The above schematic is of a circuit that will convert a 0-15Volt voltmeter to a 1.5Amp ammeter.
The Circuit is for a 'Shunt' type ammeter that allows a meter movement with a small current rating to be used to measure a much larger current. In this example 1 milliamp through the meter coil will represent 1.5 amps by shunting most of the current through the 0.1 Ohm resistor.
The meter used in this example is a meter from Radio Shack Part number 270-1754. This meter is scaled from 0 to 15 volts and has a coil resistance of 85 Ohms. A current of 1 milliamp will drive the needle full scale.
By adding two components and a little art work to the meter face you can have a 1.5 amp meter that would perfectly match with a voltmeter made with the same unit.
In this example when a current of 1.5 amps is flowing through the circuit, 1.499 amps will be flowing through the 0.1 ohm shunt resistor (RS), and 0.001 amps will flow through the meter and the calibrating resistance (RC).
Under these conditions the voltage across the shunt will be,
Therefore; for a current of 1 milliamp to flow through the meter the resistance of the meter circuit must be as follows,
If the resistance of the meter itself is 85 ohms then the resistance of the trimmer would be,
To calibrate the circuit, another ammeter is connected in series with the new meter and a load applied. The trimmer (RC) is then adjusted so that the shunt meter and the test meter agree.
A D.P.D.T. switch and the original voltmeter calibrating resistor (Rv) could be added to the circuit allowing one meter to be used for both amps and volts. See the schematic below.
The following diagram shows a method for finding the coil resistance of a meter. Sometimes the coil resistance is marked on the meter face below the clear area of the meter's window.
This type of circuit can be used for any current range from one milliamp to thousands of amps. As long as the ratio between the shunt and meter leg resistances is correct the reading will be accurate.
This circuit can also be used with the many small, battery powered LCD millivolt displays that are now available. These meters usually come with instructions for this same type of circuit.
The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.
If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.
Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.
26 December, 2020