# Resistances of different lamps

Discussion in 'General Electronics Discussion' started by electronicsLearner77, Jul 14, 2015.

1. ### electronicsLearner77

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Jul 2, 2015
In automotives like cars trucks we have different kinds of lamps like 75W, 21W etc. I wanted to know if their resistance is same or different. Does a 21W has higher resistance compared to 75W? please help. Can it be modeled as fixed resistance?

2. ### Fish4FunSo long, and Thanks for all the Fish!

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Aug 27, 2013
E = iR
P = iE
Therfore:
P = E^2/R or P = R * i^2

So..... a 120W Bulb Nominally Rated for 12Vdc
==>> 120W/12Vdc = 10A
==>> 12V = 10A * R ==> R = 1.2ohms

A 12W Bulb Nominally Rated for 12Vdc
==>> 12W / 12Vdcc = 1A
==>> 12V = 1A * R ==> R = 12ohms
These are laws, not suggestions

Have Fun!

Fish

3. ### electronicsLearner77

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Jul 2, 2015
Thank you very much for the reply. Few questions
1. Suppose the lamp is rated at 24V but i give 12V will the lamp still glow. In case if lamp does not work for 12V what component of the lamp will stop this functionality.
2. Suppose if i give 12V by taking more current can it still work that is by making V*I constant.
3. Suppose I know only the wattage let us 120W and if i do calculations for different cases
24V
120W/24V = 5A
24V = 5A*R => 4.8Ohms

12V
120W/12=10A
R = 1.2Ohms.
How for the same load different resistances are possible with different power supplies.

4. I came to know that in cold conditions the resistance is different for lamps. Is it correct?

Where am i making mistake in understanding.

4. ### LvW

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Apr 12, 2014
Of course, it is correct. Remember that the inventor of the WIEN oscillator (Hewlett) has used a tungsten lamp as a PTC resistor for stabilizing the amplitude of the oscillator.
Each lamp of this type has a positive temperature coefficient which means: It is a PTC type thermistor (the resistance increases with temperature).
As a consequence, the resistance depends on the current resp. the applied voltage.

5. ### Martaine2005

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May 12, 2015
Yes, but dimly.
Because V=IR.

Martin

6. ### electronicsLearner77

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Jul 2, 2015
One final question is if i take a multimeter and measure the resistance across the terminals of the lamp. Will it give me the resistance of the lamp?

7. ### Martaine2005

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May 12, 2015
Yes, I just had to try that.

Martin

8. ### BobK

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Jan 5, 2010
It will give the resistance at whatever voltage it is using to test it. That will not be the same as the resistance when the bulb is operated at it's rated voltage.

Bob

9. ### Martaine2005

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May 12, 2015
That's a good point Bob.
Both the bulbs I just checked wer 240v and 40w , 60w.
But the resistance measured was for a 240v 40w bulb.

10. ### Minder

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Apr 24, 2015
But as per the previous answers, you will not be able to calculate the wattage of the lamp from this.
M.

11. ### cjdelphi

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Oct 26, 2011
You'd need to switch on the bulb until hot, then quickly before it cools down measure the resistance. ..

the resistance will change as it cools

12. ### electronicsLearner77

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Jul 2, 2015
so finally can I conclude that resistance is changing because of temperature , which follows the formula between temperature and resistance.

13. ### LvW

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Apr 12, 2014
Yes - however, have you such a formula for the device under discussion?

14. ### electronicsLearner77

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Jul 2, 2015
I thought there is a simple common formula between resistance and temperature for all the conductors.

15. ### LvW

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Apr 12, 2014
No - of course not. Each conductor has its own temperature coefficient (positive or negative). More than that, in most cases, in particular for lamps, there is specific (strong) non-linearity!.