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RE: [oletrucks] tail lights

To: "John Howard" <johnhoward37@sympatico.ca>, <oletrucks@autox.team.net>
Subject: RE: [oletrucks] tail lights
From: "Hanlon, Bill (ISS Houston)" <Bill.Hanlon@hp.com>
Date: Sat, 31 Dec 2005 14:19:15 -0600
The traditional "flasher" used in most vehicles relies on a certain
amount of current (amperage) flowing through a bi-metallic strip in the
flasher to make the lights "blink".  The strip is designed to heat up
slightly as the current flows through it.  The two different types of
metal in the strip are bonded together.  One expands faster than the
other, causing the strip to bend.  When it bends far enough one end of
it disconnects from contact, breaking the flow of current to the lights.
As the strip cools it bends the other way, contact is made again, the
lights come back on and the whole cycle starts over again.  

I am going to guess at some values here and apply them to Ohms Law.  My
guess is that each lamp has a resistance of 3 ohms.  Ohms Law is I=E/R
where I is current in amps, E is voltage and R is resistance in ohms.

In the normal 12 volt 2 lamp circuit when you activate the turn signal
12 volts flows through the two lamps (you figure parallel resistance of
the two lamps as)
        3x3
     -----  = 1.5 ohms 
        3+3
at I=E/R   or   I=12/1.5   so I = 8 amps.  Lets say it takes a half
second for the bi-metallic strip to heat up enough to break the circuit
and another half second for it to cool off, close the circuit and start
to heat again.  If one of the lamps burns out the current flow will be
reduced to I=12/3 = 4 amps, so it will take the strip much longer to
heat up.  That is why you can tell that you have a turn signal light
burned out when one side flashes slow (or not at all) compared to the
other.  This is also why your turns signals flash so fast when you have
a trailer attached.

LED lamps are much more efficient and do not use near as much current as
incandescent lamps.  Probably less than 1 amp.  Effectively their
resistance is higher.  So there will not be as much current flowing
through the bi-metallic strip.  May just flash slower, may not flash at
all.  Remember that you still have the front lamp helping to cause load.

Solution??  Replace your flasher with an electronic flasher.  It doesn't
use a bi-metallic strip.  Electronic flashers maintain a constant flash
rate no matter how small (or how big, within limits) the load on the
circuit is.  FLAPS should have an electronic flasher.  If not, check
with a store that sells trailers.  The down side to electronic flashers
is that they don't slow down when one of your lamps burns out.

oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959




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