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Electronics help, need to drop 13.2v dc to closer to 12

S

Soybomb

Diamond Member
A friend handed me some stuff and asked if I could help her out with some repairs. One of the items was a usb hard drive with a dead power supply. I dug through my box of crap and found a transformer rated for 12v 1.5A output which I suspect should be plenty for the drive. The actual output from this transformer is actually about 13.2v. I went on and wired it in just to see and no hard drive will spin up. I'm assuming the high voltage is the reason and that the original power supply is internally regulated and the inside of the unit just steps down to 5v.

What's the best way to lower that to something like 12v?
 
That would work fine, when you put a load on it the voltage will come out at 12v. Same way a 1.5v AA battery brand new is closer to 1.65v, when under load it puts out less.

Edit:

Sorry, missed the USB part though. Is the polarity of the tip correct? Look for a little symbol on the adapter that looks like two diamonds on either side of a half-circle. Whichever way the open part of the circle is facing is the tip (inside) and the other is the outside. Usually positive tipped, but it needs to match what the enclosure wants.

Just looked at my SATA dock, the power supply for that outputs both 12v and 5v, it has a special PS/2 style plug on it (multi-pin) to handle it though.
 
DC adapters like that will always put out quite a bit of extra voltage when they're unloaded, as Sphexi said.

The 12V is probably to power the hard drive's motor, which is why it needs an external adapter. The drive may get the 5V it needs from the USB port itself, though that's just my guess.

Yes, do check the polarity of the adapter versus that of the drive enclosure.
The diagram described above would look something like this.


 
Yeah polarity is correct. From a little googling it looks like the max power varation atx allows is 5% which would be 12.6. Could the load of a hard drive not be enough on a 1.5A wall wart to reduce it enough?

Would most hard drives have a voltage cut off where they won't power on above it? I've tried a few drives in it and none will spin up in the enclosure (no ide cable connected) and they all spin hooked up to a desktop psu (again, no ide cable). I tossed in a 2.5" drive (5v of course) on a converter and it spun up and even installed in windows when i plugged in the usb cable. The only possible thing I could come up with a power problem and red is 5v exactly.
 
Originally posted by: Soybomb
Yeah polarity is correct. From a little googling it looks like the max power varation atx allows is 5% which would be 12.6. Could the load of a hard drive not be enough on a 1.5A wall wart to reduce it enough?

Would most hard drives have a voltage cut off where they won't power on above it? I've tried a few drives in it and none will spin up in the enclosure (no ide cable connected) and they all spin hooked up to a desktop psu (again, no ide cable). I tossed in a 2.5" drive (5v of course) on a converter and it spun up and even installed in windows when i plugged in the usb cable. The only possible thing I could come up with a power problem and red is 5v exactly.
Click to expand...

No, I'd say sphexi is right. Under no load conditions, you'll read a noticeably high voltage.
 
Something to keep in mind is spin-up power can be pretty high on desktop drives, around 30w I think. This large spike might be too much for the power supply. Maybe it's triggering over-current protection, causing the power supply to shut down?
 
Originally posted by: frostedflakes
Something to keep in mind is spin-up power can be pretty high on desktop drives, around 30w I think. This large spike might be too much for the power supply.
Click to expand...

the PS does look poorly regulated so this may well be it.

I wouldn't use a shoddy PS with a HDD. you may risk data loss.
 
Maybe the enclosure itself is defective in some way then.

What was the original power supply rated? (amps)

 
Originally posted by: frostedflakes
Something to keep in mind is spin-up power can be pretty high on desktop drives, around 30w I think. This large spike might be too much for the power supply.
Click to expand...

I was actually looking at that out of curiosity thinking maybe I really underestimated draw. The 160gb western digital lists at .75A for 5v and .9A for 12v. One of the other older drives I've tried though is a 13gb deskstar that lists .3A for 5V and .4A for 12v so I'd think it should at least spin. Hmmmmm.
 
Originally posted by: Soybomb
Originally posted by: frostedflakes
Something to keep in mind is spin-up power can be pretty high on desktop drives, around 30w I think. This large spike might be too much for the power supply.
Click to expand...

I was actually looking at that out of curiosity thinking maybe I really underestimated draw. The 160gb western digital lists at .75A for 5v and .9A for 12v. One of the other older drives I've tried though is a 13gb deskstar that lists .3A for 5V and .4A for 12v so I'd think it should at least spin. Hmmmmm.
Click to expand...

The combined rating 0.75A + 0.9A = 1.65A, assuming you're connecting the 5V rail (somehow) to the same power supply. The initial spin-up usually require much more current than normal usage. When your current increases, the power-supply might compensate by dropping the voltage.

How is the 5V pin on the drive powered?

You should check what the 5V and 12V rails are when you connect the hard drive to the power supply when the drive is supposed to be running.

Can you check to see if the 5V and 12V rails are drawing current? This requires that you connect your multimeter (in current mode) in SERIES with the 5V or 12V rail.

http://img132.imageshack.us/my...mage=dmmcurrentes2.gif

Also, make sure you're not switching the 5V and 12V pins. Applying 12V to the 5V pin will most likely kill the drive instantly.
 
No I haven't bothered to test for draw yet. I also assumed that the amperage listed on the drive was the peak draw, but don't know if thats how they're labeled or not. There is a little power supply unit in the enclosure itself. I'm guessing the oem power supply puts out a regulated 12v and the assembly inside steps it down to 5v.

Over voltage or under current....hmm maybe just time to give the gift of a new enclosure and call it a day 😀
 
A hard drive uses both 12V and 5V, natively. That 4-pin connector it takes feeds both 5V and 12V directly to the drive circuitry.

There may have been something wrong with the enclosure which caused the power supply to fail - or else the original diagnosis you were given was incorrect.

But it's almost certainly not failing to spin up from over-voltage. I've measured as much as 16V from a 12V power adapter when it was unloaded.


 
My suggestion would be to pull the drive out of the enclosure and see if you can get it to spin up with an ATX power supply. See if the thing even works before trying to get it running in the enclosure.
 
Originally posted by: MrPickins
My suggestion would be to pull the drive out of the enclosure and see if you can get it to spin up with an ATX power supply. See if the thing even works before trying to get it running in the enclosure.
Click to expand...

It does, they all spin just fine on a normal power supply. Something about the power from the enclosure doesn't seem to be doing it for the drives.
 
Maybe you can post a few pictures of the enclosure circuit board, the power plug you're using, and whatever else might be useful.

For the circuit board, post pictures of the top-side and the back-side too.
 
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