Power adapter advice - Magic smoke from 12V 1.5A portable hard disk - Page 1
Forum section: electronics / general

author: mroguy
author_rank: Newbie
timestamp: July 01, 2021, 06:19:59 pm
content: Hi all,I plugged a 12V 5A power adapter into a portable hard disk which has 12V 1.5A (1.5A) written on the back. It started smoking and is now completely gone.This was following the advice of a tech guy at my local Apple store who told me that as long as the Voltage matches then there will be no issues. I've seen other posts online that back this up but obviously something has gone wrong here.Can anyone give crystal clear advice on the rulset for mixing adapters? I've got about 20 portble hard drives and no adapters for them (lost during a home move). Do I need to match the exact Ampage on these as well as the Voltage. Have I just been unlucky here or is there a fixed rulset for this sort of thing.Hoping some of you guys can advise.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 01, 2021, 06:22:00 pm
content: Polarity must match! Not all barrel connectors are wired center positive.

author: mroguy
author_rank: Newbie
timestamp: July 01, 2021, 06:24:08 pm
content: Thanks BrokenYugo. Going to as a dumb question but how do i tell if the adapter polarity matches the hard drive? the 12V has the minus symbol on it. Next to that there are three dots with a +.Does that help me figure it out?

author: tooki
author_rank: Super Contributor
timestamp: July 01, 2021, 06:34:08 pm
content: You should find one of the two symbols here:They need to match on adapter and device.If an adapter lacks a symbol, it’s easy to measure with a multimeter. Determining what an unmarked device requires can be trickier.

author: Ian.M
author_rank: Super Contributor
timestamp: July 01, 2021, 06:34:21 pm
content: Quote from: BrokenYugo on July 01, 2021, 06:22:00 pm*THIS*However its incredibly annoying when a device has its power requirements marked on it without its DC jack polarity. In many cases there's an I/O connector somewhere on the device with a nominally grounded shell, so if the DC jack polarity isn't marked its a good idea to test continuity between each contract of the DC jack and the other connector shell. If there's good continuity, odds are that contact is DC negative. However if there's any doubt, open up the device and trace the circuit from the DC jack inwards. There's usually a polarized capacitor, an anti-parallel or series diode or an identifiable regulator IC you can look up specs for which will give you the DC jack polarity and even some idea of the operating voltage if the label is missing.Your portable hard drive may be salvageable - there are often protection diodes that fail shorted if a high current reversed polarity supply is used. Don't give up on it without investigating it and posting good closeup photos of the damage here.

author: mroguy
author_rank: Newbie
timestamp: July 01, 2021, 06:49:03 pm
content: I drilled the portable hard disk out of its case and bought a docking station for it. Unfortunatley it doesn't power up in the docking station (AX-U3DS012NEW) so i think it's gone.The power adapter I used does not have polarity on it. However, that may be because it was shipped with it's specific device. I took it from a BT (british telecom) wireless router. In fact the only markings on it are BT 200-240V 50/60Hz 12V 5A Efficiency Level VI see loads of posts online about adapters and checked with a local shop about plugging it in. I did not see any mention of polarity on either which is a bit annoying. ... just multiple sources saying... bang it in and if the voltages matches then no problem...Thank you so much for the info... I will spread this knowledge far and wide

author: mroguy
author_rank: Newbie
timestamp: July 01, 2021, 07:08:30 pm
content: Now looking for a switchable adapter with interchangable sockets. Loads on amazon but not found one yet where you can switch polarity also.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 01, 2021, 07:21:14 pm
content: If you have a multimeter (anything will do), you can measure an unmarked adapter quite easily. Just plug the mystery adapter in and probe the connector's outside and inside (stick a pin in it if the probe is too big), common/black to outside, red to center. A positive reading indicates center positive, a negative reading (or the needle of an analog meter trying to go the wrong way) indicates center negative.

author: Ian.M
author_rank: Super Contributor
timestamp: July 01, 2021, 07:49:39 pm
content: Quote from: mroguy on July 01, 2021, 06:49:03 pmIf you are lucky there may be a shorted protection diode on the hard drive itself somewhere near its power in connector. Getting it back to life to recover data *MAY* be as simple as locating the protection diodes on the +12V, and +5V rails, testing if they are shorted, and if so, desoldering them and trying it again. If you do manage to revive it, and intend to keep it, post a good closeup of the PCB + the failed diode(s), and we can probably identify suitable replacements.

author: james_s
author_rank: Super Contributor
timestamp: July 01, 2021, 08:15:36 pm
content: Most AC adapters are center-positive, but not all. Music gear in particular is almost always center-negative and I have fixed a number of synths and samplers that somebody plugged into the wrong adapter assuming that if the plug fits it will work. If an adapter is not clearly labeled you should *always* check it with a meter to confirm. It's actually a good idea to do that anyway just to be sure that it's operating properly and putting out the voltage it says it should be.

author: mroguy
author_rank: Newbie
timestamp: July 01, 2021, 08:24:05 pm
content: Thanks everyone, Ian... I'd like to say I would do that fix but I'll probably never get around to it.This thread has been a real eye opener. I'm surprised that the devices arn't smart enough to detect the correct polarity. Surely it cant be that difficult to build in some sort of safegard. Anyhow. thanks again for responding.

author: james_s
author_rank: Super Contributor
timestamp: July 01, 2021, 11:14:38 pm
content: It isn't that difficult, but it costs money, isn't foolproof and it adds another potential failure point. The manufacture doesn't really care, they sell you a device that comes with a power adapter and they expect you to use the power adapter they give you.

author: tooki
author_rank: Super Contributor
timestamp: July 03, 2021, 06:38:00 pm
content: Quote from: mroguy on July 01, 2021, 08:24:05 pmNo, but in addition to the things james_s said, it also adds potential energy losses.The center-negative configuration used to be dominant because it enabled a simple switched contact in the DC socket to switch between batteries and the AC adapter. We didn't always have "smart" electronics with the benefit of cheap but insanely complex ICs to handle every little thing. With smarter electronics came the switch to center-positive, which has some different advantages (namely, that "ground" is what's facing outward).Quote from: mroguy on July 01, 2021, 07:08:30 pmYou mean a universal AC adapter with interchangeable DCplugs? If so, they usually don't bother with a polarity switch, because they simply design the interchangeable plugs to attach in two orientations for the two polarities.

author: amyk
author_rank: Super Contributor
timestamp: July 04, 2021, 01:54:20 am
content: Quote from: tooki on July 03, 2021, 06:38:00 pmI don't think centre-negative was dominant for anything except audio and some telephony equipment, where it was used for some reasons having to do with shielding, noise reduction, and electrolytic corrosion. A switched contact could just as easily switch the other rail too...

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2021, 04:14:41 am
content: Quote from: tooki on July 03, 2021, 06:38:00 pmYou can do that with center-positive too. It doesn't matter whether you switch the positive or the negative, the batteries are isolated and floating, they don't know what "ground" is. I don't see how it can make any difference either way.

author: tooki
author_rank: Super Contributor
timestamp: July 05, 2021, 11:49:06 am
content: On a DC barrel jack, you can only switch the outside contact. There’s no way to switch the center one.Regardless, the point is that it used to be common. That it’s inconvenient today to encounter both is undisputed, but it is what it is.Ground facing outward on a barrel plug has an advantage: if it shorts against another connector, it’s less likely to cause a short (since most plugs, if they have any exposed metal, have it as ground). Not all power supplies are floating.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2021, 06:29:35 pm
content: Quote from: tooki on July 05, 2021, 11:49:06 amSure, but what difference does that make? When would it ever matter whether you disconnect the battery positive or the battery negative when you plug in the external supply? Either way accomplishes the same goal of disconnecting the battery from the circuit.I don't recall ever seeing a barrel plug used on a power supply that was not floating. I would argue that it is an inappropriate connector to use for such a supply.