# What type of resistor do I need?

4 views

### johna...@verizon.net

Feb 14, 2005, 7:45:39 PM2/14/05
to
I'm upgrading the tail lamps on my vehicle and one set of bulbs are not
being used in the new configuration. However, these two bulbs were monitored
by the vehicle's bulb failure system.

I'd like to exchange the bulbs with two resistors (to trick the Bulb Failure
Module), but I'm not sure what kind to use.

They are 5 Watt bulbs.

Thanks
-John

### Anthony Fremont

Feb 14, 2005, 8:30:23 PM2/14/05
to

"neveru...@aol.com" <johna...@verizon.net> wrote in message
news:TCbQd.7072\$uc.3273@trnddc01...

It may not be that simple. Light bulbs have a much lower resistance
when they are off than they do when they get hot. Depending upon the
method of detecting bulb failures, the circuit may rely on the off
resistance (which is what I suspect) or it may detect the high current
flowing thru the wire when it's on (I doubt it). At any rate, you will
need rather large (physically) resistors to dissipate the power (like
10W to be safe). 33 Ohms would be close to the on resistance of a bulb.
When the bulb is off, it's likely to be about 1-3 Ohms.

I would be inclined to leave the bulbs wired into their circuits and
paint them black.

### Rich Grise

Feb 14, 2005, 8:31:01 PM2/14/05
to

If it's a 12V system, then each is drawing P = I * E; I = P / E: 5/12 =
.417 amp. So the resistance is, lessee R = E / I = 12/.417 28.8 ohms.
So 27 or 30 ohms, 10 watts. You could use 5 watt resistors, but they'd be
at their max. rating.

Good Luck!
Rich

### johna...@verizon.net

Feb 14, 2005, 9:23:06 PM2/14/05
to
The system only detects bulb failure when the lights are on. And the newer
unit does not allow clearance for this particular bulb.
"Anthony Fremont" <sp...@anywhere.com> wrote in message
news:PgcQd.58070\$sr1....@fe2.texas.rr.com...

### johna...@verizon.net

Feb 14, 2005, 9:29:26 PM2/14/05
to
To possibly clarify something...I know that on the vehicle, the 55W fog
lights (optional), if not fitted, were replaced with a 1000 ohm (don't know
the wattage rating, etc.) resistor to override the bulb failure system. I'm
trying to apply the same concept, but I don't know the appropriately rating
of resistor needed for a 5W bulb. Obviously I need just enough current
bridged to make the module think a bulb is in place.

Don't know if that tells you how 'sensitive' the BFM is or if it makes any
difference. Not very educated in this area, just pick a little
up along the way.

"neveru...@aol.com" <johna...@verizon.net> wrote in message
news:TCbQd.7072\$uc.3273@trnddc01...

### Mac

Feb 14, 2005, 11:00:45 PM2/14/05
to
On Tue, 15 Feb 2005 02:29:26 +0000, neveru...@aol.com wrote:

> "neveru...@aol.com" <johna...@verizon.net> wrote in message
> news:TCbQd.7072\$uc.3273@trnddc01...
>> I'm upgrading the tail lamps on my vehicle and one set of bulbs are not
>> being used in the new configuration. However, these two bulbs were
>> monitored by the vehicle's bulb failure system.
>>
>> I'd like to exchange the bulbs with two resistors (to trick the Bulb
>> Failure Module), but I'm not sure what kind to use.
>>
>> They are 5 Watt bulbs.
>>
>> Thanks
>> -John
>>
> To possibly clarify something...I know that on the vehicle, the 55W fog
> lights (optional), if not fitted, were replaced with a 1000 ohm (don't know
> the wattage rating, etc.) resistor to override the bulb failure system. I'm
> trying to apply the same concept, but I don't know the appropriately rating
> of resistor needed for a 5W bulb. Obviously I need just enough current
> bridged to make the module think a bulb is in place.
>
> Don't know if that tells you how 'sensitive' the BFM is or if it makes any
> difference. Not very educated in this area, just pick a little
> up along the way.

I was going to do the calculation, as Rich did elsewhere in the thread.
But if you already know that a 1000 Ohm resistor will fool the foglamp
failure detector, I would strongly suggest that you start with that value.
If it doesn't work, go to a smaller value until you find one that does
work.

For power, use this formula:

P=V^2/r, where V is 12 V, and r is 1000 Ohms.

So that is P=144/1000 = 0.144.

So the first thing you should do is try a 1/4 Watt, 1000 Ohm resistor.
This is a common type which can be found at Radio Shack (if they have
Radio Shacks where you live).

If that doesn't work, try lower values of resistance.

Note:
If you go below 680 Ohms, you better switch to 1/2 Watt resistors,
and if you go below 300 Ohms, switch to 1 Watt resistors,
and if you go below 180 Ohms, switch to 2 Watt,
and if you go below 82 Ohms, switch to 4 Watt.

With a 4 Watt rating you could probably go all the way down to 43 Ohms.
Based on what you have said so far, I doubt very much you'll need to go
below that.

Good luck!

--Mac

### Rich Webb

Feb 14, 2005, 11:17:37 PM2/14/05
to
On Tue, 15 Feb 2005 00:45:39 GMT, "neveru...@aol.com"
<johna...@verizon.net> wrote:

>I'm upgrading the tail lamps on my vehicle and one set of bulbs are not
>being used in the new configuration. However, these two bulbs were monitored
>by the vehicle's bulb failure system.

[snip...snip...]

And please learn to cross-post instead of multi-post. That should be
possible even with MS Outlook Distress.

Optimally, if you're not sure exactly which would be the best group, set
the "follow-ups" to your best guess and CROSS post (many newsgroups in
the same newsgroups line) to the three or four groups that you think
might be germane.

Next best, leave follow-up blank and just CROSS post to the multiple
groups. That way, at least there's only one conversation thread. It will
still show up in all of the groups but good news readers will know to
show the postings only in the first group that the reader gets to.

Worst is to MULTI post the same question to several groups. None of the
multiple threads are joined. People will reply in one group when the
question is already answered in another group. And you'll have to go
back and re-read all of the threads separately.

--
Rich Webb Norfolk, VA

### johna...@verizon.net

Feb 14, 2005, 11:22:37 PM2/14/05
to
Thanks Mac,
This is where I was thinking my steps would lead to.
I tried the 1K 1/2W 5% Tolerance Resistors. I suppose it's possible the
contact was not seated firmly enough, but I will play with it a little more
before stepping down to the 680 Ohm. Hopefully that might be the one.

Another thought: What difference does it make that the BFM is monitoring a
5W load as opposed to a 55W load like the fogs (Does that mean it would
require a different value of resistance for the BFM)?

Note: The BFM for the rear lights is seperate from the BFM monitoring the
front.

-John

"Mac" <f...@bar.net> wrote in message
news:pan.2005.02.15....@bar.net...

### Brian

Feb 15, 2005, 4:58:50 AM2/15/05
to

"Rich Webb" <bbe...@mapson.nozirev.ten> wrote in message
news:agt2115esre0e6a5j...@4ax.com...

And when you do this, some weinerhead will tell you not to crosspost :)

### legg

Feb 15, 2005, 6:28:04 AM2/15/05
to
On Tue, 15 Feb 2005 02:29:26 GMT, "neveru...@aol.com"
<johna...@verizon.net> wrote:

>To possibly clarify something...I know that on the vehicle, the 55W fog
>lights (optional), if not fitted, were replaced with a 1000 ohm (don't know
>the wattage rating, etc.) resistor to override the bulb failure system. I'm
>trying to apply the same concept, but I don't know the appropriately rating
>of resistor needed for a 5W bulb. Obviously I need just enough current
>bridged to make the module think a bulb is in place.
>
>Don't know if that tells you how 'sensitive' the BFM is or if it makes any
>difference. Not very educated in this area, just pick a little
>up along the way.
>"neveru...@aol.com" <johna...@verizon.net> wrote in message
>news:TCbQd.7072\$uc.3273@trnddc01...

Sounds like the bulb detector is using the lamp's presence as a
pull-up or pull down, when unpowered, to detect continuity vs open or
blown fuse.

1K should work in the second case also, regardless of power, if so.

RL

### Ian

Feb 15, 2005, 7:51:18 AM2/15/05
to
news:hOeQd.26963\$wc.5483@trnddc07...
Do you know how the BFM works? On an old Volvo I had, it sensed
the difference between currents flowing in pairs of bulbs of the same
wattage
on the offside and nearside of the car. When both bulbs were not present,
the BFM did not indicate a fault.

Regards
Ian

### Rich Webb

Feb 15, 2005, 5:19:26 PM2/15/05
to
On Tue, 15 Feb 2005 03:58:50 -0600, "Brian" <br...@w3gate.com> wrote:

>And when you do this, some weinerhead will tell you not to crosspost :)

True enough. It's almost always better to be familiar enough with the
newsgroup to know that the post is on-topic and post to that single
group.

Of course, if a weinerhead does complain that crossposting is worse than
multiposting, there's always that other handy usenet tradition of adding
the w.h. to the "improve my signal-to-noise ratio" file... ;-)

[Traditionally, follow-ups to this posting should either point out
grammatical and spelling errors or note that it's not on topic.]

### Mac

Feb 15, 2005, 10:05:41 PM2/15/05
to

Well, I just don't know how the BFM works, so it's hard to say. If I had
to design a detector, I would probably just apply a small current to the
circuit, and then set a threshold voltage above which was a fault, and
below which was a good bulb.

But I think that would only work when the lights are off. You say this
system only works when the lights are on. So, maybe it works by measuring
the current when 12 Volts is applied to the bulb.

Or maybe it compares the two sides. Did you read Ian's post about his old
Volvo?

Anyway, good luck!

--Mac

### johna...@verizon.net

Feb 16, 2005, 1:04:02 AM2/16/05
to
I've tried many of the combinations suggested by everyone, but I'm afraid
I've been unable to fool the BFM with a resistor(s). I was, however, able to
wire and conceal a small 'dim' 5W bulb into the circuit...looks and works
fine. Thanks for everyone's expertise!
-John

"neveru...@aol.com" <johna...@verizon.net> wrote in message
news:TCbQd.7072\$uc.3273@trnddc01...

### Rich Grise

Feb 17, 2005, 3:54:36 PM2/17/05
to

Or, just to point out that usually the ones who bitch about crossposting
crosspost their bitch. ;-)

Cheers!
Rich