Even Goldilocks can't find the one that is just right.

December 17, 2008 11:58 AM

How closely do I have to match transformer voltage?

I have a box full of old electrical transformers and my daughter has a device that has a dead one (a replacement is on the way so this is a temp fix). I'd like to find one to match but I don't have one with the exact DC output.

That is, the device requires 4.8 I have 4.5 and 5.0. Assuming plug compatibility which one is best? My uneducated guess would be to go lower rather than over so I don't fry the delicate little circuits.

posted by trinity8-director to Technology (8 answers total)

This really depends on what sort of device you're talking about. What is the device?
posted by strangecargo at 12:24 PM on December 17, 2008

What type of device is it? Also make sure you have the right polarity as hooking that up incorrectly can fry electronics too.
posted by TedW at 12:24 PM on December 17, 2008

Previously, on AskMe.

if you're talking about the actual transformer (as in the component), then either should be fine for temporary use.

some devices have switch-mode adapters, where there is no actual transformer involved. these are less interchangeable than the older linear type that do have transformers. here is some info.
posted by ArgentCorvid at 12:41 PM on December 17, 2008

I'd be more concerned about the current. If the transformer current rating is too low, it'll smoke.
posted by Class Goat at 12:54 PM on December 17, 2008

Best answer: 10-20% in the same ballpark current range (i.e., if your old brick is a 5V, 500 mA and you have a 6V, 500 mA, you are probably fine.) If it's a regulated supply, you're OK with any current range (i.e., 6V, 2A). If it's unregulated and a much higher current supply, then the unregulated voltage will be considerably higher than the number on the brick if it is lightly loaded. (A 6V, 2A unregulated brick unloaded or lightly loaded may show 10V if read with a meter.) These are all ball park rules of thumb.

Get a voltmeter and see what the voltage output of a candidate brick is, unloaded (i.e., plugged in but not connected to your device.) If it reads what is on the brick, it's regulated. If it is a lot higher, it is unregulated.

Verify polarity of course.
posted by FauxScot at 1:02 PM on December 17, 2008

Response by poster: The device is a small digital picture frame.
posted by trinity8-director at 1:16 PM on December 17, 2008

Technically, there are a lot of pitfalls that might cause trouble. Nonetheless, if you follow FauxScot's rules, it is overwhelmingly likely that a substituted power adapter will 'just work'. So, you know, be prepared for something bad to happen, and be happy when it doesn't. :)
I've reconsidered one or two of the details in that link, but the issues are not of practical importance here.
posted by Chuckles at 4:28 PM on December 17, 2008

There was a cheap WiFi router that shipped with 12V power adapters. The correct voltage was 5V. They ran hot, is pretty much what everyone found, and the reliability problems endemic to the router were due to overheating. If you dropped the power down to the proper voltage, it would work fine. Eventually damage would have been done at the higher voltage, probably, but there can be a fair latitude before you start blowing stuff up.
posted by kindall at 10:19 PM on December 17, 2008