Burned switch contacts: is there a long-lasting repair?
Ken Bell
On an '88 Saab 9000 (that's a car, for those who are reading this
on the electronics newsgroup :-)), the electrically powered windows
were running slow, so I cleverly disassembled the window switches
and cleaned the contacts with a rotary tool wire wheel and buffer.
They looked nice and shiny and worked great for a couple of months.
Now the contacts on two of the switches are burned worse than before.
The design is not so great; the current that runs the windows goes
straight through the switch contacts, leading to a propensity for the
contacts to get burned. However, I recently read that switch contacts
are often plated to prevent this, and that I probably "cleaned" that
plating off, trying to repair the switches (which cost around $50
apiece, hence my interest in saving them).
Is there something I could apply to the contacts to keep them from
burning so fast? "Tinning" them with solder occurred to me, but if
there is something else that is better, I'd appreciate hearing about
it. Running the windows off of relays would be more sensible, but
doing that now would turn this into more of a "project" than I want
to deal with, especially since the switches are mounted in the car's
console, with no room to squeeze anything else in. Thanks.
--
Ken Bell :: ken...@panix.com :: (212) 475-4976 (voice)
======== :: sy...@giss.nasa.gov :: (212) 678-5516 (voice), 678-5552 (fax)
diana
Ken Bell wrote:
snip
> Running the windows off of relays would be more sensible, but
> doing that now would turn this into more of a "project" than I want
> to deal with, especially since the switches are mounted in the car's
> console, with no room to squeeze anything else in.
Looks like a permanent fix will be a project. A friend of mine did the
relay trick and you can check out his project at
http://www.geocities.com/MotorCity/6539/srvby5.html
Perhaps you too could put the relays somewhere besides the console, just
run
the wires to your "patched" switches. You'll still have to repair or
replace your burned switches with some decent used ones, but you won't
have all that current to burn them up anymore. I did keep a couple
switches I had working for a while by building up some solder on the
contacts, but that didn't work long.
Heck, I was too poor to buy relays or replace the switches when they
finally burned out in the Toronado, so I had to take the wires off the
switches, leave them taped and hanging out the armrest panel. I'd untape
and short them together to roll the windows up and down. Still remember
my rhyming color code "red to brown is window down". :-)
Diana
--
Hobbyist Electronics, Digital Art --> http://lonestar.texas.net/~diana/
Mike Walsh
You probably need to add arc suppression in the switch circuit. If you
show a wiring diagram to anyone who knows electronics they can tell you
how to do it. All you should have to do is add one component. Either a
diode, capacitor or varistor.
Mike Walsh
Palm Beach Gardens, Florida, USA
mike...@pb.seflin.org
Rich Compelling
Once you mess with contacts, they're as good as gone. The relay idea is
probably the easiest. You can buy 12V relays that handle 30 amps relatively
cheap. I buy them from a local car alarm/stereo shop for $5 a piece. The
relay coil typically draws less than 1 amp, and you can add a 'snubber'
across the relay coil to supress inductive kickback. Usually, 100 ohm
half-watt in series with a 0.1 uF ceramic or monolithic cap wired in
parallel with the relay coil will do it. A 1 amp rectifier ill do it, too.
(Just make sure it is not forward biased ;->) If you can run a 12V bus to
the doors themselves, put the relays in the doors so the window switches
only switch the < 1 amp of relay current.
But then again, it sounds like a project....
Good Luck
Ken Bell <ken...@panix.com> wrote in article
<5p6ti0$7...@panix2.panix.com>...
> On an '88 Saab 9000 (that's a car, for those who are reading this
> on the electronics newsgroup :-)), the electrically powered windows
> were running slow, so I cleverly disassembled the window switches
*snip*
Jack Stands
On 29 Jun 1997 20:10:40 -0400, ken...@panix.com (Ken Bell) wrote:
>On an '88 Saab 9000 (that's a car, for those who are reading this
>on the electronics newsgroup :-)), the electrically powered windows
>were running slow, so I cleverly disassembled the window switches
>and cleaned the contacts with a rotary tool wire wheel and buffer.
>They looked nice and shiny and worked great for a couple of months.
>
>Now the contacts on two of the switches are burned worse than before.
>
>The design is not so great; the current that runs the windows goes
>straight through the switch contacts, leading to a propensity for the
>contacts to get burned. However, I recently read that switch contacts
>are often plated to prevent this, and that I probably "cleaned" that
>plating off, trying to repair the switches (which cost around $50
>apiece, hence my interest in saving them).
>
>Is there something I could apply to the contacts to keep them from
>burning so fast? "Tinning" them with solder occurred to me, but if
>there is something else that is better, I'd appreciate hearing about
>it. Running the windows off of relays would be more sensible, but
>doing that now would turn this into more of a "project" than I want
>to deal with, especially since the switches are mounted in the car's
>console, with no room to squeeze anything else in. Thanks.
>
>--
>Ken Bell :: ken...@panix.com :: (212) 475-4976 (voice)
>======== :: sy...@giss.nasa.gov :: (212) 678-5516 (voice), 678-5552 (fax)
Switches and the like suffer badly from operating an induction circuit,IE coil, starter torque
motors, fans, solenoids and electric running motors.
The reason is BACK E.M.F.
The winding inside the load emits a large reverse current spike when the switch is disconnected.
This is what burns the contacts more than current demands from a heat or resistance electrical load
IE rear demister.
This back E.M.F. can be diverted through a reverse biased diode fitted accross the offending
induction load.
What is Back E.M.F. ? It is the energy left in the induction windings that increases in voltage as a
spike when the magnetic flux lines collapse.
this energy cannot disipate to the circuit as that is open so it returns down the line.
In an ignition system the condensor makes use of this buy oscilating it to increas the speed of
cross lines of magnetic flux at low engine speeds.
Another way to stop Back E.M.F. is to have a ballast resistance or voltage drop this will have a
reverse damming affect and reduce Back E.M.F. greatly, unfortunatley many systems cannot accept a
drop from 12 volts and require all available current, your windows are one.
Reducing the current by fitting relays is the best solution however reverse biased (placed) diodes
will reduce the burning affect accros contacts.
It is true of the plating on the terminals, not much you can do there.
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Gary Tait
In article <33b944cd...@news.ozemail.com.au>,
#Switches and the like suffer badly from operating an induction circuit,IE
coil, starter torque
#motors, fans, solenoids and electric running motors.
#The reason is BACK E.M.F.
#The winding inside the load emits a large reverse current spike when the
switch is disconnected.
#This is what burns the contacts more than current demands from a heat or
resistance electrical load
#IE rear demister.
#This back E.M.F. can be diverted through a reverse biased diode fitted
accross the offending
Due to the nature of window motors being bi-directional, a diode won't work,
I'd go with the relays (you can fine general purpose SPDT 30A automotive
relays for less than $10 a piece.you can put your back EMF diodes on these.
Gary Tait, VE3VBF
Kirk Kohnen
Jack Stands wrote:
>
> On 29 Jun 1997 20:10:40 -0400, ken...@panix.com (Ken Bell) wrote:
>
> >On an '88 Saab 9000 (that's a car, for those who are reading this
> >on the electronics newsgroup :-)), the electrically powered windows
> >were running slow, so I cleverly disassembled the window switches
> >and cleaned the contacts with a rotary tool wire wheel and buffer.
> >They looked nice and shiny and worked great for a couple of months.
> >
> >Now the contacts on two of the switches are burned worse than before.
> >
> >The design is not so great; the current that runs the windows goes
> >straight through the switch contacts, leading to a propensity for the
> >
> >--
> >Ken Bell :: ken...@panix.com :: (212) 475-4976 (voice)
> >======== :: sy...@giss.nasa.gov :: (212) 678-5516 (voice), 678-5552 (fax)
> starter torque
> spike when the switch is disconnected.
> heat or resistance electrical load
If this is what is truly causing the contacts to burn out, then there is a
solution. Wire up diodes across the switch contacts so that they are reverse
biased. I suspect that (since the motors are probably hefty - 10 amp or so) you
should use 10 amp diodes or so. This will cause the back emf to ba snubbed by
the diodes instead of the switch contacts.
One other possible approach would be to find low voltage MOVs (metal oxide
varistors). The high voltage versions of these are used in surge supressors.
Put a low voltage (about 20 volts or so) across each of the motors. This will
serve to snub out any voltage spikes from the motor. MOVs have the advantage of
working with either direction of current. Diodes don't.
Or, you could always simply buy a replacement switch from the dealer?
(Have you considered taking the switch to a circuit board shop and asking them to
gold plate the contacts for you? It's not hard and it isn't expensive either).
Good luck.
.
Marvin Moss
What about the old trick of putting a 10 ohm 1 watt resistor in series
with a .1 mfd. cap all across the points to help?
Marv