Light engineering

23.07.2010 18:01

My lab power supply is progressing steadily, with minor setbacks now and then. One trivial task that still needed to be done is putting lights into those nifty analog panel meters.

I ordered two instruments, a voltmeter and an ammeter, from Conrad. They come with fittings for (what I guess are) small incandescent bulbs for backlight, but no actual bulbs. Since I found incandescents a little too retro I decided to fit them with 3mm white LEDs instead. Luckily, they fit in almost perfectly.

I thought clear white LEDs I had would have a too narrow beam and wouldn't lit up the entire scale. So I decided to try my hand at Keith's LED diffusing technique.

It turned out that the single abrasive brush I had was a bit too rough and it ate through the clear plastic LED almost immediately. This was the best I could do, with the bulb still visibly deformed. I would do better to just use fine sandpaper:

Analog voltmeter, lit by one white LED

However, it turned out that when fitted in, the LEDs shine a bit upwards, so the nice bright beam on this photo turns into a single bright spot on the bottom. It actually makes no visual difference if the LED is frosted or not. So I left all subsequent LEDs in their original states.

I did notice though that most of the light was lost through the top of the instrument. So, the solution was straight-forward - put a white cardboard reflector up there.

Result as you can see is pretty good:

Analog voltmeter, lit by two white LEDs

By the way, the left LED is frosted, while the right one is clear.

Here is a close-up of the LED arrangement and the reflector on the top. I removed the metal spring fittings for light bulbs that were too tight and used LED's own leads sa simple spring latches. When the top cover presses down on them they sit pretty securely.

Panel voltmeter without the front cover

Finally, it's interesting to note that the 3 A ammeter has an internal shunt with the moving coil instrument connected directly over it, without any additional series resistance. The instrument itself has an instrument constant of 2.5 mA/A (e.g. 2.5 mA of current must flow through the coil to get 1 A reading).

These meters are advertised as accuracy class 2.5, which is pretty good considering that the shunt is a piece of metal, obviously soldered manually with little precision and gratuitous blobs of solder on both ends. Unfortunately I took the original shunt out before doing any measurements, so I can only speculate whether it actually lived up to its specifications.

Posted by Tomaž | Categories: Analog

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