Dial Lamp Puzzle.

[Joe]

When I fixed my radio-gram up I put a new dial lamp in which didn't last long. I put this down to it being a modern one which was probably designed for use in a torch where efficiency would out-rank longevity so fitted a 'vintage' one. Before doing so I checked the voltage on it and it was 6V.

That soon popped too so I put a diode in series with the next one. That was a bit dim so I replaced it with a 4.5V one. This looks like it's at full brightness but according to both analogue and digital meters there's only just over 2V across it.

What the devil is going on?

- Joe

[pwdrive]

From what you describe you have a half wave diode dropper.

Lawrence.

[AlanBeckett]

With the diode in series your meter won't be reading anything sensible.
I can't offhand remember the multiplier for half Sin waves, but if it looks about right does it matter?

Alan

[Joe]

It doesn't matter at all, Alan. I was just curious as to why there seemed to be more voltage than the meters were detecting. It may explain why I couldn't get the figures to add up when I was upping the serge limiter in the R33 after fitting a 1N4007.

Cheers

Joe

[pwdrive]

Hi Joe, if you scroll down the page in this link all should become apparent.

http://cool386.tripod.com/dropper/dropper.html

Lawrence.

[Joe]

Thanks, Lawrence. Most interesting.

- Joe

[pwdrive]

That's ok, if you ever use that idea for a dropper for a series connected heater chain then it would be prudent to include a suitably rated crowbar diode and fuse.
Measuring the true unfiltered output of a half wave rectifier is complicated due to the forward voltage drop of the diode ie: effective waveform not exactly 180 degrees etc.

Lawrence.

[Robert Darwent]

That soon popped too so I put a diode in series with the next one. That was a bit dim so I replaced it with a 4.5V one. This looks like it's at full brightness but according to both analogue and digital meters there's only just over 2V across it.

What the devil is going on?

Hi Joe,

In addition to the information provided by Lawrence, a quick method you can do in your head to arrive at a close value for the 'actual' voltage following a diode dropper; is to multiply whatever your meter measures by 1.57 (actually 1.11 x square root of 2), but 1.5 is more than accurate enough.

So with the 4.5V bulb and diode you are using, there will be roughly 2 x 1.5 = 3V across it. Since you say it is giving a good brightness and is being under-run by about a third voltage-wise, it should last a good while longer than the earlier ones you used.

Regards

[Yorkie]

Of course! 50Hz AC is 3.14 x RMS Voltage yes? So half wave (zero voltage on the negative going half cycle) is half of 3.14 = 1.57.

Interesting thread - it just goes to show that what we think is a simple little thing is often rather more complicated than at first sight.

The difference between theory and practice in practice, is often greater than in the difference between theory and practice in theory!

[pwdrive]

Of course! 50Hz AC is 3.14 x RMS Voltage yes? So half wave (zero voltage on the negative going half cycle) is half of 3.14 = 1.57.

Interesting thread - it just goes to show that what we think is a simple little thing is often rather more complicated than at first sight.

The difference between theory and practice in practice, is often greater than in the difference between theory and practice in theory!

I'm confused, can you expand on that David.

Lawrence.