# Resistance Of 1156 Bulb



## Shawn B (Nov 27, 2003)

So I installed led ligt bulbs for my rear signals, now when I signal right or left my lights flash at hyper speed. When the four ways are used they flash normal. I understand LEDs as I am an Electronic Engineering graduate. Obviously I need to use a resistor to get my lights to flash at normal speed again when I signal left or right. I assumed that the LED replacement bulbs I bought would have the required resistance to run in an automotive based application as they were intended, wrong. I can't seem to get a correct resistance reading from the stock 1156 bulb. Is there a way to get the proper resistance value or does someone know what value resistor to use?
Thanks in advance!


----------



## Fast2.0L (Oct 12, 1999)

*Re: Resistance Of 1156 Bulb (Shawn B)*

Well I believe the 1156 is 5 watts in power so it depends on how many watts your LED is drawing. 
Here's a place that sells the resistors near the bottom of the page.
http://www.superbrightleds.com/1157.htm
6ohm 50watt
They recommend using a heavy duty flasher instead though.


_Modified by Fast2.0L at 3:52 PM 1-5-2006_


----------



## EternalMind (Jun 8, 2005)

*Re: Resistance Of 1156 Bulb (Fast2.0L)*

i would recommend the new flasher...otherwise you are looking at rather large resistors (for power dissipation) and a lot of heat http://****************.com/smile/emthdown.gif


----------



## Shawn B (Nov 27, 2003)

*Re: Resistance Of 1156 Bulb (EternalMind)*


_Quote, originally posted by *EternalMind* »_i would recommend the new flasher...otherwise you are looking at rather large resistors (for power dissipation) and a lot of heat http://****************.com/smile/emthdown.gif 

How large of a resistor? 6 ohm? Heat where in the flasher?


----------



## dennisgli (May 17, 2003)

*Re: Resistance Of 1156 Bulb (Shawn B)*

So an 1156 is around 27watts at 12.8volts - you're an EE and you don't know what the resistance is?







BTW, you can 't measure the operating resistance of an incandescent light bulb with a typical ohmmeter.


----------



## Shawn B (Nov 27, 2003)

*Re: Resistance Of 1156 Bulb (dennisgli)*

OK and 1156 is 27 watts at 12.8 volts. Thanks ! Yes I am an EE but it has been a while, I don't even work in the electronics field any more. It's been quite a few years since then. Thanks


----------



## sleepygti28 (Feb 24, 2005)

*Re: Resistance Of 1156 Bulb (Fast2.0L)*


_Quote, originally posted by *Fast2.0L* »_
http://www.superbrightleds.com/1157.htm
_Modified by Fast2.0L at 3:52 PM 1-5-2006_

http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif 
anything LED wise, i highly recommend the website above!


----------



## redmondjp (May 25, 2004)

*Re: Resistance Of 1156 Bulb (Shawn B)*

So to review what you have done and want to do:
1. You replaced your incandescent 1156 bulbs with LED-based bulbs.
2. This caused the load current for the bulbs to drop.
3. You want to know what value of resistor to put in parallel with each LED bulb to bring the current back up to what it was with the 1156s, so the flasher and bulb monitoring circuitry doesn't think that a bulb is out.
First, you can't accurately measure the resistance of the bulb directly, as the filament resistance is much higher once it is glowing-white-hot. The best way to find out the resistance is to calculate it based upon the current that the bulb draws. You can easily measure this with most multimeters that have a DC current range of at least 2 amps.
But let's say that you're lazy like me and you don't want to get out of your chair--read on . . . After doing a quick web search, looks like the 1156 bulb is a 26 or 27W bulb, and consumes roughly 2.1A at the nominal voltage at the bulb location (12.5-13.5V). OK, so let's pencil-whip this one out. I'm going to guess that your LED bulb only draws 0.1A (could be more or less, depending on how many parallel strings of LEDs it has in it). That means that we have to "throw away" 2A of current through the (added) parallel resistor in order to make the car think that normal bulbs are in place.
Using Ohm's law (V=I x R), we know I=2A, we know V=13, and calculate R to be V/I = 6.5 ohms. This is very close to the standard resistor value of 6.8 ohm. OK, now we know what *value* of resistor to use, but we still have to figure out the *size* resistor, as we're basically wasting energy here in the form of heat, and if we pick a resistor with too low of a wattage rating, it will get really hot and could melt some stuff in your car (or worse!).
So using the power forumula of P=VI, we can calculate the power dissipated in this resistor as 13V x 2A = 26W--wow! That's a lot of power we're throwing away here. Since this bulb will not be on continuously, we can get away with using a smaller size of resistor.
If we assume that the bulb is turned on for 1/2 of the time (a 50% duty cycle), then we can probably get away with a 10 watt resistor (which is still a pretty big size). [NOTE: If anybody is doing this for a brake light, you may want to use a higher-wattage resistor, as a 10W resistor will get pretty darn hot if you are on the brakes for a few minutes at a long stop light.]
So let's say a 6.8 ohm, 10 watt resistor will work. You can probably fudge the resistor value a bit and it will still work fine, maybe as high as a 10 ohm or as low as a 5 ohm (but the lower the resistance you use, the higher-wattage resistor you will need)--if you use a 5 ohm I'd go up to a 15 or 20W just to be safe.
Any way you slice it, these resistors will get warm. If you really want to be safe and not worry about this, use 25W resistors and they won't get very warm at all.
Edit: the 6-ohm, 50W resistors mentioned above will work great and probably will barely get warm at all.


_Modified by redmondjp at 3:21 PM 1-5-2006_


----------



## Shawn B (Nov 27, 2003)

*Re: Resistance Of 1156 Bulb (redmondjp)*

Thanks! I got out the old text books and your overview of electronic theory above is correct. Yes a 6 ohm resistor at 50 watts will be more than enough and shouldn't get warm at all.


----------



## L33t A2 (Aug 5, 2003)

*Re: Resistance Of 1156 Bulb (redmondjp)*


_Quote, originally posted by *redmondjp* »_That means that we have to "throw away" 2A of current through the (added) parallel resistor in order to make the car think that normal bulbs are in place.

when you say parallel resistor, how would that be wired in?
in the positive wire going to the led bulb?
or add another wire with that in it also going to the bulb...
or add a wire from the positive source, not going to the bulb but to a ground ??


----------

