Why are resistors added to LED turn signal bulbs poll

[kioolt]

I thought that this would be a good poll to find out if people understand the true reason that resistors have to be added when switching to led turn signal bulbs.

[MarkAZ]

A short rant...

 

To me, a vote is basically an opinion. It this case, it's a matter of physics, not opinion. LEDs are high impedance (resistance) devices which causes the controller to think the bulb is burned out because so little current is flowing to the LED. A resistor with a lower value in parallel with the LED will effectively lower the resistance of the circuit and therefore increase the current flowing thru the circuit so that it looks like there's a functioning bulb there. Look up the formula for resistances in parallel to see how it works.

 

There are some LED devices that have a parallel resistor built in so that an external resistor is not required but it's doing the same thing.

 

 

[kioolt]

When I posted this poll I was wanting to get a discussion started on this because most people THINK that the resistor that is added increases the resistance. I thought by doing this poll this forum could educate people that that is not true.

 

Look at the following website for more information.

 

https://www.carbuyingtips.com/articles/blog/how-to-change-your-car-turn-signals-bulbs-and-upgrade-to-leds.htm

[Hank R1200RT]

Question should have "for BMW Canbus motorcycles" added.

 

Don't use a resistor with a normal LED and you will release the magic smoke.

 

Oversimplifying a lot.

[TestPilot]

The CAN bus has nothing to do with the requirement for load resistors with LEDs. The CAN bus simply transmits data from one electronic unit to another. All motor vehicles are required to change the turn signal flashing rate when a burned out bulb (or very low current draw) is present in the circuit (see Federal Motor Vehicle Safety Standard 108).

 

[kioolt]

The led's that this poll is meant to be for are those bulbs packaged for use on 12vdc. These would not require any additional resistors to stop the magic smoke.

[kioolt]

There is a difference between the way an electronic flasher and a old time thermal flasher act with a blown bulb. A thermal flasher will stop flashing at all with a blown bulb (higher resistance) and will flash faster with a higher current draw (lower resistance). Reasons for lower resistance could be additional bulbs added to the circuit (e.g. trailer lights),a short in the wiring or wrong bulbs. In contrast an electronic flasher will flash faster with a blown bulb (higher resistance) and will not change flash rates with a lower resistance. The faster flash rate is a built in design to let the operator know he has a blown bulb. Modern BMW bikes have electronic flasher units. The resistor that are added to stop the fast flash rate are not increasing the resistance, they are decreasing the resistance. If you look at the way the resistors are added you will see that they are in parallel to the bulb. The current through the bulb remains the same. You have not reduced it any. You have however added an additional path for current to flow through the resistor. Higher current flow equals lower resistance. It all got to do with parallel circuit resistance.

 

So, basically I'm trying to state that the second answer is the correct one. The resistance is high and the resistor is added in parallel to the bulb to lower the resistance so that the flasher relay does not think a bulb is blown.

[terryofperry]

"So, basically I'm trying to state that the second answer is the correct one. The resistance is high and the resistor is added in parallel to the bulb to lower the resistance so that the flasher relay does not think a bulb is blown."

 

Why are you saying the resistance is high?

 

Terry

[kioolt]

Led's by their very nature use less current for an equivalent amount of light. Why do you think LED bulbs are being pushed for energy savings reasons in your house? Because they use less current for the same amount of light produced. The same is true for any voltage led which includes motorcycles. With the voltage being the same, anytime the current is decreased the resistance is increased. It's just basic ohms law.

[TestPilot]

Led's by their very nature use less current for an equivalent amount of light. Why do you think LED bulbs are being pushed for energy savings reasons in your house? Because they use less current for the same amount of light produced. The same is true for any voltage led which includes motorcycles. With the voltage being the same, anytime the current is decreased the resistance is increased. It's just basic ohms law.

 

Except that LEDs cannot be modeled as a fixed resistance. LEDs are current-driven, not voltage-driven like incandescent bulbs. Too much current and the LED blows up. The voltage drop across an LED is virtually constant regardless of the current, which would be modeled as a variable resistance.

 

Two resistors are required when using an LED as a turn signal. One is in series with the LED to limit the current. The second is in parallel with the resistor/LED combination to increase the total current and prevent erroneous failure warnings. LEDs design for motor vehicle use often have these resistances built in so no additional circuit elements are required.

 

[kioolt]

Two resistors are required when using an LED as a turn signal. One is in series with the LED to limit the current. The second is in parallel with the resistor/LED combination to increase the total current and prevent erroneous failure warnings. LEDs design for motor vehicle use often have these resistances built in so no additional circuit elements are required.

 

I agree that the series resistor is built into the bulb to limit the current but I can't without reservations say that LEDs designed for motor vehicle use have the parallel resistor built in. If they did they would produce just as much heat as a standard bulb. I have also read that the bulbs that claim that they will work without producing fault codes is just a means of charging you more for their bulb. I don't know if this is true.

[terryofperry]

Led's by their very nature use less current for an equivalent amount of light. Why do you think LED bulbs are being pushed for energy savings reasons in your house? Because they use less current for the same amount of light produced. The same is true for any voltage led which includes motorcycles. With the voltage being the same, anytime the current is decreased the resistance is increased. It's just basic ohms law.

 

Except that LEDs cannot be modeled as a fixed resistance. LEDs are current-driven, not voltage-driven like incandescent bulbs. Too much current and the LED blows up. The voltage drop across an LED is virtually constant regardless of the current, which would be modeled as a variable resistance.

 

Two resistors are required when using an LED as a turn signal. One is in series with the LED to limit the current. The second is in parallel with the resistor/LED combination to increase the total current and prevent erroneous failure warnings. LEDs design for motor vehicle use often have these resistances built in so no additional circuit elements are required.

 

Exactly. Without having the data sheet to your particular lamp or more precisely, lamp array, one cannot answer the question. As Testpilot stated, quality replacements need no further circuitry to work properly on today's vehicles. I understand what you were trying to accomplish with your question and I applaud you but it is not that simple. The resulting circuit is a series-parallel circuit as Testpilot stated. Generally speaking, a red led is designed to have a forward voltage drop of 2 volts +/-, a green may be 3 volts +/-, a white could be 5 volts +/-. Underdriving the lamp will yield little to no light output, overdriving the lamp will yield a greater light output resulting in a lower life expectancy and greater heat loss. Any energy savings go out the window when you add your load resistor(s).

 

It is not as simple as it seems.

 

Terry

[kioolt]

I believe it's safe to say that for those led turn signal bulbs that cause your turn signals to hyper flash it's not because the resistance in lower, it's because the resistance is higher. That was my original intend of this thread/poll.

[kioolt]

Although the poll at this time is 11 for low resistance and 10 for high resistance I see no one explaining why they believe it is low resistance. Does someone care to chime in?

[Bill R MZ]

you will release the magic smoke.

OK, I voted.

Reading on using LEDs, I'm not any where near trying to explain why you need to use a resistor....

However, I fly electric power RC sailplanes...I have "released" a lot of "magic smoke" from wires...

And watched LiPo batteries "vent with flames"...

And this is called "fun"...

Bill R

 

[MarkAZ]

OK...let's say the turn sig circuit is designed for a 24 watt bulb. That would mean the controller is expecting a current of about 2 amps, assuming a 12v supply. The LED 'bulb' I used in the turn sig on my R1150RT draws .100 amp at 13.5v, way below what the controller is expecting so it will think the bulb is missing or burned out and the turn sigs will 'hyperflash'. That means the resistance of the LED bulb is about 135 ohm (by Ohm's law, resistance = volts/amps). If we put, say, a 6 ohm resistor in parallel with the bulb it will appear as a total resistance of just under 6 ohms (for two resistors, R1 and R2, in parallel, the total resistance = R1 x R2 / (R1 + R2) so in this example, total = 810/141 = 5.74 ohms). In a 12v circuit that would give a current of about 2 amps, just like the 24W bulb so the controller wouldn't know the difference.

 

[it turns out, tho, that there's a simple mod to the turn sig controller on the 1150 that makes the resistors unnecessary, so that's what I did.]

 

'The good thing about science is that it's true whether or not you believe in it.'...Neil DeGrasse Tyson

[kioolt]

Do you know where the instructions for the turn signal mod can be found?

[MarkAZ]

Yep...

 

Here: Turn Sig Mod

 

Scroll down a bit for details. It's a long thread but you might want to skim thru it all.

 

I only had to cut the first trace. Just be careful, the traces are very small. Use a sharp Xacto knife.

 

There's some more info in a thread I started in the Oilheads forum. Look for 'LED Turn Sig Bulbs - Need a Resistor?'. It's a couple pages into the Oilheads forum.

Edited June 21, 2017 by MarkAZ

[kioolt]

https://mattgadient.com/2006/10/12/from-regular-to-led-bulbs-how-much-of-a-power-savings-can-you-expect/

[MarkAZ]

Yes, LEDs definitely take a lot less power. On cars and more modern bikes (oilheads and newer) that's not such a big deal as their charging systems are pretty good. I'd guess it would make more of a difference for an airhead with a more marginal charging system or other vintage bikes or cars.

 

The other thing is that if you have to add a resistor you don't really save any power, it's mostly dissipated in the resistor. And the resistors are fairly large and get pretty hot so you have to be careful where you mount them. I would not have put LEDs on my 1150 if I had to use resistors. That turn sig mod is great, tho.

 

For me the main attraction to LEDs on the bike is that they're considerably brighter. It was worth the 15 mins it took to do the mod...and it will still work OK with incandescents, if necessary.