Calculating the bulb power for 12V knowing the parameters for 24V

Question

Hello all. I am a mechanic, car diagnostician. I have a question about the 12V tester. The point is that there is a 24V 3W bulb inside. My question is: How do I calculate (using the formula) the power of this bulb for a voltage of 12V? For example, to match and buy a bulb for 12V and .... W. I...

Ture11 · Accepted Answer

The most appropriate pattern. At 12 volts you should have 1.5 W. As for the reason why you get these results - I would bet that the cable with the alligator clip has a lot of resistance.

Anonymous · Accepted Answer

But what is the calculation here? that someone figured out that he would make a 12 / 24V universal tester does not mean that you have to follow the measurements in the 12V installation. Find the smallest available power in such a housing because probably less than 5W for 12V is not available and you will have a 12V tester. Remember that you release LIN / CAN with something like that if you accidentally touch the wrong cable.

Jawi_P · Accepted Answer

Buy a normal 12V 250mA or 3W tubular bulb, there are plenty of them on the net.
Although when it comes to the sampler itself, why replace the bulb? This one is so universal that, as you have experienced for yourself, it lights at 12 and at 24V.

jack63 · Accepted Answer

These calculations suck. Bulbs have a resistance strongly dependent on temperature, i.e. in consequence of the flowing current. Moreover, this correlation is non-linear.
Measure the resistance of the bulb with an ohmmeter, you will be surprised ...
Attention eurotips as the most appropriate!
Anyway, who is sensible in the 21st century uses a signal finder light bulb ???

Freddy · Accepted Answer

Why do you want to lie to the customer, such bulbs are available without any problem and cost about PLN 3.

Adamcyn · Accepted Answer

Then read what the famous outlaw, excellent electronics engineer - a colleague wrote on a similar subject Quarz:

Adamcyn · Accepted Answer

This is quite a complex topic and it starts with the distinction between DC or AC voltage.

krzysiek_krm · Accepted Answer

Hello, If the light bulb had a constant resistance (which is of course not true because its resistance depends on the temperature), the current intensity would be twice lower. Simply Ohm's law: I = V / R. As for the power, P = V * I = V ^ 2 / R = I ^ 2 * R, so the electric power for a constant resistance is proportional to square voltage or square current. This is called the Joule-Lenz law in physics. You decreased the voltage twice, the power decreased four times, approximately, of course, without taking into account the change in resistance. best regards

vodiczka · Accepted Answer

So you already know how to deal with it For illustration - the characteristics of the 25W and 100W main series bulbs. Red curve - 100W Blue curve -25W

marekkgb · Answer

First of all, thank you all for your prompt reply.
Buddy Jawi_P
with the current current flowing through the tester I will not do any harm to the ECU. However, 0.2 A will surely close each relay coil.
My friend Ture11
I can measure the resistance of the bulb itself and the wire without load. But since the current flows through the non-load resistance will be close to zero. Currently, the resistances in automotive installations are rarely measured. I focus on the voltage, voltage drops and voltage type.
Buddy jack63- what's the deal with using a light bulb as a signal finder - if you can ask?
I use a 21W bulb to check the circuits under load. Moreover, if I am not satisfied with the result, I can always measure the voltage drop under the load of this bulb on each element of this circuit.

Continuing with the main question how to convert 24V 3W to 12V. Such an example:
,, The customer comes to me with the above-mentioned bulb - I tell him that they are no longer available (or horrendously expensive). How am I supposed to choose a 12V bulb for him?

Thanks

Adamcyn · Answer

Your calculations are correct. For 12.0 volts, use the same method. Using the formula instead of measuring will be more difficult because:

marekkgb · Answer

Thanks Adamcyn

I understand the non-linearity of resistance. But I mean, no one on the forum will answer the following question.

Calculate how much electricity is consumed by a 24W 3W bulb connected to a 12V voltage source?

After all, I should probably remember that I will not mention schools, not studies.

For me, this question is very simple, but I have a problem with the answer.
best regards

Added after 6 [minutes]:

Buddy Freedy
It was a hypothetical query. I have already ordered this bulb - even a few. And it's about my 12V tester.
How to calculate theoretically the equivalent of 24V 3W.

marekkgb · Answer

Buddy Adamcyn. I am completely satisfied with your answer.
I understand and thank you.

Added after 6 [minutes]:

It's still such a small question.
Will every current collector or circuit component follow the same principle as you quoted?

marekkgb · Answer

And that also explained a lot, my friend Krzysk. Thank you for your help. Marek

marekkgb · Answer

Yes. thanks to everyone I close the topic

Calculating the bulb power for 12V knowing the parameters for 24V

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