Power adapter voltage question.

October 25, 2005 4:52 PM

I need a 6V positive-tip power adapter for a USB hub. I have a bag full of old adapters of varying voltages. I know the 6V means that it should not exceed the 6 volts, but can I use a lesser-volt adapter?
posted by Edward King to computers & internet (8 answers total)

but can I use a lesser-volt adapter?

Usually yes, it won't hurt anything. Usually. If there's a switching power supply inside the router converting the voltage to something else this could be bad. Otherwise at worst it just won't work.

Check that your power supply outputs the required current (or more) before using it though. If it's not enough then don't use it. If it's LOTS more, then it will already be outputting somewhat more than its rated voltage under the small load unless it's regulated (unlikely), which, in this case, works out well for you.
posted by shepd at 5:02 PM on October 25, 2005

usb devices want 5v, but that doesn't automatically mean your actual hub will be happy with <6 v. worth a try tho.br>
or, what shepd said.
posted by dorian at 5:05 PM on October 25, 2005

Why would switching power supplies be hurt by undervoltage, again?
posted by Krrrlson at 7:43 PM on October 25, 2005

For the same power, under voltage means over current. Too much current means too much heat, and you get a failure.

There should be an internal current limit that will keep the device from failing, but it isn't always the case.
posted by Chuckles at 7:53 PM on October 25, 2005

this is why the regulated multi-voltage adapters at ratshack cost more than the non- ones: with the regulated ones your device will only get as much current/voltage as it actually draws, even tho the adapter, while set at the proper voltage, may be rated at much higher current.
posted by dorian at 8:03 PM on October 25, 2005

That previous comment was in answer to Krrrlson, by the way.



It is also worth noting that power brick replacement is always a little risky. The unloaded voltage of a line frequency transformer type power brick is much higher than the rated voltage of the brick (rated voltage is measured at rated current, not at no load). This isn't true for a switching power supply type power bricks because they have much better load regulation (the voltage doesn't change as much when the load current changes).

If you replace a 250mA line frequency transformer with a same voltage 3A line frequency transformer, you can cause a failure because the device being powered doesn't draw enough current to pull the output voltage down.

If you replace a switching supply based brick with an identically rated (both current and voltage) line frequency transformer, you can create the same problem. Devices designed for the constant output voltage of a switching supply may not be designed to take a line frequency transformers higher low load voltage.

That is all really technicality though... Just try it and see what happens! Nothing in life is guaranteed after all.
posted by Chuckles at 8:12 PM on October 25, 2005

You all have been great. Thanks! I really need to bone-up on basic electricity.
posted by Edward King at 8:54 PM on October 25, 2005

I know the 6V means that it should not exceed the 6 volts, but can I use a lesser-volt adapter?

Actually, that is not correct. The hub most likely needs a minimum of 6V input to operate correctly. This is because the hub must supply a precise 5V to any device plugged into it. This is done with a linear voltage regulator in the hub that generally requires at least 1V of headroom to produce 5V.

A 6-volt brick is most likely unregulated which means that it outputs about 9V with no load and 6V at its maximum rated current. A self-powered USB hub requires at least 500 mA for each downstream port for a total of 2000 mA for a four-port hub. So this means you need a supply that will output at least 6V at 2000 mA. Both of these values should be printed on the back of the brick.

Using a supply with less than 6V will not harm anything, but might not work very well either, including the devices you plug into it. So could you use a 9V supply? Well that might be a bit more risky. Although the hub's voltage regulator can probably tolerate a higher voltage input, it won't be able to handle the extra heat generated by the higher voltage, causing it to shut down.

But, heh, if you don't mind the possibility of seeing a little smoke, I'm all for experimentation.
posted by JackFlash at 12:57 AM on October 26, 2005