Well, you may be right, my memory is a bit fuzzy on the equations these
days, but I knew what I meant, and I suppose you did as well. In any event,
I'm glad you were able to set the record straight and clarify my point.
Tinning the contacts might be ok, but be careful that you don't make an OK
situation into a disaster. If you don't solder well, this ought to be
avoided.
--
-----------------------------------------------------------
Marc Siegel, ABSnet Internet Services eMail: smarc@abs.net
Baltimore, MD http://www.abs.net voice: 410/361-8160
-----Original Message-----
From: Tim Economu [mailto:economu@whidbey.com]
Sent: Thursday, April 04, 2002 8:23 PM
To: smarc@abs.net
Cc: MG List
Subject: Old switches
When you said:
Just remember, that POWER = CURRENT x RESISTANCE (P=IxE).
You probably meant to say that POWER = the square of the current times the
resistance of the circuit, or P = !^2 X R, or put another way power
generated in the circuit is proportional to the voltage across and the
current thru the circuit., P=IXE.
The first equation is why you need good switch connections, the I squared
exacerbates any extra resistance in the switch by producing more heat. But
the resistance is not just from the connections....it's also from the switch
contacts themselves. A good first order fix for an old switch is to remove
the contacts and solder plate them. The tin and lead will form new switch
contacts and will give some more years of switching. But a better fix would
be to send the switches out and have them replated tin or gold. Anyone know
who might be doing plating these days??
Putting a cap across the switch might provide some help for arcing...but you
would have different values for each circuit which could be a real pain.
Also the switches don't generally arc much when closed, mostly when opened.
Oh and one more thing. The equations you are using are Ohms law and they are
simply :
E I
------
R
So knowing any two values will get you the third. Any questions?
Kind regards,
Tim
+++++++++ contact info ++++++++
Tim Economu
Senior Design Engineer
Offgrid Engineering LLC
4131 Springwater Rd.
Clinton, WA 98236 USA
phone: 360-579-2117
economu@whidbey.com
http://www.offgridengineering.com
+++++++++++++++++++++++++++
Like most people, the switches in my MGB didn't work so well. There are a
couple related reasons for this. First off, the switches are, well, just
old! Oxidation, dirt, heat, and pitting (from arcing) have contributed to
this problem. While replacement switches aren't terribly expensive, it is
possible to rebuild them in many cases.
After very carefully dis-assembling them, I cleaned them out with a mild
degreaser, and polished the internal contacts. Reassembling them with a
proper lubricant, or at least with some di-electric grease, restored them to
a smooth operation with a very low electrical resistance (more on this
later).
The interesting point to note is that after talking with some other people,
it seems that nobody took the additional steps needed to truly solve the
problem. Most just bought new switches and plugged them in. The reason this
is a problem is because of resistance, namely, the terminals on the wires
that mate with the spade lugs on the switch. The wiping action of the
contacts is simply not enough to overcome 20 - 30 years of oxidation and
related scum build-up. So what you essentially wind up with is a relatively
high resistance connection at the switch. Running current through a resistor
(or a bad connection) generates HEAT, which will burn up your new or
repaired switch. I suspect that caused a lot of the problems in the first
place. If your light switch feels warm or hot after the lights have been on
for a few minutes, you have a problem! This will destroy your switch, wires,
and connectors! Just remember, that POWER = CURRENT x RESISTANCE (P=IxE).
The dirtier your connection, the higher your resistance, thus the more heat
generated at that point!
So what to do? I guess that depends on your level of ambition. Don't neglect
to (gently) polish the spade terminals on your switches. Bright and shiny is
good. But the problem is those dirty, abused connectors that you're about to
plug back into your switch! The best solution is to cut them off and crimp
new ones on. Use high quality insulated terminals. Depending on the severity
of the past problem, the wire may be black and discolored inside the
insulation for an inch or more beyond the wire end. If your wire isn't
bright and shiny where you crimp on a new terminal, you have not solved your
problem. Being that there is only just so much slack, you may need to try to
make do, so carefully clean the end of the wire with emory cloth before
crimping the new terminal on. I recommend the use of di-electric grease on
the terminals to prevent future oxidation. Perhaps put a small amount on the
wire before you insert it into the terminal.
The cheesy method would be to take a thin ignition file and clean up the
terminal carefully. You may need to close the terminal up a little bit to
improve the connection - it should be snug, but not require undue force to
install. Realize that if the wire is under the crimp of the terminal is
blackened, you haven't solved anything!
I have also considered putting a small capacitor across the switch to
eliminate arcing when the load is switched on/off, but that may be serious
overkill. Another solution, not for purists though, is to install a relay
under the bonnet, and use the dash switch to drive the very low current
relay. I suspect this may be the overall best solution, especially if your
wiring is really bad. The small coil on a relay will be much more tolerant
of a high resistance switch circuit that the headlamps themselves would be.
I'll bet your lights would be a lot brighter this way!
- --
- -----------------------------------------------------------
Marc Siegel, ABSnet Internet Services eMail: smarc@abs.net
Baltimore, MD http://www.abs.net voice: 410/361-8160
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