, go where it gets its power from, disconnect the 29VAC transformer, plug in your 5VDC power supply, and you'd be down to cutting and connecting cables to re-use the wires in the walls. Otherwise, run a new AC outlet to the closet in question.
    – Ecnerwal
    Commented Jan 1, 2022 at 23:25
  • Search that thing for a UL, CSA or ETL mark. If none, throw it in the trash and use any USB adapter with such a mark. Ikea's are excellent, and Apple's larger blocks are the best in the business and readily available "new unused" on eBay cheap. (One comes with every iPad, and schools throw them away in favor of gang chargers). Commented Jan 1, 2022 at 23:42
  • tie the AC into a full bridge rectifier w/ smoothing capacitors, then run the unregulated DC output to a DC to DC buck converter to output 5v. you can buy rectifier modules, and use a 24vdc to USB adapter, commonly sold for RVs, to get the 5v; it shouldn't mind the slightly higher input voltage.
    – dandavis
    Commented Jan 2, 2022 at 0:03
  • I would look more closely and probe around that old ADP alarm system. Something is providing it ~120V and it is getting stepped down to 29V. Could be that 120V is headed into a battery somewhere and then the 29V is coming off the battery to the alarm system. Commented Jan 2, 2022 at 0:06
  • 1
    29VAC does not come from a battery.
    – Ecnerwal
    Commented Jan 2, 2022 at 0:38

4 Answers 4


Though you know the voltage you measured is 29 VAC, you must know the current rating of the old ADP alarm system to be sure that it can provide sufficient current to operate the new alarm. If the proposed buck converter were 100% efficient, which they are not, it would need to provide 5 VDC at 1.5 A, that is 7.5 W. You'll need to trace the connection of the ADC wire to its power supply to determine if that's feasible.

However, since you'd need to find that power supply anyway, why not replace it with the 5 VDC power supply, and use the existing wiring to carry that 5 volts to where it's needed? You'd need to connect each end to a USB-to-parallel wire connector, using the standard pinout.


You're making it harder than it is. Everything conforms to standards:

  • The alarm input is standard USB, not "random 5 volts".
  • That "29 volt" is 24VAC nominal, commonly used for thermostats and doorknobs. The transformers are readily available.

Solution 1: Find the 24VAC's source and wire it into that.

Most likely the 24VAC is coming from a transformer either plugged in or hardwired into AC mains. It's no problem converting from AC mains to 5V "USB style" power: For a plug-in solution, you have it. For a hardwired solution, run approved wiring (e.g. 12/2 NM cable) to a standard junction box, and then install a "USB outlet". This is very easy to do "Code compliant".

enter image description here

Solution 2: Step up from 24VAC to mains, then use a mains USB outlet.

This is a little trickier to do Code compliant, because it requires careful match-up of junction box, outlet and transformer. You simply use a common 24V thermostat transformer, backwards.

You must select a transformer designed to fit onto a junction box "knockout hole", the North American standard is the "1/2 inch trade size knockout" which is about 7/8" diameter actually. Then use a tall junction box which has 1/2" knockouts. The transformer goes into a side knockout, so the AC voltage is inside the junction box and the 24 volt is on the outside.

Fit the USB receptacle into the junction box in the normal way. Link its hot-neutral to the transformer's. Use an all-plastic box as there'll be no way to earth/ground this (but you are double-isolated by the 2 transformers, so it shouldn't be a problem).

So entirely inside the junction box, you have 120VAC wires between transformer and USB receptacle. (or your nation's standard mains voltage).

It is very important to use a USB-Only receptacle like pictured above, and not one with a standard mains socket. The transformer link can only support 20-30 watts, so both transformers will fry and catch fire if you plugged in a 120V/230V load!

All of the above should be done with approved equipment (North America: UL/CSA listing). Note that all CE marks must be presumed to be counterfeit, unless the item was sourced from an actual bricks-and-mortar supplier inside the EU. A Redoute retail shop is fine. Mail-ordering Siemens equipment is fine. Mail-ordering Amazon from 3rd party seller "TEKSHOO", nope - that CE stamp will be faked.


If it is a good alarm system I would suggest installing an outlet as the best solution, not necessarily the easiest or cheepest. Running 5V any distance to power electronics is not a good solution. You also have the possibility of reversing polarity and destroying the alarm unit, that may only take a few milliseconds to happen. You can get adapters to convert hard wiring to the appropriate USB connector. and use the existing wiring but be careful and check polarities before connecting to the alarm. You can always power it with a 12V wall wart and use a buck converter placed at the alarm site to supply the 5V. Some may be purchased from your from your favorite china supplier that will even have the USB connector on them. I purchased one about 4 years ago and it works great.


The usual dc-dc modules...

enter image description here

enter image description here

These will most likely have a counterfeit LM2596 because they all do, and garbage quality capacitors. They will probably not deliver a stable 5V at the rated current and when the capacitors die, output voltage can go over the set value which will fry your alarm. The ones I tested had a fake LM2596 that ran at the wrong frequency, which saturated the inductor, the caps were burning hot due to being too cheap, then died, etc. It's just trash.

Anyway. The peak voltage of 29VAC is about 40V so if you use your AC-DC module you should check the black cap on the left has at least 50V rating, otherwise it will pop. It says it has a LM2596HV which is good to 60V, but who knows what's inside that chip. National Semiconductor was bought by TI in 2011 so if the chip has the National logo on it, at least you can be sure of one thing, it sure didn't come from the official manufacturer!

You can know if yours will die quickly by putting a finger on the caps: if they're warm, it will die.

Here's what I would do...

Locate the source of 29VAC, there's a transformer somewhere. Since you no longer use it, might as well remove it.

If the wires are good for mains voltage, and there isn't other stuff using 29VAC wired to them, then you can just reuse them for mains after replacing the transformer with wire nuts. Now you have mains where your alarm is.

If the wires are in a conduit, you can pull them out and install mains rated wire instead.

Otherwise, I'd replace the 29VAC transformer with a 12VDC power supply, and near the alarm, a 12V to 5V converter that isn't a counterfeit LM2596 module. For these, the highest chance of it not dying due to garbage quality capacitors is to buy one without electrolytic capacitors. If you can find a fixed 5V one without potentiometer, that's another failure point removed. Example, and another from a proper brand. Suggestion: a car cellphone charger, that's 12V to 5V...

  • 1
    I see where you're mentioning cheapo step-downs simply to warn about them. Good. Honestly if you're removing the 29VAC transformer why not just put a listed USB receptacle there "and done with it"? Having an intermediate 12V step seems unneeded, if you are worried about voltage drop over distance, just carry the 5V on enormously thick wires such as 14 AWG. (where would you get that I wonder :) Commented Jan 2, 2022 at 23:37
  • It really isn't that expensive to do it right either -- Altronix makes a 24VAC/DC to 5VDC converter (their VR1TM5) for ~$30 through security equipment suppliers Commented Jan 3, 2022 at 2:28

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