How can I increase the full scale of an analog voltmeter and analog current meter or ammeter?

Question

I have been researching about this for a long time and all I am finding is about 1mA full scale reading. I do not know what it means even. Why do we need to flow 1mA all the time? If I want to create a meter that can read 5mA, can I do some calculations using Ohm's law and make a circuit according to that and it would work?

I have no idea how can I even create an ammeter that can read 5mA current and a voltmeter that can read 20V of current. What are the conditions that I need to take in mind? These internet sources do not help at all.

You do know that these days the viable solution here is a DMM (Digital Multi Meter) that has range selection and function selection options that can address all your needs at rather reasonable cost. — Michael Karas, Nov 04 '22 at 06:01
Series and parallel resistances will get you what you need. 0 to 5ma? 1 ma goes through the meter, and 4ma goes through the parallel shunt resistor. 0 to 20V? Choose a series resistance that limits the current Through the meter to 1ma at 20V. Don’t forget to account for the resistance of the meter in all this math, it’s certainly not zero. — Bryan, Nov 04 '22 at 06:09

score 6 · Answer 1 · edited Nov 04 '22 at 06:54

First it probably helps to understsand how analog ampere and volt meters work. They are basically the same with (in many cases) slightly different parameters.

Wikipedia has a nice explanation of an ammeter.

There are other kinds of constructions with specific strengths and weaknesses but this is the most common one for analogue devices.

It's a copper coil (red line) mounted on an axle with an attached indicator inside a permanent magnet cage. The turning part is spring loaded (green part) so it returns to 0 if there is no current through the coil.

For how exactly it works, read the Wikipedia article. For our purposes it's enough that the turning angle of the indicator needle is proportional to the current through the coil. We always need a current through the coil.

The full scale current tells us, how much current needs to go through the coil to give us a full scale reading. Since a copper wire also has a resistance we cvan calculate the resistance of the coil with Ohms law.

Let's say we have an ammeter with full scale reading at 1mA at 100mV. This means that the coil has a resistance of 100 ohms.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

You now want to have it show full scale at 5mA. This means that we need to divert 4mA somehow to another path so that at 5mA current we have only 1mA gong through the coil of our ammeter.

We do this with parallel resistors. We know we have 1mA going trough the ammeter at 100mV. Now we need to find a resistor that lets 4mA pass at 100mV. $$R = U / I = 100mV / 4mA = 25 ohms$$

schematic

^{simulate this circuit}

That's how the range of an ammeter is extended.

For a voltmeter we do basically the same math but with resistors in series that take up the "excess voltage".

If you want a measurment range of 20V on that same hypothetical ammeter we've been using, the voltage drop across the ammeter is 1mA at 100mV. This means that our series resistors will need to drop 19.9V at 1mA.

$$R = U / I = 19.9V / 1mA = 19.9kOhm$$

schematic

^{simulate this circuit}

Oh God!! You are amazing. Thanks a lot. It was so confusing when I was reading some books — Obiick, Nov 04 '22 at 14:16
@Obiick happy to be helping... Voting up good answers and accepting the answer helps people having the same question in the future. — kruemi, Nov 04 '22 at 14:19
I have just a quick question. Why its says that ammeter should have to connected with the device in series and voltmeter in parallel? — Obiick, Nov 06 '22 at 04:58
@Obiick it basically boils down tu kirchhoffs laws: https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws . Short: Current is basically counting, how many electrons per seconds go trough. It's like counting cars that go trough a tunnel. You count the cars on the road in front of the tunnel or after. But not on the small road that goes over the hill... — kruemi, Nov 07 '22 at 05:35

score 3 · Answer 2 · answered Nov 04 '22 at 06:09

A typical 1 mA meter has a full scale voltage of 50 mV (50 ohms), so you can add a resistor in parallel to read higher current. So to read 10 mA you would need a resistor such that the parallel combination is 50 mV at 10 mA, or 5 ohms. But you actually need about 5.55 ohms to get that value. This resistor is known as a shunt, and for higher current, such a 5 A, you would use a 5 A 50 mV shunt, which is actually 0.01 ohms.

For a voltmeter, it is convenient to refer to a 1 mA meter as 1000 ohms/V, so a 1000 ohm resistor in series will make a meter with 1 V full scale. And 100,000 ohms will be 100 V full scale.

How can I increase the full scale of an analog voltmeter and analog current meter or ammeter?

2 Answers2