Choosing the Right Resistor for 12V 0.3W LED Indicators: Replacing 12V 21W Motorcycle Bulbs

Question

Hello,
Can someone help in choosing a resistor? I want to replace the two indicators on the back of the moto. Earlier bulbs were 12V / 21W. The new LED indicators are 12V / 0.3W - What resistor should be used in parallel to make it work and keep the old flashing frequency? They flash too fast (I say right away that the change of the circuit breaker / relay is gone for sure)

WRadek · Accepted Answer

For Elektronik987:
Unfortunately, you do not know how the flasher unit on motorcycles works. And your argument is generally correct, but not in this case.

To lukasixthm:
Downward - that is, towards less and less resistance, i.e. less and less Ohms, but more and more power dissipated on the resistor. So talk less about more watts.

As long as you do not replace the circuit breaker with one adapted to the LED indicators, you will not have savings on electricity. But if you don't care about it, you have to hit this resistor.

lukasixthm · Accepted Answer

Buddy Elektronik987, the resistor in question is as if instead of a light bulb. So in parallel with the "LED bulb". This is because inserting the LEDs changes the blinking frequency. The breaker is designed in such a way that it breaks faster after, for example, a light bulb burns out. I don't know if it's a deliberate intention, I didn't delve into it.

When connecting the LED, we have to cheat the circuit breaker with this resistor with a resistance similar to the resistance of the hot fiber (10ohm). Then the supply voltage (14V) is deposited on it, so it must be of correspondingly higher power (20W).

Buddy Mromat. When I said down, I meant lowering the resistance and therefore increasing the watts. Ohma law. Little off top - I recommend you the ElektroDroid smartphone application. It will help you with many simple problems.

lukasixthm · Answer

There are special interrupters for LED turn signals, and if you do not need to replace it, you must select the resistor experimentally. Start with 680ohm 2W and run down if it was still too fast.

Mromat · Answer

Down? So reduce Ohms and Watts? Can you tell me what this is coming from? What patterns? I found something that the old bulbs gave a current of 1.75, so from 21W they gave about 7 Ohms, so a similar resistor (8-10 Ohm) should be used? I found that people wrote 25 W 8 Ohms but I would like to know at least in theory what it comes from.

Anonymous · Answer

Dude, you don't really know what this is about. Tell me what do you need a resistor for these LEDs? You would need a resistor in the case of a voltage higher than the supply voltage of the LEDs. The LED itself will take as much current as it wants because it has some resistance (you will calculate the current flowing in the circuit at the given resistance using the formula I = U / R), so since we have the power of one LEDA, i.e. 0.3 W, and the formula for the power is P = U * I. So I = P / U and all 0.025A comes out. If you were to worry about your turn signals, and in the installation of a motorcycle or scooter, it is possible that you have a peak voltage around 14.5V, in this case you only need a resistor, you will calculate it using the formula R = Usas-Uobc / Iobc. Substituting to the formula you will calculate R, which is a resistor that will limit the current to the set value and there will be a voltage drop on it equal to the difference of supply and load voltages. The resistor is 100R / 1W. About 0.6W will be emitted on the resistor, so it should be used for at least 1W in reserve

Mromat · Answer

Thank you all. I bought what the seller suggested in the electronics store - 50W6R8J (apparently 6.8 Ohm I was surprised myself that it has just such a value, but it supposedly comes from 12V / 1.75A) and it works as it should - the lighting frequency is appropriate. The resistor has a heat sink, and apparently more power in my case means less resistor heat and a higher price. Fellow motorcyclists on foreign forums put 25W8R resistors on 1-3W LEDs and they also worked.
Question - do such combinations affect the service life of the circuit breaker?

bambus94 · Answer

It affects, but in my opinion positively, because the cold filament of the bulb is about 1R, which causes a short increased power consumption. And the resistor has a constant 6R8 and the diode is a fraction of this current.

Choosing the Right Resistor for 12V 0.3W LED Indicators: Replacing 12V 21W Motorcycle Bulbs

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