Episode: 3145
Title: HPR3145: A light bulb moment, part 1
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr3145/hpr3145.mp3
Transcribed: 2025-10-24 17:45:56

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This is Hacker Public Radio Episode 3145 for Friday 21 August 2020. Today's show is entitled
A Light Bulb Moment. Part 1
It is hosted by Mr. X
and is about 11 minutes long
and carries an explicit flag. The summary is
finding the working voltage of a bulb.
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Hello and welcome Hacker Public Radio audience. My name is Mr. X
and welcome to this podcast.
As usual I'd like to start by thanking the people at HPR for making this service available to us all.
HPR as a community led podcast provided by the community for the community.
I mean you can contribute to why you just pick up a microphone, any recording device
and send something in. I'm sure you must have something that's worth well sending in.
If I can do that sort of thing I'm sure you can.
And right now I'm sure a lot of you all have a bit more spare time on your hands
because of the thing that's going on. The thing that I won't talk about.
Anyway, on with the show.
So this podcast came about because of an email I received from a good friend Dave Morris
on the 4th of March, no less.
This is currently the 30th of March.
I just think we've had to pull this show together.
And I haven't noticed when I shall actually get uploaded.
Anyway, that's sort 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 and 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 out.
Well, first of all, before I go any further, I should probably say I'm not a bulb expert.
However, I said about doing until investigating.
What I discovered was that conquering 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 working at temperature will give a misleading answer.
This is because the resistance of a standard bulb changes with temperature.
And I can demonstrate this by measuring the resistance of two bulbs I have lying around the house.
So the first bulb I have is a clear traditional bayonet tungsten filament house bulb
rated at 40 watts. I live in the UK so its operating voltage is 240 volts.
I use my trusty fluke 77 digital o-meter but any meter capable of measuring resistance will do.
The bulb at room temperature merged a resistance of 108 ohms.
The other bulb was a halogen E14 screw in bulb rated at 28 watts.
And its operating voltage, like the previous bulb, was 240 volts.
Its measured resistance at room temperature with 144 ohms.
Okay so from this we can do a few calculations.
There's a couple of formulae that we could put to use.
There's the standard ohms law of VIR and the power PVI sort of thing.
So what I've done is I've drawn this diagram, the diagrams for the two formulae.
And I'll include that in the show notes.
These were created using LibreOffice draw version 6-073.
It's not too bad actually. I haven't really tried it before.
So that was a bit of fun.
So an easy way of dealing with these formulae is to...
I've described the diagrams that I've created.
If you think of a neculatical triangle as a good pick word.
And you've got this triangle and you split it horizontally with a line.
So you've got two sections, you've got an upper section and a lower section.
It's a horizontal line that bisects the equilateral triangle in the mid point.
So that makes sense.
Now on the bottom area, bottom section, draw a vertical line to split that lower section in two.
So you now have three areas atop, bottom left and bottom right.
In the top, you put a V for volts.
On the bottom left, you put an I for current or amps.
And in the bottom right, you put an R for resistance and ohms.
So we know the voltage and we know the resistance.
What we don't know is a current.
So with one of these diagrams, what you do is if you want to know the current,
you stick a thermal over the I and what's left is a V on the top and the R underneath.
So it's V divided by R.
So 240 volts divided by R, which is 108 ohms.
That comes out with a current of 2.2 amps.
So that bulb, if it had a steady resistance of 108 ohms,
would in theory draw 2.2 amps.
Now the next formula, which again, the same triangle,
but you put different letters on it, you put P, V and I,
P for power, which is measured in watts.
That's in the top segment.
The bottom left segment, you put V for volts.
And the bottom right, you put I for current.
So putting a thermal over the P, the top section,
you can find that by multiplying V times I.
So that's 240 volts times 2.2 amps,
because remember we're decarculated that.
And that comes out at 520 watts,
where that's not quite right, is it?
520 watt bulb, I don't think so.
I think it was a 60 watt, did I say it was a 60 watt?
But did I say 40 watt in fact?
So that's complete nonsense.
So what happens is as the resistance of the bulb,
sorry, as a attempt of the bulb goes up,
the resistance also goes up.
So you can't do that.
Again, we'll do it for the halogen one as well.
So we've got 240 volts, V, divided by R.
It's 240 volts, divided by 144 ohms.
That comes in at 1.7 amps.
That's for the halogen one.
So then you go to the second formula, which is V times I,
for the power or watts.
So that's 240 volts times 1.7.
And that comes out at 408 watts, again.
Rubbish, that's not really good.
So I did a wee bit digging around,
and as per usual, I used Google and came up with somebody
who asked a very similar sort of questions on candelpowerforums.com.
They obviously specialize on an electrical bulbs and whatnot.
I don't know what their 40 is, but yeah,
it seems like there's a forum for just about anything you can think of.
So one person, I'll put that link in the show notes obviously.
One person and a 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 has dimmable dashboard lights.
Changing these bulbs can be a real pain and can be expensive.
Sometimes a stated working voltage for a bulb can be misleading.
For example, a car bulb will often state a working voltage of 12 volts,
but it's likely to be running at a higher voltage
when the engine is running due to the operation of the car's 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 batch at a time
until the required brightness is achieved.
It isn't so simple if the bulb requires a higher voltage.
In the end, I just passed this information on to Dave,
wishing him luck and 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 ideas on finding the working voltage of a bulb.
This way, it can be shared with rest of us during the community news.
Better still, it's sending your own show.
So there we go, that's the end of this podcast.
This will probably just, but might well be a multi-part show
because there's quite a bit to do bulbs.
I'll maybe cover them in later shows.
So stay tuned.
Anyway, thanks for listening to this,
and I hope you found it kind of interesting.
And if you want to contact me,
I can be contacted at www.hpr.googlemail.com.
That's MRX-AT-HPR-TheAT symbol-googlemail.com.
So until next time,
thank you, and goodbye.
Thanks for watching.
Thanks for watching.
