April 25, 2024. Meter Magic

At times we need to include some form of metering in our homebrew rigs. No, we will not be installing a Nano VNA in a transmitter. But often a current meter is needed like in a tube type amplifier.

Real panel meters like the Simpson, Tripplet, GE, and Westinghouse of old are still hanging around but can be expensive especially for some specific ranges that may be needed. Yes, you can buy the Chinese ones for a $10 --but just not the look of the old-style round meters.

So with a bit of meter magic I am going to share how you can take a basic 0-1ma meter of old and make it read up to 200ma (or any range) for this specific application.  Enter the Meter Multiplier which is all based on simple mathematics.

Basic electronic mathematics states the current entering a node equals the current leaving a node. So for our example I1 which is the source current is the same value as the sum of I2 and I3 which are leaving the node.  Our 1st equation. 

Now by Ohms law E=IR, the voltage drop with I2 across R1 is the same value as the voltage drop with I3 and R2. They are a parallel circuit to ground starting at the same node. Our second equation: I2XR1 = I3XR2.

Now we have some other known values to finalize our solution and that is we know that the maximum current is 200ma and at that current we want our ammeter to read full scale at 1ma.

To make our life easy we pick the value of R1 to be 1 Ohm and a good reason is that at 200ma the voltage drop across R1 is only 200millivolts (0.2 volts). Another assumption is that at the full 200ma of total current that I3 will be 1ma which now sets the value of I2 at 199ma.

We are on a roll. R1 = 1 Ohms and I2 = 199milliamps so the voltage drop across R2 is also 199millivolts and with 1ma of current E=IR says that R2 is 199 Ohms. 

Now a bit of recognition that we have not addressed the internal resistance of the meter itself and we may not need to know that value. If we are supplying 200ma of current all we need to do is make R2 a trim pot and with the full 200ma of current adjust the trim pot wired as a variable resistor so that the meter reads 1ma. In essence we are taking the internal meter resistance and the adjusted R2 so that their series sum is 199 Ohms. 

So that works for 200ma. But suppose the current is only 100ma which should make the meter read half scale or  0.5ma. Does it? 

Our branching tells us the 0.5 ma is flowing past R2 and so our voltage drop is 0.0005 X 199 = 0.0995 Volts across R2 and that is the same voltage across R1. Thus 0.0995 Volts and 1 Ohm says the current is 99.5ma.Thus 99.5 + 0.5 = 100ma. QED

The quiescent bias value for the tube amp is 20ma. So how would that work. The meter scale at 1 ma indicates 200ma of current so a scaling would say the 20/200 is 1/10 of 1 or 0.1 ma on the meter scale.

Lets check that. (E=IR & we know I and R) Thus 0.0001 X 199 = 0.0199 volts. In our other branch we have 0.0199 Voltage drop and 1 Ohm which gives 19.9 ma, 19.9 + 0.1 = 20ma. QED.  

We could actually make a look up sheet that would give you an equivalent value of current as read on a 0-1ma DC ammeter.

The best for last: what we really are doing is taking the ratio of 1ma of meter current equals 200ma of circuit current. This means you take every circuit current and multiply it by 1/200 for the meter reading. That would be easy to tell you that up front but our several calculations show you the math behind that and you need to do the math to initially find the value of R2.

 I guess I have to say it or it will get missed. If you need a 0-500ma range then do the calculation of R2 for 500ma and then all values of circuit current are multiplied by 1/500 (0.002) and the circuit current is translated to a range of 0-1ma. Cool beans and slightly ahead of the innovation.

Keep in mind no Nano VNA's were used or needed for this sharing.

73's

Pete N6QW

	Current	Meter
	10	0.05
	20	0.1
	20	0.1
	40	0.2
	50	0.25
	60	0.3
	70	0.35
	80	0.4
	90	0.45
	100	0.5
	110	0.55
	120	0.6
	130	0.65
	140	0.7
	150	0.75
	160	0.8
	170	0.85
	180	0.9
	190	0.95
	200	1

A small test to see if you are watching this closely. I entered two values of 20ma in the Excel spreadsheet, so the tabular data is missing the 30ma value -- but the chart will tell you that. It is 4AM and I really am awake.