So I have this annoying situation in my house that seems fairly unique and I couldn't find any thread asking this question, maybe because it's stupid. Anyway, if you want to skip all the background (tl;dr), here's my question:
What happens if I plug a standard AC-DC converter (100-240 v input to 5 v 200 mA output) into a low-voltage AC supply current (18 v AC)?

Here's why I am asking:
I have an old defunct intercom system that ran on 18v AC and wherever the transformer was installed (not in the breaker box) it got covered up when the garage got drywall installed a few years ago. This was before I purchased the house, so I have no clue where the transformer is.

I pulled the intercom control and radio out of the wall, and it's just about the right size to mount a tablet with a dashboard to control some automated devices in my house (z-wave & zigbee). I was hoping to install a receptacle in the box behind the intercom and use the tablet's power supply to keep it powered all the time.

The tablet power supply has standard input parameters (100-240 v, 50-60 hz, 0.3 A). So - what happens if I plug the power supply rated for higher voltages to an 18 v supply?

My multimeter doesn't read Amps or frequency on AC, so I am guessing these are within range. My best guess is that the power supply would just draw less on the lower voltage and charge slower? Since the tablet is plugged in all the time, no biggie. Even if I could find the transformer, I am not sure I'd want to bypass it, because the wiring into the intercom was low-voltage, probably 16 g an not shielded well - I would not trust it for high voltage.

Anyway, my solution is to plug in the converter and see, but I thought I'd post this questions first and get some feedback from folks who know what they are talking about. Thanks in advance for helping!!!
Matt

 

It won't work ,its a switched mode psu, designed to run from 100v to 230 v, so anything less will not work....

 

It would be relatively easy to rectify the 18vac to dc, add a filter cap, and then regulate the dc down to 5v. There are bridge rectifiers available in 4-lead packages, and buck converters are cheap.

 

It would be relatively easy to rectify the 18vac to dc, add a filter cap, and then regulate the dc down to 5v. There are bridge rectifiers available in 4-lead packages, and buck converters are cheap.

Thanks for the advice, I am looking into that. I'm more of a software guy, not a ton of electronics experience and not wanting to burn down my house (it's the only one I have). After doing some more searching, I am guessing that my cheap-o multimeter is actually reading the voltage wrong - 16v AC seems to be a much more common low voltage current to use in homes in the US - for things like thermostats, etc.

I found this already put together and it prevents me from having to design a circuit. http://www.mikroe.com/add-on-boards/power-supply/5v-3-3v-reg/ Can't beat the price. Think it will work?

 

I found this already put together and it prevents me from having to design a circuit. http://www.mikroe.com/add-on-boards/power-supply/5v-3-3v-reg/ Can't beat the price. Think it will work?

It looks perfect except for the input voltage being right on the upper limit. If you could find one with a 20v (or more) input rating, that would be great. You should run it under load for several days and keep a close check on the temperature before you bury it in the wall.

Legal disclaimer: I am not a professional, and you should have a professional electrician approve the device and perform the installation, being sure that it is safe and meets all building codes for your location.

 

It looks perfect except for the input voltage being right on the upper limit. If you could find one with a 20v (or more) input rating, that would be great.

yep, there's a slightly more expensive version that's 9-23vAC, I will go with that.

You should run it under load for several days and keep a close check on the temperature before you bury it in the wall.

Yep, it will be installed behind the tablet which is installed on a hinged frame in a shielded/insulated box, so worst case scenario I burn up a $50 pos tab. I will run it for a couple of days and check the heat and unplug it at night and when I leave.

Legal disclaimer: I am not a professional, and you should have a professional electrician approve the device and perform the installation, being sure that it is safe and meets all building codes for your location.

Lol, of course. I never am actually going to do this ...if anything happens it will have to do with the intercom system.

 

Sounds good. Good luck.

 

It won't work ,its a switched mode psu, designed to run from 100v to 230 v, so anything less will not work....

IWHT: the tablet probably recharges from a power unit the same as laptops, the most common voltage I've seen on laptops is 19V.

18V might well be somewhere near what is required - but its important to not lose sight of the fact that at 18VRMS the rectifier will put 1.414x that on a reservoir capacitor. So some form of regulator is likely needed.