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hpr3145 :: A light bulb moment, part 1

Finding the working voltage of a bulb

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Hosted by MrX on 2020-08-21 is flagged as Explicit and is released under a CC-BY-SA license.
Hardware, electrical. (Be the first).

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Duration: 00:10:49


This show came about because of an email from Dave Morriss on the 4th March 2020

The upshot of it was that Dave came into possession of an illuminated lantern that he wanted to use. He had a problem however as the lantern in question was sealed and he didn't know the working voltage of the bulb within. He asked me if there was a way of working it out.

Well first of all before I go any further I should probably say I'm not a bulb expert, however I set about doing a little investigating. What I was about to discover is that calculating this is not as straightforward as you might think.

One thing you might think to do is to use a meter to measure the resistance of the bulb. Even doing this however is fraught with difficulty because measuring the resistance of a cold bulb not at working temperature will give a misleading answer. This is because the resistance of a standard bulb changes with temperature.

I can demonstrate this by measuring the resistance of two light bulbs I have lying around the house.

One is a clear traditional Bayonet tungsten filament house bulb rated at 40W. I live in the UK so its operating voltage is 240V I used my trusty Fluke 77 Digital Ohmmeter but any meter capable of measuring resistance will do. This bulb at room temperature measured a resistance of 108 ohms

The other bulb was a Halogen E14 screw in bulb rated at 28W and its operating voltage like the previous bulb was 240V. Its measured resistance at room temperature was 144 ohms)

Diagrams Created using LibreOffice Draw "Version:"


Calculations For (tungsten filament bulb)

V/R = I (Amps)
so 240V / 108 ohms = 2.2 Amps

V x I = P (Watts)
so 240V x 2.2 Amps = 528W

Calculations For (halogen bulb)

V/R = 240V / 144 ohms = 1.7 Amps    (halogen)

V x I = W
so 240V x 1.7 Amps = 408W

As per usual I used google and come up with somebody asking the same sort of question at yes it seems like there a forum for just about anything you can think of.

One person on the forum made the point that there really isn't such a thing as a working voltage it just depends on how long you want the bulb to last. For this reason I always turn down the brightness on any car I own that his dimmable dashboard lights changing these bulbs can be a real pain and or can be expensive.

Sometimes the stated working voltage for a bulb can be misleading for example a car bulb will often state a working voltage of 12V but is likely to be running at a higher voltage when the engine is running due to the operation of cars alternator.

Another person suggested trying to estimate the length of the filament and using this to determine the working voltage.

Yet somebody else suggested just using a variable supply and to just slowly increase the voltage until a reasonable amount of light is generated. This might be quite simple if it's a low voltage bulb and you could even use some batteries lying around connecting them in series adding one battery at a time until the required brightness is achieved.

It isn't so simple if bulb requires a higher voltage.

In the end I just passed this information onto Dave wishing him luck in his path to discovering the optimal working voltage for his lantern.

Feel free to write a comment if any of you out there have any other idea on finding the working voltage of a bulb this way it can be shared with the rest of us during the Community News, better still send in your own show.


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