[mythtv-users] HD-PVR power supply (wall wart) gone bad

Stephen Worthington stephen_agent at jsw.gen.nz
Sun Dec 16 08:52:49 UTC 2018

On Sat, 15 Dec 2018 18:06:13 -0700, you wrote:

>On 12/3/18 8:37 AM, Allen Edwards wrote:
>> I will repeat, you will almost certainly burn out your device if you 
>> power it from +12 volts.  I just looked at your cable and it looks 
>> fine.  Therefore I would ask why do you think the old supply is bad?  
>> Did you use a voltmeter or an oscilloscope to test it.  If so, you 
>> should be able to test the output of your cable.  If you didn't test it, 
>> and given you want to put +12 into a +5 device, I would guess you did 
>> not test it, perhaps it is the PVR that is bad and not the power supply.
>> The other possibility, mentioned already, is that there are ground 
>> problems.  The wall wart is almost certainly not grounding your PVR as 
>> the supply from the computer is.  If your PVR is grounded on a ground of 
>> different potential than where the computer is grounded that could cause 
>> problems. Plug them both into the same outlet and see if that helps.
>> Allen
>No worries, I never planned to use the +12V because it seemed like a 
>mistake that someone on the list said to use it.
>I had a bad power supply and a "good" one, I just wanted to fabricate a 
>cable so when my next supply went bad I would be ready. I'm already on 
>my 4th so it's probably a matter of time. Currently I'm using my "new" 
>wall wart power supply since my newly fabricated cable isn't working so 
>I did test the new cable after I soldiered it. Some interesting readings:
>Old wall wart reading at 1/2 inch from the brick: 5.14 V
>New wall wart reading at the barrel connector: 5.09 V
>Newly fabricated cable that uses PC power supply (at the barrel 
>connector): 5.04 V
>I suspect it may be a current/amperage issue and am hoping to make time 
>to move to a different connector on the PC power supply.

The PC power supply should have no trouble at all delivering enough
current for an HDPVR.  It should also deliver very well regulated
voltage very close to 5 volts.  But that will be at the connector you
are attaching your new cable to.  If that is a Molex connector (old
IDE style), then they normally use quite heavy duty cable, so no
matter how much current you draw the voltage should remain stable.  If
you are connecting to one of the new SATA power cables, then they are
much smaller gauge wire and you can get a voltage drop if your cable
is drawing too much current.  And if your own cable is using wire that
is too small a gauge, then you can get voltage drop there.  So what I
would recommend when building one of these cables is to connect to one
of the Molex connectors, and use the same gauge of wire as is going
into the Molex connector for your own cable.

If I remember correctly, the HDPVR wall warts are good for 2 amps, but
they will likely only need to deliver all of that for the inrush
current at turn on.  The specs for +5 V power available at a PC power
supply Molex depend on the power supply you are using, but should be
good for a lot more than 2 amps - enough more that if something goes
wrong with the HDPVR, I hope it has an internal fuse, because the PC
power supply will likely be able to cause the HDPVR to burn rather
than have the PC power supply detect a problem and shut itself down.

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