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'[OT]: Turn signal Lightbulbs'
2000\06\20@210137 by

Greg Wrote

> I have a 12V 27W light bulb in my car turn signals.
> The bulb measures 1 Ohm.  So I think theirs about
> 2.2 Amps current flowing.
>
> 1) I cant even get them to light outside my car
> using my power supply.  I am using some regular alligator
> clips and touching them to the light bulb.  The supply is set to 12 Volts
> and im measuring 3 amps.  I am a little hesitant to increase the current
> because I dont know what im doing...Whats a safe way to get them to
> light?  Shouldnt they be slightly lit if I only supply 1 Amp of current?
>

1. Alas a light bulb is a non-ohmic conductor, like you measured,
the cold resistance is 1 ohm, if it stayed 1 ohm then 12 amps
would flow at 12 volts = 144 watts, as it gets hotter the resistance
increases and bulb settles at 27 watts or (volts x volts / watts) 5.3
ohms.
So youre 3 amp supply just cant persuade the bulb to get hot
enough to increase its resistance.

The second part about more resistance to change the flashing
rate... I would be tempted to remove the flasher unit and put an
electronic one in its place, there have been many such units
published in electronic hobbiest magazines, with clever little bulb
failure warnings etc..
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________
Lightbulbs have an extremely nonlinear resistance- they will draw much more
than their rated current on turn-on.  If you want to be really careful- add
a large capacitor to the output of the power supply, turn on the supply, and
then attach it to the lightbulb, the capacitor should absorb the initial
high current demand of the lightbulb to get it to the ~2.5A steady state
condition.

One way to effectively increase the resistance would be to PWM (pulse width
modulate) the light bulb- using a transistor to rapidly cycle the lightbulb
on and off.  Though you aren't really increasing the resistance (unless you
look at the circuit over time in terms of power delivered to the bulb), this
should do what you want.

Matt Bennett

{Original Message removed}
> I have a 12V 27W light bulb in my car turn signals.
> The bulb measures 1 Ohm.  So I think theirs about
> 2.2 Amps current flowing.
>
> 1) I cant even get them to light outside my car
> using my power supply.  I am using some regular alligator
> clips and touching them to the light bulb.  The supply is set to 12 Volts
> and im measuring 3 amps.  I am a little hesitant to increase the current
> because I dont know what im doing...Whats a safe way to get them to
> light?  Shouldnt they be slightly lit if I only supply 1 Amp of current?
>
> 2) Whats a cheap way to increase the resistance on the line so they will
> blink faster.  I havent seen pots that can handle 30W and I hear they are
> quite expensive (\$45).  Is there some other ways that I can vary the
> resistance (variably by hand or digitally) on the line?
>
> Greg
>
If your car uses a bi-metalic type flasher unit, then they can be carefully
tweaked to flash faster or slower by judicious bending of the bimetalic arm
inside.  Adding resistance just to speed up the flashers is not the way to
go.  The lamps will be dimmer and therefore less easily seen in bright
light, and the resistor will have to dissipate a lot of power.

We have 22 Watt flasher lamps over here in the UK, wonder if they are the
same fitting?  (Bayonet cap type)  That would speed up the flashers without
signifgicantly reducing light output.

Mike

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