VIA Epia + CF won't work with power supply [solved]

jesnow · l33t Joined: 26 Apr 2006 Posts: 892

I'm building a media player for under my TV (why do they call them set-top boxes, when no modern TV has a big enough space on top to put a box?).

It's based on the VIA EPIA board that until last year was my primary computer. I got a 233x 4GB CF card drive for root, plenty of space, as long as portage and /usr/src are off somewhere else, either on the net or on a drive that only gets woken up when the system is being maintained.

But the first time I tried it out, the computer shut down for no reason all the time. I soon realized it was shortly after the HD went to sleep. When I tried it without the HD, the PS wouldn't start at all.

It turns out that switching power supplies can be damaged if run with a load that is too small, and the EPIA+CF wasn't enough load! So I guessed: the HD is rated 12V*0.5A + 5V*0.6A is 8W. At 12V that would be 144/8 = 18 ohms. I went out and got two 10-ohm resistors (rated 10W) and hooked them up in series across the 12V (yellow) and ground (black) on a male drive connector and plugged it in.

Worked!

But the resistors got quite warm, and it seems to me throwing away electricity like that is a waste, so I hooked up a 50-ohm pot in series too, and started turning it up. Interestingly, my cheap dvm had trouble getting an accurate reading at just a few ohms, so I had to get the good one from work. The two "10 ohm" resistors together were 26 ohms (5.5W). I started geting nervous when the PS made ominous noises around about 50 ohms total (3W), but kept running until 72 ohms, when the pot maxed out (2W). I guess 3w of waste I'll have to live with.

At least until I get a low-wattage PSU.

NeddySeagoon · Posted: Sat Jan 31, 2009 2:14 pm Post subject:

jesnow,

Switched mode PSUs are not normally damaged by low loads but cheap ones do not not regulate well at low loads, they tend to provide too high a voltage.
Such conditions are normally detected and the PSU shuts down, only to restart as soon as the voltage falls again.
This can happen 1000s of times a second, as a result, the PSU emits a horrible screaming noise that has to be heard to be believed.
PSUs do the same thing when they are overloaded too.

You are in good company with the extra resistive load. The original IBM PCs that hab a hard drive as an optional extra were fitted with the same sort of ballast resistor when the hard drive was not filled.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

jesnow · l33t Joined: 26 Apr 2006 Posts: 892

The elegant solution is the picopsu, which is available in 60-120W sizes, and ccoupled with a 12V brick can easily power a HTPC. I'll bet I can get away with a 20W brick.

http://www.google.com/interstitial?url=http://www.mini-box.com/s.nl/it.A/id.417/.f

For the moment, I'm sticking with the reistors, as I have no idea if my wife will *like* having a computer attached to our tv. And probably the teenager hogging it even more.