Im working on led tail lights for my vehicle. the standard bulb is a 21w 12v bulb. what i need to try and figure out is whether or not i am able to recreate this same load using leds without a load resistor. the led in question is a 3.2v forward 20ma. i would like to avoid a load resistor due to its high heat output. Basically i need to fool my car into thinking that it is a normal bulb. im still working on finding the specifics of the circuit for the car so i know what range i need to be in to be successful, but in the meantime im just wondering if there is actually any way to do what i have suggested.


closed as unclear what you're asking by Olin Lathrop, PeterJ, Daniel Grillo, Keelan, Chetan Bhargava Sep 2 '14 at 17:38

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    \$\begingroup\$ Please use proper English capitalization. \$\endgroup\$ – Nick Alexeev Sep 2 '14 at 4:16
  • \$\begingroup\$ Why are you interested in finding a method that doesn't use resistors? Are you interested purely for academic reasons? There are other methods that don't use load resistor but do use resistors for other purposes. If you tell us why you're asking what you're asking, we can possibly help you more. \$\endgroup\$ – Dan Laks Sep 2 '14 at 4:54
  • \$\begingroup\$ its not that i dont want to use resistors. its that i want to try and avoid bulky load resistors that produce alot of heat. my tail lamps on my car have the feature built in that if a bulb goes out it throws an error code on the dash. basically i was curious to see if there is a way to wire leds to obtain the same load as the regular incandescent bulb to be able to avoid a load resisitor. \$\endgroup\$ – Geoffisacnt Sep 2 '14 at 12:50
  • \$\begingroup\$ Unless you can determine how the car knows whether the lamp is blown or not, then putting in the resistive equivalent of the lamp is probably the only way you can keep it quiet. If you have access to the maintenance manual and you can post the relevant circuit diagram(s) maybe we can figure out how to fool the car into thinking there's a good lamp out there. \$\endgroup\$ – EM Fields Sep 2 '14 at 13:53
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    \$\begingroup\$ Olin: His post seemed innocuous enough that the only disrespect I see is yours, since if you can't be "bothered" to read through the thing, then you're being elitist and making unwarranted assumptions about its content and, therefore, flaming gratuitously and out of ignorance. \$\endgroup\$ – EM Fields Sep 2 '14 at 14:03

You don't draw voltage, you draw current from a voltage source/supply, and you can connect as many LEDs in parallel as you want to - up to the current limit of the supply - but you must first connect a current-limiting resistor in series with each LED and then connect each of the strings in parallel, like this:

enter image description here


This won't work as your Source is of 5V. If you connect 2 LED groups in parallel then effective voltage required will be still 3.4V but current will increased to 40mA. This arrangement will need series resistor as well to avoid excessive current from LEDs. If you connect LEDs in series then required voltage to ON LEDs will be 6.8V which is more than 5V source. You'll not able to see Light from LED. Better to use series resistor to make circuit easy.

  • \$\begingroup\$ i planned on using resistors. was just wondering if there was anyway to not have to use a load resistor to stop the error messages and hyper flashing \$\endgroup\$ – Geoffisacnt Sep 2 '14 at 4:33
  • \$\begingroup\$ @Geoffisacnt I highly doubt that the resistor is causing that. Why don't you tell us in your original question EXACTLY what you are doing, what warnings are coming up, and why you think no resistor is the answer to the issue? \$\endgroup\$ – KyranF Sep 2 '14 at 9:41
  • \$\begingroup\$ thanks for the response. No i know the resistors wouldnt cause that. the car expects to see a certain load from the incandescent bulb or it throws an error message saying the bulb is blow. thats what im trying to avoid. i know i can put a load resistor in the mix and fix the issue, but load resistors get hot and are kind of bulky. i was just wondering if there was anyway to avoid the volt resistors. im working now on getting the exact specs on the tail light circuit...i apologize for the delay \$\endgroup\$ – Geoffisacnt Sep 2 '14 at 12:52
  • \$\begingroup\$ so far what i have found is that the stock incandescent bulb is a 25 watt that draws 1.9 amps. is there anyway to mimic this with the leds listed or do i need more info on the bulb for you? \$\endgroup\$ – Geoffisacnt Sep 2 '14 at 13:02
  • \$\begingroup\$ Geoffisacnt: Please take the time and make the effort to write your posts following conventional English practices, otherwise they could be construed as being disrespectful. \$\endgroup\$ – EM Fields Sep 2 '14 at 14:12

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