Need resistor help please (35w x 12v)

[Nate Miller]

Hello,

I recently bypassed my vehicles electrical system for my headlights and
now my vehicle comes up with headlight out warnings. I have been told
if I installed resisters in the original wiring to simulate the load of
the light bulbs then my problem will go away.

Based on ohm calculations, I've figured to simulate 55watt on a 12 volt
circuit I'd need a 2.6ohm resistor. Optionally I can change the
computer to think they are different lights and only need 35watts,
which would require a 4.1ohm resistor.

I see resistors all over the net, radioshack, etc, but they are tiny
little things that I do not think are sufficient for this application.

Two things I would appreciate help on:
1. Are my calculations correct? And is there a specific type of
resistor i need to simulate 12v 35watt or just any 4.1ohm resistor?

2. Where's a good source for the resisters I need?

Much appreciated!

 

[Ben]

Hello,

I recently bypassed my vehicles electrical system for my headlights and
now my vehicle comes up with headlight out warnings. I have been told
if I installed resisters in the original wiring to simulate the load of
the light bulbs then my problem will go away.

Calculations look OK to me.

But where would you leave the heath of 55 or 35 Watt on electric power ? You
would need resistors like these:

http://www.action-electronics.com/resist50.htm

mounted at a metal surface, for each light.

(Or: don't you use the headlight switch and has the vehicle only to check
the lights when starting the vehicle, leaving the resistors "unpowered" ? )

Regards,
Ben

 

[Tim Shoppa]

2. Where's a good source for the resisters I need?

The best way to dissipate 35W on a 12V supply is to use automotive
lamps.

Ironic, ain't it?

Tim.

 

[Locu]

Thanks for the source for those resistors! I wonder, just how hot do
those get? The only place to mount them is on the inside plastic of
the headlight unless I breach the shell of the headlight to run wires
outside (which kinda defeats my oem look purpose and potentially allows
moisture in).

The vehicle checks the lights whenever they are switched on, so if they
are switched on before i power the vehicle it throws the warning up
until the vehicle is off, and also if i turn them on while driving, it
then springs the warning on me right then. If a bulb burns out while
driving, it pops the warning too I believe, so it seems to be
constantly monitoring.

My other option is to disable the bulb out sensor, but nobody happens
to know where it is in my car (2002 Audi A4).

 

[Locu]

re: automotive lamps..

This is certainly an option, i could mount a lamp in there. Any lamps
that are small, and burn really efficiently that don't create much
heat?

 

[Dave Platt]

re: automotive lamps..

This is certainly an option, i could mount a lamp in there. Any lamps
that are small, and burn really efficiently that don't create much
heat?

Compact fluorescents are a good deal more lumen-efficient than
incandescents. Unfortunately, they're not small and are somewhat
fragile.

High-output LEDs are better than small incandescents, by a factor of
2:1 or 3:1. They still dissipate quite a bit of heat.

And, in truth, it doesn't really matter at all how lumen-efficient the
bulb is, if you're planning to have it in an enclosed space. The
light emitted by it will strike the inside of the enclosed space, be
absorbed, and will turn into heat.

If you're running 3 amps at 12 volts into a lamp, you're going to have
36 watts of heat to get rid of, one way or another. Through one path
or another, every bit of power you pump into the lamp ends up as heat,
somewhere.

I'm curious - what is/was your original goal. You mentioned having
"bypassed" your original lights - just what did you do, and what are
the advantages of having done so?

 

[Locu]

Ahh, good point on the light striking surface and turning into heat.. I
hadn't thought of that.. pretty futile maneauver except perhaps to
spread the heat a little i guess.

Well, I installed HID (xenon) lights in my non-xenon equipped vehicle.
The projectors are designed for xenon's though, just it didn't come w/
that option. I bought an OEM kit, and everything is there, except the
stock wiring doesn't support the high amp load the startup of the HID's
draw which can be upwards of 100watts I understand. So I used the
existing wiring to a relay, to turn on/off my wiring harness which
brings power direct from the battery thru fuses to the HID lights.

Now since I bypassed the cars internal wiring, the car see's no light
bulbs in there, so it complains.

 

[Locu]

I just presented this question to xenondepot.com, I guess a leading
supplier of headlights in the US. Their response is:

Hello Sir,
We do not sell this however you need to purchase 2 x 15ohm resistors ( 1 per side).

Their calculation seems to be way off from ours, any ideas?

 

[Graham W]

I just presented this question to xenondepot.com, I guess a leading
supplier of headlights in the US. Their response is:


Their calculation seems to be way off from ours, any ideas?

It may only be a case of satisfying the monitoring circuit's detection
current rather than simulating the high-power consumption of a real
main-beam lamp.

I'd suggest you substitute whatever you have in the nature of small
panel lamps, dome-light bulbs to see if any of them will do the trick.

 

[Clint Sharp]

I just presented this question to xenondepot.com, I guess a leading
supplier of headlights in the US. Their response is:


Their calculation seems to be way off from ours, any ideas?

Maybe the headlight warning system doesn't need to 'see' the full
headlight load, maybe 15 ohms is enough to fool it, personally I'd try
something higher than that, reducing the value until you get a reliable
no fault indication. Bear in mind you need to dissipate the heat
generated and you need to increase the wattage of the resistor as you
lower the resistance, a higher ohm value will produce less heat and be
physically smaller in a lower wattage package.

 

[Ross Herbert]

Hello,

I recently bypassed my vehicles electrical system for my headlights and
now my vehicle comes up with headlight out warnings. I have been told
if I installed resisters in the original wiring to simulate the load of
the light bulbs then my problem will go away.

Based on ohm calculations, I've figured to simulate 55watt on a 12 volt
circuit I'd need a 2.6ohm resistor. Optionally I can change the
computer to think they are different lights and only need 35watts,
which would require a 4.1ohm resistor.

I see resistors all over the net, radioshack, etc, but they are tiny
little things that I do not think are sufficient for this application.

Two things I would appreciate help on:
1. Are my calculations correct? And is there a specific type of
resistor i need to simulate 12v 35watt or just any 4.1ohm resistor?

2. Where's a good source for the resisters I need?

Much appreciated!

I am curious as to why you want to by-pass the car electrical system
which monitors lamp current. I assume that by-passing still means you
have the original lamps in operation.

If this is the case it seems rather self defeating to bypass the
monitor circuitry while having to install dummy load resistors which
will consume (waste)the same amount of energy as the original lamp
load in addition to the original lamps themselves. Without a very good
reason, it would appear to me to be futile to double the normal lamp
current drawn from your battery.

 

[Brian]

I just presented this question to xenondepot.com, I guess a leading
supplier of headlights in the US. Their response is:


Their calculation seems to be way off from ours, any ideas?

This assumes that the load sensors in the ciruitry will "sense" that
headlights are installed with 15ohm loads which could very well be true
as it comes from xenondepot. With a 15 ohm resistor and the vehicle
voltage around 14V, Ohms law tells us this resistor needs to be rated
at over 13W.

Watts = Volts^2 / Ohms 13.066 = 14^2 / 15

You can order 15 Ohm / 25 Watt rated resistors from Mouser.com for
$2.18 US. Part # is 71-HL25-06Z-15. You will need 2 of these, 1 for
each headlight.
http://www.mouser.com/index.cfm?han...uctid=193771&e_categoryid=359&e_pcodeid=71046

If you go this route, make sure you order the correct resistance. If
you put that part # into Mouser's part search, their first few results
will be the incorrect ones as they show the 150 Ohm and 15k Ohm
resistors first.

If you want to experiment a little, you can try going to any electronic
store around you like a radioshack and picking up a few 1.5k resistors
either 1/4 or 1/2 watt will do as you will be only passing .13 watts
through them at 14V. These come in packs of 5 or so from Radioshack
for 99 cents. It is quite possible that your sensors will see even
this small of a load as a working light. If this doesnt work then your
only out a buck and I would suggest going with the 15 Ohm resistors.

--Brian

 

[Locu]

Brian,

Excellent info! thank you! Last night I plugged a 2 watt lightbulb
into the headlight circuit and still had the warning, then i plugged a
5 watt bulb in, and warning went away! that tells me I dont have to
generate a full 55watts, which is a really good thing, that would be a
lot of heat!

 

[Brian]

Brian,

Excellent info! thank you! Last night I plugged a 2 watt lightbulb
into the headlight circuit and still had the warning, then i plugged a
5 watt bulb in, and warning went away! that tells me I dont have to
generate a full 55watts, which is a really good thing, that would be a
lot of heat!

Given that information, a 33 Ohm 10 Watt resistor will allow you to
pass 5.9W through the resistor and be well within the rated power
dissipation for the resistor. If you can find a 47 Ohm 5 Watt
resistor, you *might* be able to save a few cents but doing this your
only going to have a 4.1W load. Just pointing out your options.

Here is a link to a 40 cent 33 Ohm 10 Watt 5% resistor.
http://www.mouser.com/index.cfm?han...uctid=207351&e_categoryid=202&e_pcodeid=52610

--Brian

 

[Franc Zabkar]

Ahh, good point on the light striking surface and turning into heat.. I
hadn't thought of that.. pretty futile maneauver except perhaps to
spread the heat a little i guess.

Well, I installed HID (xenon) lights in my non-xenon equipped vehicle.
The projectors are designed for xenon's though, just it didn't come w/
that option. I bought an OEM kit, and everything is there, except the
stock wiring doesn't support the high amp load the startup of the HID's
draw which can be upwards of 100watts I understand. So I used the
existing wiring to a relay, to turn on/off my wiring harness which
brings power direct from the battery thru fuses to the HID lights.

Now since I bypassed the cars internal wiring, the car see's no light
bulbs in there, so it complains.

I'd suggest modifying the device that monitors the lamps, assuming it
is accessible from a maintenance point of view.

Let's assume this device works by monitoring the voltage drop in a
series resistance by means of a comparator, as follows:


BAT+ O--+- Rsense --+-O O---- 55W lamp ----O BAT-
| |
|___ ___|
| |
__|____|__
\ /
\ Comp /
\ /
\ /
\/


In this case you could remove the sense resistor and transfer it to
the new, heavier gauge, relay switched lamp circuit, as follows:

+= Rsense ==+
BAT+ O==+= Rsense ==+=O O==== 100W lamp ====O BAT-
| |
| |
O O

O O
| |
|___ ___|
| |
__|____|__
\ /
\ Comp /
\ /
\ /
\/

To maintain the same voltage drop for a 100W lamp as for the original
55W one, just use the equivalent of two identical sense resistors in
parallel. Doing it this way might only require a cheap 5W Al-clad
resistor.

- Franc Zabkar

 

[Franc Zabkar]

I'd suggest modifying the device that monitors the lamps, assuming it
is accessible from a maintenance point of view.

Let's assume this device works by monitoring the voltage drop in a
series resistance by means of a comparator, as follows:


BAT+ O--+- Rsense --+-O O---- 55W lamp ----O BAT-
| |
|___ ___|
| |
__|____|__
\ /
\ Comp /
\ /
\ /
\/


In this case you could remove the sense resistor and transfer it to
the new, heavier gauge, relay switched lamp circuit, as follows:

+= Rsense ==+
BAT+ O==+= Rsense ==+=O O==== 100W lamp ====O BAT-
| |
| |
O O

O O
| |
|___ ___|
| |
__|____|__
\ /
\ Comp /
\ /
\ /
\/

To maintain the same voltage drop for a 100W lamp as for the original
55W one, just use the equivalent of two identical sense resistors in
parallel. Doing it this way might only require a cheap 5W Al-clad
resistor.

Another approach is to leave the original resistor within the
monitoring device and add a second resistor to the heavy gauge
harness. The downside is that the current would then be shared between
two paths. To avoid disturbing the module, you may be able to
determine the value of the internal resistor by metering the BAT and
LAMP OUTPUT pins.

- Franc Zabkar

 

[Locu]

Well, here's some new oddness to this problem.

I ordered for testing 2 x 15ohm 25watt 1% resistors... they were about
$5 each, which I'm ok to spend just to learn..

I wired them up tonight to my drivers side stock wire headlight, and
they are definitly receiving power, they get very hot within about 30
seconds.. to hot to touch. Turn the lights on and the computer still
spits out the bulb out warning.

So, in series with the resistor I put the 2watt lightbulb, still got an
error. In series I then put the 5watt light bulb, and error goes away.

I'm not super savvy with power stuff, but could perhaps anyone tell me,
is the resistor drawing a different type of current/power/something
than the lightbulb? Clearly the resistor at 25watt is drawing more
power than the 5watt lightbulb.

Anyone got any idea what gives?

- Nate

 

[ehsjr]

Well, here's some new oddness to this problem.

I ordered for testing 2 x 15ohm 25watt 1% resistors... they were about
$5 each, which I'm ok to spend just to learn..

I wired them up tonight to my drivers side stock wire headlight, and
they are definitly receiving power, they get very hot within about 30
seconds.. to hot to touch. Turn the lights on and the computer still
spits out the bulb out warning.

So, in series with the resistor I put the 2watt lightbulb, still got an
error. In series I then put the 5watt light bulb, and error goes away.

I'm not super savvy with power stuff, but could perhaps anyone tell me,
is the resistor drawing a different type of current/power/something
than the lightbulb? Clearly the resistor at 25watt is drawing more
power than the 5watt lightbulb.

Anyone got any idea what gives?

- Nate

You did not say if you installed the 15 ohm resistors in
series or in parallel.

When you finally got it working you had the resistor
(2 15 ohm resistors) at either 7.5 ohms or 30 ohms in
series with either one or 2 bulbs.

To get an answer, please tell us what the final
circuit was. How many bulbs were in series with
the two resistors, and were those resistors in series
or in parallel?

Ed

 

[Locu]

This is 2 completely separate circuits I'm speaking about, one for the
left headlight, and one for the right headlight. So, effectively if I
get one working the same setup on the other will work as well (cross
fingers). So, each side will only have 1 resistor. The resistor is a
15ohm 25watt resistor.

When I did my tests last night, the first test which still made my car
beep and cry about the light out was just a single resistor, and that
is it. Here is my best text drawing.

+ ---------------
|
15ohm
|
- ---------------

My 2nd test, in parallel behind the resistor I put the 2 watt
lightbulb, and still got the error from the car. Drawing:

+ ------------------------
| |
15ohm 2watt bulb
| |
- ------------------------

My 3rd test, same as the 2nd test except with a 5 watt bulb. This made
the error go away.

+ ------------------------
| |
15ohm 5watt bulb
| |
- ------------------------


Thing is, in my earlier tests before I had resistors to play with, this
setup also made the error go away:

+ ------------------------
|
5watt bulb
|
- ------------------------

The resistor in parallel doesn't seem to do anything. =o Although, it
gets very hot so it's definitly sucking power.

 

[ehsjr]

This is 2 completely separate circuits I'm speaking about, one for the
left headlight, and one for the right headlight. So, effectively if I
get one working the same setup on the other will work as well (cross
fingers). So, each side will only have 1 resistor. The resistor is a
15ohm 25watt resistor.

When I did my tests last night, the first test which still made my car
beep and cry about the light out was just a single resistor, and that
is it. Here is my best text drawing.

+ ---------------
|
15ohm
|
- ---------------

My 2nd test, in parallel behind the resistor I put the 2 watt
lightbulb, and still got the error from the car. Drawing:

+ ------------------------
| |
15ohm 2watt bulb
| |
- ------------------------

My 3rd test, same as the 2nd test except with a 5 watt bulb. This made
the error go away.

+ ------------------------
| |
15ohm 5watt bulb
| |
- ------------------------


Thing is, in my earlier tests before I had resistors to play with, this
setup also made the error go away:

+ ------------------------
|
5watt bulb
|
- ------------------------

The resistor in parallel doesn't seem to do anything. =o Although, it
gets very hot so it's definitly sucking power.

OK! My guess is that the sensing circuit is looking for
a very low resistance. Once it finds it, it stops looking.
The bulb, when cold, presents a very low resistance, far
lower than 15 ohms. It appears that satisfies the circuit.

It very well may be that the circuit needs to stay "satisfied"
by a higher resistance. You could test with a switch between
the 5 watt bulb and the 15 ohm resistor. One the circuit is
"happy" switch the bulb out of the circuit to see if the
circuit stays "happy". Of course, the 15 ohm resistor is
not the equivalent resistance of the bulb (once the bulb is hot)
When hot and dissipating 5 watts at ~13.8 volts, the bulb is
drawing ~362 mA. That makes the resistance equal to about
38 ohms.

If you want to go with a resistor load instead of the 5 watt
bulb, you may be able to use something conceptually like this:

+----> To headlamp
|
| RY1
Switch | N/C
+ -----o o----+--------+---o-o---+
| | |
R1 [30R] [RY1] [1R] R2
| | |
Gnd --------------+--------+---------+

The 30 ohm resistor can be the two 15 ohm 25 watt resistors
you already have, but it will dissipate only about 7 watts.
That keeps the heat down a bit. The 1 ohm resistor is 20 watts
or more. There needs to be some time delay before the relay
energizes, so RY1 should be energized by an adjustable time
delay circuit. Also, 1 ohm is a guess.

The nice thing about the circuit is that the heavy
power dissipation in the 1 ohm resistor occurs only
for a brief time, until the relay is energized.

And it could be that you don't need the 30 ohm resistor
(2 15 ohm resistors in series) at all - the new style
headlamp may draw enough to keep the sensing circuit
"happy", once the initial test is satisfied. You could
test by placing the 5 watt lamp in the circuit, turning
things on, and then switching the 5 watt lamp out.
If the sensing circuit does not object, than all you need
is the time delay relay (delay on) and a suitable load
resistor (the one ohm resistor I guessed at)

With some experimention with the length of the time delay,
and the value of the 1 ohm resistor, and possible elimination
of the 30 ohms resistance, you may find an ideal method
to make the car's computer happy with your new headlights.
Ideal would be R1 completely out of the circuit. If it
has to be there, the higher the resistance the better,
as long as it yields 100% correct operation. (The higher
the resistance, the lower the heat produced.)

You can make your own time delay circuit:
Sw
+ ----o o---+-----------------+-----+
| | |k
| [RY1] [D] 1N4001
+------+ | |
| | +-----+
[POT]<---+ |
| /
+---+---[Zd]---[330]---| NPN
| | 6V \e
[C] [10K] |
| | |
Gnd ---------+---+-------------------+

You can start with a 10 k pot and a 1000 uF cap and
adjust for a good time delay. Reducing the value
of the cap or lowering the setting of the pot will
shorten the time delay. Once you have adjusted the
pot and have the thing working 100%, you can replace
the pot with a standard value fixed resistor closest
to the pot setting.

Ed