I ginned up a humidistat using an arduino and some relays powered by my furnace 24VAC system (R and C). Yesterday we had a power outage (lights flickered off and on once or twice then the power was out for a few hours) precipitated by some utility work in our neighborhood. Something about that seems to have fried my power converter, as now when I hook it up to R and C on my furnace, the 3 amp breaker on the furnace trips. So my question is how can I prevent this from happening again in the future?

This is the actual power converter I'm using.

I'm not sure if I had a current spike or a voltage spike, nor do I really know how the power converter works, so I'm not sure really what to even search the internet for. I've heard of such things as isolation transformers, and that possibly a capacitor across the the two leads coming into the transformer might cushion the voltage spikes. Or maybe just a fuse?

Can anyone offer advice?

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    Can you link the data sheet? We need to see some stuff about cooling and ventilation. Commented Apr 19, 2023 at 5:38
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    The instructions in the listing say to connect the input to an AC battery. In a car. And to not reverse the polarity of the two red wires. There is a picture of a snow blower, and a European consumer panel. It's all complete gibberish. What expectations could you have of this "working" at all?
    – jay613
    Commented Apr 19, 2023 at 8:53
  • I would put a UPS in, for the lightweight but critical services like Furnace controller, modem/router (AP/phones) etc.
    – Criggie
    Commented Apr 19, 2023 at 22:18
  • I'm going to take the advice and get a plug in 120v adapter. Thanks very much for the advice!
    – nickvans
    Commented Apr 20, 2023 at 6:09

4 Answers 4


While, generally speaking, there aren't major safety issues with low-voltage random stuff like this power supply, the same general design and quality issues that affect 120V (and up) powered equipment do apply. Just reading through the Amazon page, I am not impressed - i.e., if the product quality is comparable to the quality of the advertising copy then I wouldn't touch it with a 10' pole.

Fortunately, what you are looking for can be provided by a 120V plug-in power pack, and you can find those with UL or ETL listing at a reasonable price without too much trouble. Those can often be specified with extra protection, and also plugged into a surge protector. Something like this 4A UL listed power supply from Adafruit or this 2A UL listed power supply from Walmart. You can also use USB power supplies, which are available pretty much anywhere, but getting more than the baseline USB power involves some communications, while a simply 5V supply will supply up to the rated value just by connecting it.

The situation would be different if your primary power was 24V AC (or anything other than typical 120V-240V AC). But in your case using the 24V AC thermostat transformer is a convenience, but doesn't actually save you any money. 120V AC -> 5V DC has become such a commodity item that even a good quality (UL or ETL listed) power pack can cost less than a 24V AC -> 5V DC converter.

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    Thanks so much for the recommendation. You're right using the 24V was just convenient because that's what was right there. Running an extension cord over there to plug in a 2A UL power supply wouldn't be too unsightly. I even had one I was about to throw away that came with something or other. Thanks for the recommendation!
    – nickvans
    Commented Apr 19, 2023 at 4:10
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    I would not run an extension cord. Instead plug the power pack directly into a receptacle (or into a surge protector plugged into a receptacle) and extend it to your destination using thermostat wire or similar. The low voltage stuff is much safer to route in strange places, etc. Commented Apr 19, 2023 at 4:17
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    Use reasonably beefy (probably 18ga minimum) wire if running the 5 volt supply very far. Some 5-volt devices may be unreliable if the combined voltage drop on the positive and negative supply "rails" exceeds half a volt or so.
    – supercat
    Commented Apr 19, 2023 at 22:56

The reviews on that model aren't very good. A few one star reviews saying it failed in a short amount of time. I've usually powered my Arduino's with a power supply that plugs into an outlet. My recommendation is to find a power supply with better reviews. You'll probably wind up paying a little more. If nothing else in your house failed, I'm guessing it wasn't much of a spike.


24 VAC will go into 5 VAC in the transformer. After that you will have rectifier bridges (4 diodes) as first DC component, followed by capacitors to smooth it out. This would be the most simplest (old) form.

Unfortunately the transformer store some capacity (inductive currents) and power on/off will create spikes that might kill that transformer. You could protect it by using some coils (to limit the inductive currents) and some very fast fuse.

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    I don't think there's enough space for a 25VA transformer in there - it's much more likely to rectify the 24V then use a buck converter to get 5V. Commented Apr 19, 2023 at 14:33

While I agree with much of what has been answered already, an answer to the question asked (even if using a 24V-5V converter is nuts, especially if it's flambe-quality import junque) would be to install a "Whole House Surge Suppressor" at the main 240/120V power panel. Since these are now required by modern code, they are relatively inexpensive and commonly available - if your panel is not itself an obsolete fire hazard there are many that will slot right in to a pair of breaker spaces, and protect (to the extent they manage) everything in your house.

Including flambe-quality import junque but of course, it might just fail anyway, as it's flambe-quality and not listed by any reputable testing lab. So follow the other answers about that piece of equipment, but consider putting a suppressor in your main panel even if code didn't require that when your panel was installed, so it's not required that you retrofit one. It's still a good idea. And it no longer requires hunting down oddball industrial market surge suppressors, like in the old days before NEC required it.

  • Thanks for the recommendation. I'll do some looking into it. Are there any breaks or anything that I should watch or for? Or maybe certifications I should look for?
    – nickvans
    Commented Apr 20, 2023 at 6:07
  • Certifications are the usual: UL or ETL (or CUL or CSA in Canada) etc. "Nationally Recognized Testing Laboratories." CE is NOT one of those. Beware of Import Junque that might flambé.
    – Ecnerwal
    Commented Apr 20, 2023 at 11:57

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