[urq] Marker Light Challenges

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Fri Jun 17 12:03:34 EDT 2005

Nope!  Think about it ... the only thing the switch/flasher can do is to
connect the circuit to the battery.  What you say about the other
filaments glowing is actually true; the technique sets up a voltage

The primary characteristic that makes this work is the fact that the
marker is a comparatively low power bulb ... combined with the fact that
you have a number of high power bulbs in parallel in the path to ground.
You may want to sit down and draw the effective circuit diagram to see
how this works.  When the headlights are off, the former ground signal
for the marker bulb receives power from the turn signal circuit.  The
path through the marker bulb to ground is via the parking lights.  You
were correct in your understanding that when the headlights are on the
markers are made to blink off by applying battery voltage to both
terminals ... but the comment about the flasher interrupting the circuit
is incorrect.  When the headlights are on and the turn signals are not
energized, the marker bulb receives ground via the front and rear turn
signal filaments in parallel.  

I wanted to comment on the statement that "filaments will never be low
resistance" ... it turns out that they always are!  The standard
filament for a brake or turn signal bulb is around 25 watts.  This means
that the filament is only around 0.5 ohms ... pretty low resistance by
itself, and in most cases we're talking about at least two such
filaments in parallel.  The characteristics of an incandescent filament
are such that when the filament is cold it has an even lower resistance
(which is why most bulbs burn out when the light is switched on).  I
don't know the type of the marker bulb, but I suspect it is rated at 5
watts or so ... which would imply a resistance of 2-3 ohms ... which is
still pretty low, but comparatively high.  

While I find the marker flash functions perfectly well using this
voltage divider concept, there are some who do not like the fact that
the voltage divider guarantees that the marker bulbs never see "full"
voltage.  If you want to make sure that you have a full battery supply
to the marker bulbs you can insert a relay into the circuit, and have
the relay supply switched battery power to the marker bulb.  What you do
is basically to wire the relay coil up as the marker light would be in
the above situation ... one of the coil terminals goes to the headlight
source and the other to the appropriate turn signal source.  Some relays
have built in diodes to eliminate the inductive kick back voltage, make
sure you don't use one of those relays in this example as the current
will flow in different directions depending upon whether the headlights
and/or the turn signals are energized.  If you are considering
installing headlight relays, you can incorporate the marker relays (2)
in the same box and run the two turn signal inputs and marker bulb
outputs and end up with fully bright marker lights.  

Steve B
San Jose, CA (USA)
> Steve Buchholz wrote:
> > ...the marker light gets a low resistance to ground
> > through the paralleled filaments for the turn
> > signals, and when the headlights are off the marker
> > light gets a low resistance to ground through the
> > headlight and taillight filaments
> I always thought it was grounding back through the parking light
> or flasher (whichever function isn't active).  The filaments will
> be low resistance, and if the current flowed through them they'd glow
> dimly (as would the marker) since you've got multiple filaments in
> series.  With the lights on and the flasher operating, the flasher
> actually interrupts the circuit because you have +12V on both sides of
> the marker filament.  Consequently, the marker flashes alternately
> the normal turn sigs when the lights are on.  Of course, I could be
> completely off target with all of the above.
> I definitely like the flasher function.  I'm a bit concerned about
> the current will go if the switch/flasher grounds become marginal, but
> it's an easy mod to reverse on the fly.  There's a little superstition
> involved too, because the incident with my '85 happened the day after
> did this.

More information about the urq mailing list