Monday, April 2, 2012
Silly pointless stuff... (panel meter)
This one has a little offset (doesn't quite rest at zero) but the for now that doesn't matter. The range seems to go to 20 milliamps.
But... if we put the panel meter in the circuit the needle slams full scale instantly (oh no 3.27mA should be less than 20mA - what's wrong?)
Our little meter doesn't say what the actual resistance is (we could measure it but that's means we have to do a bit of math (very simple math) and today I don't want to do math for this.
When we do this it comes to about 117 ohms (and that's easy to make with a 100 ohm and a 15 ohm resistor in series).
Just for the heck of it the math for figuring out the extra resistors is easy: The meter is 73.9 ohms, the resistor we want in parallel is 9.93 ohms and we want a 20 fold change in the scale... so (73.9 + x + 9.93)/9.93 = 20 -> 83.84 + x = 20(9.93) -> x = 198.6 - 83.84 -> x = 114.86
So all this is kind of silly - it's a very simple thing to do but it's fun sometimes to do things in a different way than usual. Now everyone seems to like digital displays - they are great, but there's something about the old style panel meters that I have a fondness for...