26 Apr
2011
26 Apr
'11
11 a.m.
[Using car bulbs as dummy loads]
Tony, very helpful advice, as always.
Thank you.
On a purely practical note what do you do about connecting the bulbs to the
Yes, it;s often the practical details that are missing (I can think of
several books that are seriosuly lacking in such information). I will
make some suggestions...
connectors? I'll need to find a suitable bulb
holder, guessing somewhere
like Maplin or RS would have something, although a quick search at Maplin
Car bulbs often have non-stnadard caps. The 'stop/tail' bulb has a
B15-size (SBC) bayonet cap, but with offset pins so it can't be fitted
the wrong way round in the car. So it won't fit a normal lampholder.
Headlight bulbs are even worse.
You won't find the holders in an elecrtronics catalogue, and if you ask
to buy a spare light cluster at a car dealer, be sitting down when they
quote the price :-). More seriously, I suspect you can get such things
from breakers yards at a fairly sensible price.
[ FWIW, when I replaced a headlamp unit on my father's car, I kept the
wiring and connectors from the old one. I have no idea if I'll ever need
them, or what for, but I don't throw things out if I can see a possible
use for them :-)]
Some headlamp bulbs (although I've not found a 6V one that this applies
to) have terminals that will take 1/4" faston ('lucar spade') terminals,
which fives an obvious way of connecting to them.
Most of the time, though I cheat!. It's only a temporary job after all.
The contacts on the base of the bulb are soft solder. The cap itself
solders easitly too. I jsut tin the neds opf the wires and solder them to
the bulb connections.
didn't show anything that looked suitable. But
what do you do for the end
that you connect to the power connector? I am sure that with all the
different types of connectors you encounter it might be difficult to come up
with a reliable connection to the connector that will work with all or most
connectors, do you do something custom for each PSU you test or do you have
a more generic solution?
This dependso n the machine, and in particualr how many I am likely to
have to repair and how the PSU outputs are arranged.
For example, if it's a separate SMPSU unit with a barrier strip or
terminals for the outputs (as in a PERQ AGW3300), then I would just
connec the lamp wires to the barrier strip.
If it's a machine where there's a PSU regulator board and I am going to
have several to repair (say an HP9826/HP9836, I own 3 such machine), I
might make up a specail test unit to connect to the PSU board edge
connector. The uint woudl cotnain dummy loads (possibly wirewound
resistors, not lamps), monitor bulbs/LEDs and sockets to connect a
voltmeter for testing. MAking such a unit is quite easy, but it can be a
lot of work (in the case of the HP machines I just mentioned, designing
it took about 10 minutes, soldering it up took about the same time, but
drilling the box and making the spacers to fit the connectors at the
right heights took a morning.)
SOmetimes I just 'tack-solder' the load wires to points on the PSU board
or backplane PCB. I don;t normally use crocodile clips, which have the
amazing habit of springg off just when you don't want them to :-)
My PDP8s and PDP11s have PSUs that are happy without dummy loads, you cna
run them with no laod and just chekc the voltages. If I had one that
needed a dummy load (or if I wanted to check on-load anyway), I'd
probably make up a usnit ti the 15 pin plug wired to laod
resistors/lamps. It's the sort of machine that is quite common, so it
would be worth making up a test box.
-tony