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Testing a blown AC adapter

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.

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14 thoughts on “Testing a blown AC adapter”

  1. Man this brings to mind a really funny, yet stupid human story. Working at a camera shop I see all sorts of idiots.

    We had this guy who for about a year came in always looking for Nikon Nikkormat F’s. It is one of nikons very first SLR camera models.

    Almost every camera from that era have what is called a PC socker. A small plug to hook up an external flash. A good portion of cameras from that time didn’t have hot shoes on them to mount a flash.

    Anyway, there is a type of plug on some flash cables that looks like a house hold ac plug. The cords connect a camera to a flash or some other device to fire the flashes.

    This guy for some reason kept taking those cords, plug the PC end into the camera, the other end into the wall outlet! Sparks always flew, the flash wiring was fried! Ther cameras still fired as they were all mechanical except for the flash wiring and light meter. Of course after these episodes, the meter didn’t work either. He bought like 3 or 4 in one year.

    None of us ever asked why he just didn’t stop plugging them into the wall outlet. We got a few repaired for him, but he stopped that as the cost got out of hand.

  2. I’ve sometimes had good luck just getting one of those Radio Shack (blush) 1-amp multi-voltage wall warts. Granted, it can be hard to overcome the shame of walking into a Radio Shack in the first place, but in these days when electronics as a hobby is practically extinct, it can boil down to either that or some industrial vendor’s minimum order.

  3. For devices like answering machines and cordless phone bases, those multi-voltage adapters are fabulous, since they come with a variety of plugs and you can reverse the polarity. My answering machine runs off one of those, since I lost the original in a move.

    And most consumer electronics stores carry them, if you want to avoid paying an inflated Radio Shack price, though Radio Shack will probably have a better selection and/or higher-amp units.

    Unfortunately, finding an AC adapter at Radio Shack that has enough oomph to run a hub or a scanner isn’t very likely.

  4. Dave,

    Great article on a subject that many would like to read more about! Like you said, you are not an electrical engineer but you still manage to pull off a description of this issue to your readers, making good sense and keeping it simple as well.

    I got a few pointers, mostly additions to what you have written:

    1. Your rules for substituting simple adapters (point 1). Your idea on voltage being able to vary a bit is true of course, but the rules are in reality pretty much fixed since most components (like IC’s) have specifications on how much the voltage can vary. Your hub, for example, probably doesn’t have a 5 volt regulator that can fix any voltage variations so I would in your case recommend an adapter where you could be pretty sure that the voltage wouldn’t dip too much.

    1. Point 4 is correct only if you are using direct current along with set resistance (things like chopped current don’t factor in – simple adapters like the ones you are talking about would fit this picture).

    1. The power calculation you used on your hub. You said it uses 5 volts, 3 amps and thought it was strange. Well, using your formula, voltage x current = watts. That would mean that your hub draws 15 watts (3 x 5 =15). My switch uses 12 watts average.Specifications tell me 1 amp current is needed along with 12 volts (1 x 12 =12). This should show you that your hub isn’t so far off.If you consider the function of the hub/switch then 12-15 watts isn’t that much at all.

    2. Which leads to a question. Why does my switch require 1 amp, while your hub requires 3, while they are similar in their power consumption? That has to do with construction. Many components are rated for 5 volts usage (IC components). If you are using an input voltage that is higher then you need to have a voltage regulator that shifts from your higher voltage to the lower (5v) one. This mean s extra components which costs money and space. Regulators are really cheap but if you consider a company building 200.000 hubs and the regulator costs them a dollar then you are looking at a cost of $200.000 dollars. Consider the same issues as when motherboard manufacturers set 5 PCI ports on their motherboard instead of 6 because the actual socket costs them 20c in quantity of 100.000…..

    3. The trick you pulled with your adapter, to pull the cord before it boiled over (figuratively speaking) was good. Although in my opinion and experience you could have just put your hand over it and pulled the plug once you felt it getting warm. Adapters are really simple constructions (using windings) and they can take a real beating before breaking (also talking from experience – I have designed a couple). This doesn’t work on adapters that generate an average current that is so low that your unit won’t heat up.

    4. Your ideas on current and voltage multiplied being equal to power delivered is true for very simple designs. However, remember the power supply we got in our computers. They are switched which means that easy calculations don’t work. They pretty much work for simple adapters like the ones driving our modems, hubs, switches, USB hubs and so on but for more advanced, switched power supplies those ideas are not so simple.

    I will gladly give more feedback if you or your readers are interested. And last but not least, I hope this rant has made sense since the window I am writing this in is so small that it doesn’t give me any idea on what I have written looks like! Consider that a constructive criticism on your new site ๐Ÿ™‚

    Keep up the good work.

    Dave T.

  5. OK, I made some edits. I said “watts” once where I meant “amps,” and I didn’t explain amps right, so I recounted a demonstration my dad did with a garden hose about 15 years ago. So now you EEs can have some more fun with the journalist. ๐Ÿ™‚

  6. OK, OK. What do you call a journalist hanging from a power line? Just kidding. ๐Ÿ™‚ Maybe the next time Dave is over, I’ll have to re-live that prank with the charged capacitor and the metal chair…

    And Dave T., let’s not get too far into the technical stuff. You’ll start reminding me of things like Wheatstone bridges, wave inverters, thermistors, and transistor beta calculations. I *did* throw away my EE degree, after all. ๐Ÿ˜‰

    And you’re right: this is one friggin’ small edit box…

  7. Steve,
    Once I had started writing this old stuff came back in waves! I couldn’t hold back :-)Hope I didn’t bore you too much with my rant ๐Ÿ™‚

    Dave T.

  8. Hi, your conversation is very interesting. I bought a used external iomega zip drive 250MB without ac adaptor. I thought it would be easy to find a adaptor… but not really.

    The power requirement on the back of the device said :
    5V „Ÿ„Ÿ„Ÿ„Ÿ 1.0A

    I found a 5V GlobTek adaptor with 2.4A. If I understand the first paragraph in this discussion, it doesn’t hurt to have more amperage (tell me if I’m wrong). But my problem is with the polarity. I don’t know what is the right polarity for the zip drive.

    Does anyone know what is the proper polarity to use with a iomega zip drive ? (or where I can find this information)

    You can also send me a email at

    thanks, Mario Cote

  9. Mario, my stock ZIP power supply shows ‘center positive’ for the plug.

    …and you should be okay with the heavier amperage; that’s not too far outside the design parameters for the power supply and the ZIP should load it down just fine. If you were a paranoid, your concern would be overvoltage due to too light a load (unlikely: If you’re concerned about that, just add a temporary pigtail to the plug wiring and put a voltmeter on the system. Of interest, the external battery packs for the ZIP drive spec an output of 6 volts).

  10. USB and Firewire are just data connectors. USB connectors are small and rectangular, Firewire connectors are rounded on one end. USB can carry power, though not a large amount. I don’t know about Firewire.

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