What sort of store sells 3V DC adaptors?
July 30, 2012 8:15 AM   Subscribe

I need to buy a three volt DC adaptor with a barrel shaped connector. (Picture the normal 12V DC adaptor, but with a smaller connector on the end that plugs into the device.) And I need to get it this afternoon, in between running some other errands. Where should I look? What sort of store carries this sort of thing?

I don't think I've ever seen odd-sized power supplies sold separate from the device that they power — but then, I've never really looked for 'em either. The device I need this for is something I bought online, so I can't just go back to the store where I got it.

I'm in Austin, TX if it matters.
posted by nebulawindphone to Shopping (20 answers total)
Radio Shack, who carry less and less components as time goes on, is worth a shot for something this ubiquitous. Give them a call before showing up and they should be able to tell you if they carry loose adapters at all.

Also, Fry's would almost certainly have it, but I don't know how near to one you are.
posted by griphus at 8:19 AM on July 30, 2012

Radio Shack carries a wide range of adapters, or at least they used to.
posted by brianogilvie at 8:20 AM on July 30, 2012

Best answer: I was going to suggest Fry's also. What you need is an universal ac to dc adapter that comes with a variety of DC plugs (and goes as low as 3 volts)
posted by bluefrog at 8:23 AM on July 30, 2012

Here is something from RS. A bit overpriced, but convenient if you're in a hurry.
posted by bluefrog at 8:28 AM on July 30, 2012

Radio Shack, Best Buy, or your local AAA or other travel agency office will likely have these.
posted by valkyryn at 8:35 AM on July 30, 2012

Best answer: Also, you didn't note it, but sure you get the correct amperage and connector width as well. Both of these things vary.
posted by griphus at 8:42 AM on July 30, 2012

Response by poster: I was just about to ask that as a followup question, actually. How do I work out what amperage I need? It doesn't seem to say on the device itself, and I can't find any info online. (Under "power requirements" the manufacturer's brochure just says DC 3.0 V when using the AC power adaptor and doesn't tell you how many ants it draws.)
posted by nebulawindphone at 8:47 AM on July 30, 2012

Response by poster: (Ants? Amps. Please do not infest my recorder with ants. I do not think that will help.)
posted by nebulawindphone at 8:47 AM on July 30, 2012 [2 favorites]

Best answer: According to this it's a 3V, 1A, center positive. I can't vouch for the veracity, obviously.
posted by griphus at 8:56 AM on July 30, 2012

Response by poster: Ah, never mind. I dug up the actual manual and it's in there. One amp. Mkay.

But I'm still curious, in a teach-me-to-fish sort of way. Would there have been any way I could have calculated this or made a reasonable guess? (Based on the type of device it is, maybe? Or the number and type of batteries that it takes? Or...?) Or is this something that is unguessable and absolutely has to be specified?

(Electricity fascinates me, but it also makes me feel like a total moron.)
posted by nebulawindphone at 8:57 AM on July 30, 2012

Watts = Amps x Volts, so Amps=Volts/Watts
posted by bluefrog at 9:07 AM on July 30, 2012

Aren't modern DC adapters generally current-limited devices? That is to say, they will only deliver as much amperage as they are asked for, regardless of the load? I know older transformer-based adapters had to be correctly matched to keep them from blowing out whatever they were attached to but I think with modern switchmode adapters the amperage rating is more of a capacity than a fixed output. So you can just err on the safe side and try to get an adapter that seems a bit more powerful than you'd really need.

Someone please correct me if I'm wrong.
posted by Scientist at 9:09 AM on July 30, 2012

Best answer: There's a semi-standard diagram convention on devices that use barrel connectors for power, something like:

3V 1A - -C *- +

where the very bad ASCII-art is supposed to be a representation of the tip/ring in the connector. If it's not on the device, check the manual.

What you need to know to replace one of these is:
  1. Voltage — if it needs 3V, 4.5 V or 6V, that's what you have to give it. Not below, not above. You can't tell how well regulated your device is internally, so stick with exactly how it's rated.
  2. Power/Current — sometimes the rating is given as current (in your case, 1A), sometimes as power (3W). As bluefrog said, there's a simple relationship between these in DC. As long as your adaptor provides at least enough current as the rating, you should be good. You can be a bit over in current, and you'll be fine (so 1.5-2A is okay). Going too large means you're paying for what you don't need.
  3. Polarity — the tip/centre can be positive or negative. Centre negative (+ -C *- -) is rarer, but happens. This matters, and is sometimes switchable for those multi-adaptors you can buy.
  4. Connector size — the great thing about barrel connectors is that there are so many standards to choose from. Don't try to fudge the wrong size.
And yes, there are also AC power adaptors which provide low-voltage AC; beware of these in the surplus store. These are rare for devices that can also run off batteries.
posted by scruss at 9:23 AM on July 30, 2012 [1 favorite]

Here are the symbols scruss is trying to draw.

Also, a follow-up question for scruss (that the OP might find useful): the spec sheet says the power consumption is 0.27W, which wouldn't work in the W=A*V equation, as it yields 11A. How does that work out?
posted by griphus at 9:35 AM on July 30, 2012

The wattage rating given in the specs is probably an estimate of real-world power consumption over time, but the power supply needs to be able to provide enough current for short spikes of high demand.
posted by contraption at 9:58 AM on July 30, 2012

Scientist - you are not completely wrong, though cheap adapters which deliver higher voltages under lower load still exist. The key word here is regulated. An unregulated adapter might be built such that it's 5v at 1A but you'll get a higher voltage under less load.

It's also more common for electronics to have on-board regulation to deal with imprecise input. The question becomes HOW imprecise.
posted by phearlez at 10:14 AM on July 30, 2012

Best answer: Actually, griphus, bluefrog has the second equation wrong: P = I×E, so I = P/E = 0.27/3 = 0.09A. This is a tiny amount, so basically almost anything would work. We love devices with no moving parts!
posted by scruss at 10:32 AM on July 30, 2012

Radio Shack. And bring the device with you, to make sure the tip fits in.
posted by Marky at 11:10 AM on July 30, 2012

Response by poster: Right. So to make sure I'm getting all this:

I went and bought an adjustable adapter with regulated output. It's got a 3V setting, and it promises you can use it up to 2500 mA on any voltage setting.

It sounds like this might be overkill in the sense that I could have gotten away with something way dinkier. But if I'm understanding correctly, it won't fry my gear or catch on fire or anything really upsetting like that. Yes?

If you still have the packaging for your M10, check it first. My M10 came with an external 3V power supply.

Yeah, mine did too, but it's stashed in a box somewhere and I'm leaving for the field tomorrow. :)

posted by nebulawindphone at 1:34 PM on July 30, 2012

Response by poster: Beautiful. Thank you!
posted by nebulawindphone at 3:16 PM on July 30, 2012

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