[mythtv-users] HDPVR problems - power supply again?

[Stephen Worthington]

On Wed, 14 Aug 2019 11:35:21 -0400, you wrote:

>On 8/14/19 6:36 AM, Stephen Worthington wrote:
>> So the real problem with using a PC power supply only comes
>> if you connect the HDPVR to a standard power connector (ether Molex or
>> SATA) and do not have any current limiting in the connection you use
>> (such as a properly rated fuse or circuit breaker).  The PC power
>> supply can then supply a great many amps into the HDPVR, and if the
>> HDPVR has a fault that draws huge current, that can cause a fire.
>
>
>Having powered my HD-PVR from one of my PSU's molex connectors for
>years, I'm curious about this.
>
>Given the over-current protection (OCP) features that my PSU itself
>advertises, plus the fact that that system is on a properly limited (via
>circuit breaker) circuit, I'm not terribly concerned about the HD-PVR
>bursting into flames.
>
>But let's say I had a cheap PSU on a circuit without a breaker.  Is
>there any reason to fear the molex-connected HD-PVR any more than any of
>the other PC components driven by that same PSU?
>
>
>-WD

I am afraid you clearly do not understand how current protection
works.

If the HDPVR is rated at 2 amps and it draws 15 amps from the PC, it
can certainly burst into flames.  The PC power supply will not be in
an over current situation - it can easily supply 15 amps.  Each of the
devices you connect to a PC power supply (eg hard drive) will
typically have a fuse on its circuit board that will limit the current
in situations like this.  But if the HDPVR was relying on having a
power pack that supplies no more than 3 amps (say), then it may not
have an onboard fuse.  It would be expecting the fuse to be in the
power pack.

This is just the same situation as with mains AC power.  The breaker
or fuse for a set of AC sockets may be rated at 20 amps.  But each
device you plug into one of the sockets on that fuse will be required
by law to have an internal fuse that limits its current to a maximum
of what one socket can handle, which here in New Zealand is 10 amps.
And usually the fuse will actually be rated for the device's normal
maximum, so for a low power device like an FM radio it might be 0.5
amps.  Without such fuses, a faulty device can overheat a socket and
start a fire, but the fuse or breaker on that circuit will not blow as
the current is still well less than what it is rated for.

So I would recommend that anyone who wants to power an HDPVR directly
from a PC power supply fits an inline fuse in the cable used to do
this.  That fuse should be rated at the current a standard HDPVR power
pack is rated to supply.  Failing to do this may well be illegal
depending on what the electrical wiring regulations in your locality
are.