Friday, March 20, 2020

Analogue Meter

A few years ago, I noticed an analogue meter with an amusing front panel.  Since it is very old, I expected that it may not work right, so I bought two.  They have been sulking in a dark corner of my Junque Bochs for a long time, but I found them again when rummaging for something else and decided that I have to find a use for the poor things.  One turned out to be intermittent, the other is perfectly fine.

Et Voila! A super inaccurate Volt-Amp Meter:

Volt-Amp Meter

It doesn't look too bad mounted inside a Perspex cube that I got at Daiso some weeks ago.

Now the old movement is actually useful again and it has measurement ranges for 2V, 20V and 200V DC and AC, plus 20mA DC.

It is a 20mA movement, so I used 1k, 10k and 100k resistors and a couple diodes to give it a 2V, 20V and 200V scale.

Which is more accurate: My uncalibrated 4 digit digital meter, or my uncalibrated analogue meter?

Component Tester
BTW, I found a very nifty little component tester on AliExpress, the DTU-1701, which goes for the princely sum of about $20.  It is called a Transistor Tester, but it can test anything: Resistors, Capacitors, Coils, Diodes, Triacs and even Transistors too...

DTU-1701 Component Tester

It is very simple to use: Hook the component to any of the three leads and press the test button.  It then displays a graphic of what it found.  To me, it is especially nice for winding RF inductors, which are usually a bit of a chore to measure.

I used to measure coils with my oscilloscope:
Every scope also has a square wave output, used to calibrate the probes.  You could however use it to disturb and pulse test anything and one simple way to test a coil (or capacitor) is to make a LC tank circuit, then excite it and look at the frequency of the ringing.


The ring frequency can then be used to calculate the inductor/capacitor value:

fr = (1/2π) √((1/LC)

That of course depends on having a good known coil/capacitor to test against and most have a tolerance of +-20% or +100 -50%, so this little tester is rather easier.

Have fun!


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