AC adapter basics

AC adapter basics:
It seems that the world now revolves around AC Adapters or ‘Wall Warts’ as they tend to be called. There are several basic types. Despite the fact that the plugs to the equipment may be identical THESE CAN GENERALLY NOT BE INTERCHANGED. The type (AC or DC), voltage, current capacity, and polarity are all critical to proper operation of the equipment. Use of an improper adapter or even just reverse polarity can permanently damage or destroy the device. Most equipment is protected against stupidity to a greater or lessor degree but don’t count on it. The most common problems are due to failure of the output cable due to flexing at either the adapter or output plug end. See section below on repair procedure.

1. AC Transformer. All wall warts are often called transformers. However, only if the output is stated to be ‘AC’ is the device simply a transformer. These typically put out anywhere from 3 to 20 VAC or more at 50 mA to 3 A or more. The most common range from 6-15 VAC at less than an Amp.

Typically, the regulation is very poor so that an adapter rated at 12 VAC will typically put out 14 VAC with no load and drop to less than 12 VAC at rated load. To gain agency approval, these need to be protected internally so that there is no fire hazard even if the output is shorted. There may be a fuse or thermal fuse internally located (and inaccessible).

If the output tested inside the adapter (assuming that you can get it open without total destruction – it is secured with screws and is not glued or you are skilled with a hacksaw – measures 0 or very low with no load but plugged into a live outlet, either the transformer has failed or the internal fuse had blown. In either case, it is probably easier to just buy a new adapter but sometimes these can be repaired.

Occasionally, it will be as simple as a bad connection inside the adapter. Check the fine wires connected to the AC plug as well as the output connections. There may be a thermal fuse buried under the outer layers of the transformer which may have blown. These can be replaced but locating one may prove quite a challenge.

2. DC Power Pack. In addition to a step down transformer, these include at the very least a rectifier and filter capacitor. There may be additional regulation but most often there is none. Thus, while the output is DC, the powered equipment will almost always include an electronic regulation.

As above, you may find bad connections or a blown fuse or thermal fuse inside the adapter but the most common problems are with the cable.

3. Switching Power Supply. These are complete low power AC-DC converters using a high frequency invertor. Most common applications are laptop computers and camcorders. The output(s) will be fairly well regulated and these will often accept universal power – 90-250 V AC or DC.

Again, cable problems predominate but failures of the switching power supply components are also possible. If the output is dead and you have eliminated the cable as a possible problem or the output is cycling on and off at approximately a 1 second rate, then some part of the switching power supply may be bad. In the first case, it could be a blown fuse, bad startup resistor, shorted/open semiconductors, bad controller, or other components. If the output is cycling, it could be a shorted diode or capacitor, or a bad controller. See the “Notes on the Diagnosis and Repair of Small Switchmode Power Supplies” for more info, especially on safety while servicing these units.

AC adapter testing:
AC adapters that are not the switching type (1 and 2 above) can easily be tested with a VOM or DMM. The voltage you measure (AC or DC) will probably be 10-25% higher than the label specification. If you get no reading, wiggle, squeeze, squish, and otherwise abuse the cord both at the wall wart end and at the device end. You may be able to get it to make momentary contact and confirm that the adapter itself is functioning.

The most common problem is one or both conductors breaking internally at one of the ends due to continuous bending and stretching.
Make sure the outlet is live – try a lamp.
Make sure any voltage selector switch is set to the correct position. Move it back and forth a couple of times to make sure the contacts are clean.
If the voltage readings check out for now, then wiggle the cord as above in any case to make sure the internal wiring is intact – it may be intermittent.

Although it is possible for the adapter to fail in peculiar ways, a satisfactory voltage test should indicate that the adapter is functioning correctly.

AC adapter repair:
Although the cost of a new adapter is usually modest, repair is often so easy that it makes sense in any case. The most common problem (and the only one we will deal with here) is the case of a broken wire internal to the cable at either the wall wart or device end due to excessive flexing of the cable.

Usually, the point of the break is just at the end of the rubber cable guard. If you flex the cable, you will probably see that it bends more easily here than elsewhere due to the broken inner conductor. If you are reasonably dextrous, you can cut the cable at this point, strip the wires back far enough to get to the good copper, and solder the ends together. Insulate completely with several layers of electrical tape. Make sure you do not interchange the two wires for DC output adapters! (They are usually marked somehow either with a stripe on the insulator, a thread inside with one of the conductors, or copper and silver colored conductors. Before you cut, make a note of the proper hookup just to be sure. Verify polarity after the repair with a voltmeter.

The same procedure can be followed if the break is at the device plug end but you may be able to buy a replacement plug which has solder or screw terminals rather than attempting to salvage the old one. Once the repair is complete, test for correct voltage and polarity before connecting the powered equipment. This repair may not be pretty, but it will work fine, is safe, and will last a long time if done carefully.

If the adapter can be opened – it is assembled with screws rather than being glued together – then you can run the good part of the cable inside and solder directly to the internal terminals. Again, verify the polarity before you plug in your expensive equipment.

Warning: If this is a switching power supply type of adapter, there are dangerous voltages present inside in addition to the actual line connections. Do not touch any parts of the internal circuitry when plugged in and make sure the large filter capacitor is discharged (test with a voltmeter) before touching or doing any work on the circuit board. For more info on switching power supply repair, refer to the Notes on the Diagnosis and Repair of Small Switchmode Power Supplies.

If it is a normal adapter, then the only danger when open are direct connections to the AC plug. Stay clear when it is plugged in.

AC adapter substitution and equipment damage:
Those voltage and current ratings are there for a reason. You may get away with a lower voltage or current adapter without permanent damage but using a higher voltage adapter is playing Russian Roulette. Even using an adapter from a different device – even with similar ratings, may be risky because there is no real standard. A 12 V adapter from one manufacturer may put out 12 V at all times whereas one from another manufacturer may put out 20 V or more when unloaded.

A variety of types of protection are often incorporated into adapter powered equipment. Sometimes these actually will save the day. Unfortunately, designers cannot anticipate all the creative techniques people use to prove they really do not have a clue of what they are doing.

The worst seems to be where an attempt is made to operate portable devices off of an automotive electrical system. Fireworks are often the result, see below and the section on: “Automotive power”.

If you tried an incorrect adapter and the device now does not work there are several possibilities (assuming the adapter survived and this is not the problem):
1. An internal fuse blew. This would be the easiest to repair.
2. A protection diode sacrificed itself. This is usually reverse biased across the input and is supposed to short out the adapter if the polarity is reversed. However, it may have failed shorted particularly if you used a high current adapter (or automotive power).
3. Some really expensive hard to obtain parts blew up. Unfortunately, this outcome is all too common.

I inherited a Sony Discman from a guy who thought he would save a few bucks and make an adapter cord to use it in his car. Not only was the 12-15 volts from the car battery too high but he got it backwards! Blew the DC-DC converter transistor in two despite the built in reverse voltage protection and fried the microcontroller. Needless to say, the player was a loss but the cigarette lighter fuse was happy as a clam!

Moral: those voltage, current, and polarity ratings marked on portable equipment are there for a reason.

Voltage rating should not be exceeded, though using a slightly lower voltage adapter will probably cause no harm though performance may suffer. The current rating of the adapter should be at least equal to the printed rating. The polarity, of course, must be correct. If connected backwards with a current limited adapter, there may be no immediate damage depending on the design of the protective circuits. But don’t take chances – double check that the polarities match – with a voltmeter if necessary – before you plug it in! Note that even some identically marked adapters put out widely different open circuit voltages. If the unloaded voltage reading is more than 25-30% higher than the marked value, I would be cautious about using the adapter without confirmation that it is acceptable for your equipment. Needless to say, if you experience any strange or unexpected behavior with a new adapter, if any part gets unusually warm, or if there is any unusual odor, unplug it immediately and attempt to identify the cause of the problem.