<p>This show came about because of an email from Dave Morriss on the 4th March 2020</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>I can demonstrate this by measuring the resistance of two light bulbs I have lying around the house.</p>
<p>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</p>
<p>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)</p>
<p>Diagrams Created using LibreOffice Draw "Version: 6.0.7.3"</p>
<p>Formulae<br />
<img src="https://hackerpublicradio.org/eps/hpr3145/img_001.png" alt="Formulae" /></p>
<p><strong>Calculations For (tungsten filament bulb)</strong></p>
<pre><code>V/R = I (Amps)
so 240V / 108 ohms = 2.2 Amps

V x I = P (Watts)
so 240V x 2.2 Amps = 528W</code></pre>
<p><strong>Calculations For (halogen bulb)</strong></p>
<pre><code>V/R = 240V / 144 ohms = 1.7 Amps    (halogen)

V x I = W
so 240V x 1.7 Amps = 408W</code></pre>
<p>As per usual I used google and come up with somebody asking the same sort of question at <a href="https://www.candlepowerforums.com/vb/showthread.php?64832-How-to-identify-voltage-of-an-unmarked-bulb" class="uri">https://www.candlepowerforums.com/vb/showthread.php?64832-How-to-identify-voltage-of-an-unmarked-bulb</a> yes it seems like there a forum for just about anything you can think of.</p>
<p>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.</p>
<p>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.</p>
<p>Another person suggested trying to estimate the length of the filament and using this to determine the working voltage.</p>
<p>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.</p>
<p>It isn't so simple if bulb requires a higher voltage.</p>
<p>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.</p>
<p>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.</p>
