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

marekkgb 14259 15
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  • #1 16855577
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    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 am asking for this because I measured the current consumption of this bulb for a voltage of 12.5 V.
    It was 0.065A. The formula for the power P = U × I shows that this bulb has only 0.8W.
    Thank you for your help

    Added after 2 [minutes]:

    Calculating the bulb power for 12V knowing the parameters for 24V Calculating the bulb power for 12V knowing the parameters for 24V
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    #2 16855635
    Ture11
    Level 39  
    Posts: 5406
    Help: 573
    Rate: 1116
    marekkgb wrote:
    The formula for the power P = U × I shows that this bulb has only 0.8W.

    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.
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    #3 16855639
    Anonymous
    Level 1  
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    #4 16855669
    Jawi_P
    Level 36  
    Posts: 3193
    Help: 259
    Rate: 693
    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.
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    #5 16855695
    jack63
    Level 43  
    Posts: 11699
    Help: 847
    Rate: 2767
    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 ???
  • #6 16855780
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    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
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    #7 16856644
    Freddy
    Level 43  
    Posts: 19997
    Help: 1394
    Rate: 2806
    marekkgb wrote:
    ,, 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?
    Why do you want to lie to the customer, such bulbs are available without any problem and cost about PLN 3.
  • #8 16856666
    Anonymous
    Level 1  
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  • #9 16856766
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    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.
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    #10 16856792
    Anonymous
    Level 1  
  • #11 16856824
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    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?
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    #12 16856865
    Anonymous
    Level 1  
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    #13 16856884
    krzysiek_krm
    Level 40  
    Posts: 4612
    Help: 716
    Rate: 598
    Hello,
    marekkgb wrote:
    Calculate how much electricity is consumed by a 24W 3W bulb connected to a 12V voltage source?

    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
  • #14 16857073
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    And that also explained a lot, my friend Krzysk. Thank you for your help. Marek
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    #15 16857090
    vodiczka
    Level 43  
    Posts: 30170
    Help: 1183
    Rate: 4288
    marekkgb wrote:
    Buddy Adamcyn. I am completely satisfied with your answer.
    I understand and thank you.
    So you already know how to deal with it :)
    For illustration - the characteristics of the 25W and 100W main series bulbs.
    http://www.elektro.info.pl/images/photos/24/1973/__b_7618d25315538abab5c0376536a0c5a0.jpg
    Red curve - 100W
    Blue curve -25W
  • #16 16857748
    marekkgb
    Level 11  
    Posts: 41
    Rate: 10
    Yes. thanks to everyone I close the topic
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Topic summary

✨ The discussion revolves around calculating the power of a 24V 3W bulb when used in a 12V system. The main question posed by a mechanic involves determining the equivalent power for a 12V bulb based on measurements taken from the 24V bulb. Responses highlight that the power at 12V should be approximately 1.5W, although actual measurements showed only 0.8W due to the bulb's resistance being affected by temperature. Participants emphasize the non-linear relationship between voltage, current, and resistance, suggesting that the power decreases significantly when voltage is halved. Recommendations include purchasing a standard 12V bulb, as the original bulb is still available on the market. The conversation also touches on the use of light bulbs as signal finders in automotive diagnostics.
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FAQ

TL;DR: For a 24 V/3 W tungsten bulb run at 12 V, power drops about 4× because P ∝ V² for a fixed R; “electric power … is proportional to square of voltage.” [Elektroda, krzysiek_krm, post #16856884] Why it matters: This helps mechanics choose safe 12 V test-lamp replacements without damaging vehicle electronics.

Quick-Facts

Quick Facts

How do I estimate a 24 V/3 W bulb’s power when powered at 12 V?

Use the ideal resistor model: P scales with V². Halving voltage from 24 V to 12 V reduces power about 4×, so ~0.75 W. Real tungsten filaments deviate due to temperature, so treat 0.75 W as an estimate, not an exact value. [Elektroda, krzysiek_krm, post #16856884]

Why doesn’t a filament bulb scale linearly with voltage?

Tungsten filament resistance increases sharply as it heats, making the V–I relationship nonlinear. As one expert put it, “Bulbs have a resistance strongly dependent on temperature… this correlation is non-linear.” Expect measured current and power to differ from ideal calculations at lower voltages. [Elektroda, jack63, post #16855695]

My tester shows 12.5 V and 0.065 A. Is ~0.81 W correct?

Yes. Power equals voltage times current. 12.5 V × 0.065 A ≈ 0.81 W, which aligns with expectations for a 24 V/3 W bulb run at about half voltage. A moderator affirmed this calculation approach in the thread. [Elektroda, Adamcyn, post #16856666]

Can lead or clip resistance skew my current reading?

Yes. High resistance in a long lead or alligator clip can reduce current and make the bulb seem weaker. Clean contacts, use short leads, and verify with a low-burden ammeter to avoid under-reporting. This was flagged as a likely cause for low readings in the thread. [Elektroda, Ture11, post #16855635]

What 12 V bulb should I buy to replace a 24 V/3 W in a test lamp?

A practical match is a tubular 12 V, 250 mA (~3 W) bulb. It fits common test-lamp housings and delivers similar brightness at 12 V. If your tester is dual-voltage, verify fit and heat before swapping. [Elektroda, Jawi_P, post #16855669]

Do I need to replace the bulb for 12 V testing if my probe is marked 12/24 V?

Not necessarily. The stock 24 V/3 W bulb will still glow at 12 V, just dimmer, which can be useful for continuity checks without stressing circuits. Many mechanics keep the universal setup intact. [Elektroda, Jawi_P, post #16855669]

Is it safe to probe modern cars with a bulb tester near LIN/CAN wires?

Use caution. A bulb can load or short sensitive data lines (LIN/CAN), risking network faults. Prefer a high-impedance test light or DMM on communication harnesses. The thread warns that a bulb probe can “release LIN/CAN” if you touch the wrong cable. [Elektroda, Anonymous, post #16855639]

Will ~0.2 A from a test bulb close a relay coil?

Yes, many automotive relay coils draw around or below 0.2 A, so a bulb that sources that current can unintentionally energize a relay. Test downstream carefully to avoid unexpected actuation. [Elektroda, marekkgb, post #16855780]

Fast way to estimate bulb power at a different voltage (How-To)

  1. Measure current at the target voltage with a reliable meter.
  2. Compute P = U × I for that point.
  3. Validate by watching brightness and heat; tungsten is nonlinear, so don’t extrapolate far from the measured point. [Elektroda, Adamcyn, post #16856666]

Are 24 V/3 W bulbs rare or expensive?

No. Participants report they are easy to source and inexpensive, about PLN 3 each. Check automotive lighting suppliers or online marketplaces for several form factors. [Elektroda, Freddy, post #16856644]

Do all electrical loads behave like filament bulbs when voltage changes?

No. Behavior depends on component physics and whether voltage is DC or AC. Filaments are temperature-dependent resistors; electronics can be constant-power or constant-current. Start by identifying supply type and load characteristics before extrapolating. [Elektroda, Adamcyn, post #16856865]

Where can I see a bulb’s current–voltage curve to understand nonlinearity?

Review example V–I characteristics for 25 W and 100 W lamps shared in the thread. The plotted curves clearly show rising resistance with current and temperature, explaining dim behavior at reduced voltage. [Elektroda, vodiczka, post #16857090]
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