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I have an ~10-year-old hub (Surecom EtherPerfect 505ST if anyone's interested) that says "DC 7.5V" below its power connector. Unfortunately, I can't find the adapter, but I do find one that fits but outputs 12V. My question is: is a modern computer peripheral device that takes an adapter likely to be able to safely regulate down from 12Vdc to 7.5Vdc? Since it's an old bit of hardware I'm not terribly worried about losing it forever, but I have to weigh the risk of damaging it against the likelihood of success if I keep looking for the proper adapter.

(Or maybe I have found the proper adapter and the one that came w/the device supplies more than 7.5V)

  • This a similar question for PCI cards (5Volts to 3.3Volts): superuser.com/questions/839893/… . One of the conclusion (until now) that makes some sense: «[...] the motherboard is properly throttling down the voltage on that slot to 3.3 Volts». – Sopalajo de Arrierez Dec 13 '14 at 16:46
  • @Ramhound I'm pretty sure than a 12V adapter is guaranteed to deliver 12V as long as the current requirements are met. – PenguinLust Dec 13 '14 at 16:53
  • @SopalajodeArrierez Unfortunately, a motherboard is very different from a peripheral device and 5V to 3.3V is much less than 12V to 7.5V. The percent change (approximately -35%) is nearly the same, though. I'm not sure which metric is more telling – PenguinLust Dec 13 '14 at 16:56
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You should never use a significantly higher voltage rating adaptor on any kind of electrical device. If you do so you are risking damaging the equipment and anything else that you might have plugged into it.

You can, on the other hand, use a higher current rating without worrying about the device you are plugging it into. Devices only take as much current as they need.

Think of it like a pipe, with a vent at the end. The voltage is the force of the water in the pipe, the current is the thickness of the pipe. The vent will allow a particular trickle of pressure (voltage) to pass through. Too little pressure will not allow the vent to work, but too much pressure will blast the vent off and destroy it. The thickness of the pipe (current) is largely irrelevant to the pressure in the pipe, what is important is the amount of pressure that the contents are under.

An 8V adaptor might be okay, but it depends entirely on the electronics in the device you are powering. Given that a 7.5V adaptor is already pretty odd I would suspect it is asking for that voltage for a reason. 12V will almost certainly lead to an abruptly shortened life for what you are plugging it into.

  • +1 for 12V will almost certainly lead to an abruptly shortened life for what you are plugging it into. – Andrew Morton Dec 13 '14 at 18:11
  • Now that I think about it, it doesn't make a lot of sense that an ad hoc device that will be connected to a power supply w/a regulator of its own will be designed w/another regulator. Unless because of some production oddity, it would be cheaper to design that regulator and take an adapter from off the shelf. Incidentally, I've an old router that rates itself requiring 5Vdc, but the adapter I have w/it is 15Vdc. But I guess you're going to tell me that I have mismatched it. – PenguinLust Dec 13 '14 at 20:05
  • @PenguinLust the problem is that there are some devices that can be perfectly fine with doing that sort of thing, especially if they have an old cheap and inefficient regulator. The problem is that that older regulators waste all the extra voltage as heat and so work themselves into an early grave that way. Newer devices tend to use power supply circuitry that expects +/- 10% nominal voltage and can be outright killed by anything more. It's hard to know what is in what piece of kit so the rule of thumb is that even if it does work then it's still going to die sooner than it would have. – Mokubai Dec 13 '14 at 22:15

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