On Fri, 23 Feb 2007 07:25:18 -0500, CBFalconer
<cbfalconer@yahoo.com> wrote:

>kony wrote:
>> <cbfalconer@yahoo.com> wrote:
>>
>>>> Avoid the lowest cost PSUs which tend to be quite overrated,
>>>> generally most people can use a median priced unit unless
>>>> their system was particularly well endowed, but some
>>>> prettied-up generics also sell for more than they're worth,
>>>> name brands can help but first you'll need to consider the
>>>> specific parts in the system.
>>>
>>> Or you can measure it by using the wattmeter method I outlined in
>>> another thread.
>>>
>>> I suggest people build suitable test loads. You can use 12V
>>> automotive bulbs for the 12V supply, but you may have problems
>>> finding suitable 3.3v and 5V bulbs. Now make a breadboard with a
>>> suitable number of bulb sockets wired in parallel.
>>>
>>> When someone offers an XXX watt PSU, load up sufficient bulbs to
>>> dissipate XXX watts, distributed between the various lines
>>> according to the mfg spec for max load. Connect, turn the PS on,
>>> and wait. I predict 90% of PSUs will self destruct in 30 min or
>>> less. So do this test at the sellers location, before buying. If
>>> it passes, buy it. :-)
>>
>> There would definitely be quite a few PSU that fail such a
>> test, particularly those which came free-with-case, but
>> there are a couple of other problems in particular.
>
>The main point of the post was that automotive, or other, bulbs
>make a relatively cheap load, and sockets allow you to adjust the
>values. Did you ever price high wattage low ohms resistors?

They can definitely get expensive (for what they are) from
the major electronics houses.

Over the years I've used a number of different *materials*
for the load, some rather crude like the spiral steel
binding on some pads of paper (varies, but typically about
0.2 Ohm per segment), nichrome wire from a toaster or hair
dryer, etc., and the bulbs. Bulbs work fine and if that was
what I had available for a static load, I might use them but
at present I don't have but a few spares that are saved for
cars they might be used on.

Key to getting low cost power resistors is shopping around,
like the surplus electronics 'sites. A couple examples,

6 x 0.5Ohm, 15W $0.99
http://www.goldmine-elec-products.co...?number=G14375

10 x 1 Ohm, 20W $0.99
http://www.goldmine-elec-products.co...?number=G14510

While not as fancy as some of the power resistors with a
surrounding metal heatsink, those with the heatsink still
depend on being 'sunk to other metal, an addt'l cost and
time to implement. By using a greater number of the lower
cost resistors above, heat density is lowered, enough that
no further heatsink is required if only enough resistors are
used (though it can be a bit fiddly to have dozens of power
resistors lying out on a board, a proper heavy-duty circuit
board to hold them all is a bit better for repeated use).

Light bulbs are certainly another option, though they have
another issue that can make it more time consuming to
implement, that as the applied current goes up, the
resistance goes down, leaving one hoping the manufacturer's
wattage spec comes close enough to reality to estimate the
current per voltage.... at least with the automotive 12V
blubs, with a household 110V type it might be easier to just
take each in turn and measure the current through it.

Bulbs can get expensive too, a tail-light or turn signal
bulb may be cheap but the numbers required to arrive at the
rated capacity for a modern PSU, and sockets if you're not
willing to solder to them, can add to the expense.
Headlights are a little better, but the cheaper old type
with the housing and lens all-in-one are less common now and
make for quite a large setup on a bench, if you're testing a
PSU with a now-modest 18A current rating, it'll take ~ 5 @
50W. I imagine I could get some from a junk yard for
practically free, but I would like something a bit smaller
that could be placed next to a system for testing without
removing the PSU from the system, or if taking it to someone
else's system, something that looks a bit more professional
will give them less worries about what is being done.