All too often, people plug the wrong AC adapter into an electronic device. People just plug in the first adapter that fits, and usually when they do this, if the equipment wasn’t blown before, it is now.
They’re known by many names, most of them not affectionate: power bricks, wall warts… But you miss them when they’re gone.

Universal AC adapter

This “universal” AC adapter isn’t completely universal, but its adjustable voltage and polarity give it a good start

I was doing some cleaning up over the weekend when I came across my 8-port dual-speed hub. It died a couple of months ago, and rather than figuring out what went wrong, I just replaced it with a 5-port switch. But since sometimes I need that many ports, I decided to see if I could figure out what went wrong.

I suspected the problem was with the AC adapter rather than the hub itself. The reason is pretty simple: When a computer won’t power up, the first thing I do is replace its power supply. In years of repairing computers, I’ve seen plenty of blown power supplies. I don’t know if I’ve ever seen a motherboard blow except in cases of a user or technician doing something wrong. It’s not impossible for a circuit board to go bad while it’s just sitting there pulling power, but it’s rare. A good general rule of thumb for electronics: Power supplies blow a lot more frequently than any other electronic component.

So, suspecting a blown AC adapter (remember, those wall warts are power supplies), I went hunting for a substitute. This is something you should do very carefully.

This principle works not just for hubs, but for anything that uses a wall wart: scanners, cable and DSL modems, Zip drives, video games, etc.

You need to look at several things when substituting AC adapters:

1. Volts. You can sometimes get away with a slightly lower voltage than the original. You don’t want to risk going higher. Giving it too much voltage is the most likely way to blow something up.

2. Polarity. The polarity is indicated by a diagram on the adapter and/or the device that looks something like this:

- --Co-- +

This indicates that the negative power is on the barrel, and the positive power is on the tip. Reverse the polarity, and the best case scenario is the device won’t work right. The worst-case scenario is that you’ll blow something up.

If you’re handy with a soldering iron, you can, of course, reverse polarity by snipping off the connector and reversing the wires.

3. Amperage. Steve tells me amperage is charge flow, whatever that means. Think of voltage as speed. With voltage, you don’t want to be too high. With amperage, you don’t want to be too low. If the adapter’s amperage is too low, you can expect it to heat up and possibly blow the adapter.

I think my dad once explained amperage vs. voltage with a garden hose. Amperage was the amount of water coming out of the hose, while voltage was the speed of the water coming out of the hose. When he put his thumb over the hose, the water flowed faster (voltage increased) but the amount of water flowing decreased (amperage decreased).

I think I got the details right. He was the one with the bachelor’s degree in physics (not to mention chemistry and biology, and that med school thing), and I think I was 12 at the time.

Useful information that won’t make your eyes glaze over: High amperage is more likely to kill you than high voltage.

4. Wattage. Wattage is a measure of the total amount of power available. Watts = Volts * Amps. If you have two of the three ratings available, you can use simple algebra to calculate the other. The watts and amps on the replacement should be very close to or higher than the original. Remember that.

5. Last and least, the size of the plug. If the plug doesn’t fit, you obviously can’t test anything, but if the plug doesn’t fit and everything else is wrong you haven’t blown up the device, now, have you? Plug size and polarity are the two things that you can change relatively easily. You can always lop off a plug and solder the right-sized plug onto the wire.

In my case, my hub takes a 5-volt DC, 3-amp adapter. A Google search revealed those specs aren’t terribly common. I found an original OEM replacement on Netgear’s site, but we’re looking at 20 bucks and they had no stock. A 5-volt adapter usually costs about 5 bucks. And since the adapter died, I’m not sure I want an OEM replacement. When something breaks, I’m usually not inclined to buy something identical if I can help it. I found a 5-volt, 4-amp adapter that would fit, but the polarity was reversed. It was 6 bucks, but the vendor had a minimum order of $25. Rats. Did I want to spend half the cost of a new unit to find out if my old one was bad? I was starting to remember why that 5-port switch looked so attractive.

So I dug through my collection of AC adapters. Most of the stuff I had was 9v, 12v, or in one case, even 16v. Finally I spotted a 5v adapter with the correct-sized plug and the correct polarity. Hey, my Zip drive proves itself useful for something other than losing data! The only problem was it was rated for 1.5 amps–half of the original’s rating.

So I called up Steve DeLassus, since he’s got a degree in electrical engineering and I haven’t had to think about this kind of stuff since high school. Knowing that asking him a question about amps is like asking me how to figure out the verb in a sentence, I posed the elementary question and got his response:

I can’t harm the device, but I might harm the power supply. Then he wondered aloud how a hub could draw three amps. I let him ponder that question.

My response to that thought, based on my years of experience as a journalist: Since low-end hubs are commodity devices and people generally buy them on price (people generally only buy 3Com or Intel on name recognition), why would a second-tier vendor spec a power supply that’s any beefier than necessary? Maybe it’s ridiculous for a hub to draw 3 amps, but for all I know, the hub has a ridiculous design inside.

I plugged the hub into the power supply with nothing else connected to see if it would light up. And light up it did. I quickly unplugged it, since I didn’t want to melt the replacement AC adapter.

So now I know that I need a new power supply for that hub, and since I had parts laying around, it didn’t cost me anything to find out. Better yet, I didn’t blow anything up in the process and I know I didn’t blow anything up. No wondering, no worries.