Wednesday, October 01, 2008

Adapting Adapters

To re-use an AC adapter for a different gadget, you need to know the voltage, amperage and polarity of both the device and the adapter. I don't throw adapters out even if their gadget is broken--I can often re-purpose the adapter to work with a new gadget.

Adapters and most appliances that use them will have something like this:

We are concerned with Output and the funny symbol that is (on this one) in the bottom right corner.

The input doesn't really matter as long as it was designed for the US (or whatever voltage you have where you are). Output is what we are concerned with.

Voltage should match as closely as possible. Undervoltage is unlikely to harm a DC gadget, (especially one that is meant to run on batteries) although it might not work. 2 or so volts over probably won't hurt most gadgets either, but there is no guarantee. This adapter is marked 13.8v, came with computer speakers asking for 12 volts, and I used it successfully on wireless speakers asking for 15.

98% of gadgets with adapters are DC, but there are a few AC.

If no voltage is listed, but the appliance takes batteries, count 1.5 volts per battery.

Amperage (Amps, A, mA)is in some ways more flexible--the adapter should have more amp capacity than the appliance--Less is no problem at all. This is an unusually large adapter, with an unusually high rating of 1.7 amps. Most adapters are rated in mA, somewhere between 100 and 800.--1,000 mA is equal to 1 amp, so this is equal to 1700 mA. It is adequate to power anything taking this voltage with 1700mA or less current--The appliance will only take what it needs. If the gadget needs more than the adapter can supply, it will likely either burn the adapter out, or blow a difficult to replace fuse inside the adapter, without harming the appliance.

Polarity is the final concern. The symbol in the lower right corner shows the polarity at the plug--There is a negative sign attached to the C portion of the symbol, and a + attached to the center dot. This means that the center conductor or tip is the positive conductor, the outside or back is negative. AC output won't have a polarity.

If the plug fits,voltage is close enough, polarity matches, and amps is more on the plug than gadget, you are good.

Plugs are fairly easy to change--I've had cases where I had an adapter that was electrically right but had the wrong plug, and another adapter with the right plug. Swapping the two gives an adapter that works for the application. The hard part is figuring out the polarity of the plug. Any Radio Shack has multimeters, Harbor Freight has them for a few dollars--That makes verifying polarity (and actual voltage) simple.

Without a multimeter, you'll need to get creative. My experience has been that the marked wire is positive, but I won't guarantee that's always the case. (with zip cord, one wire is marked with a stripe, ridges or lettering, where the other is unmarked) That should let you deduce the proper connection. Most plugs are center positive, and most stuff under about 6 volts won't be damaged by a brief reverse polarity, but don't count on that for anything precious. If you are doing a laptop power supply, get a multimeter to be sure. If you have a Harbor Freight nearby, they have multimeters under $5.

A final method is to wrap wires to the battery clips. Most gadgets will have two obviously separate clips, and usually one or more clips that you can see are connected together. The pointed end of the battery is positive, the negative end is flat. You can usually strip a couple of inches of wire and twist it around the single clips and get your gadget running. If the clips aren't accessible, use old batteries with their ends taped to hold the wire in place.

1 comment:

  1. Also, if a device requires an adapter with an AC output, you can probably use an adapter with a DC output on it.

    If the AC-powered device has a half-wave rectifier inside it, will only work with a DC adapter of one polarity or the other.

    But if it has a full-wave rectifier inside it, either polarity of DC adapter will work just fine.

    You should not exceed voltage by more than a volt or so, and the worst mistake is to use a DC adapter of the wrong polarity.

    A simple cheap rectifier used in a "crowbar" configuration would save the device so a competent tech could fix it after a power cross, but some (mostly Chinese) manufacturers are just too cheap to add it.

    Luckily, most CB radios have them, and many automotive devices do.