A Clever Meter

Adding a meter and switch to get voltage and current readings …

The voltage voltage and current of a power supply can be read by a single panel meter fitted to the output as shown in the diagram below. The switch can be a press button, toggle, or slide switch. In the open position, the meter reads voltage (0 - 30v). When pushed, it reads the current delivered by the power supply.
The same meter and switch arrangement can be added to any power supply. In the diagram below, it is added to a 7805 3-terminal regulator.
The output voltage of a 7805 is normally 5v, but it can be “jacked up” by adding a potentiometer to the common terminal of the regulator. The output voltage can be in the range 5v to 24v, providing the input voltage is at least 5v above the output voltage. The output current will depend on the input voltage and the heatsinking of the regulator.
Only about 100mA to 200mA will be available @5v - 10v due to the voltage drop across the regulator, and a higher current will be available as the output voltage is increased. This is one of the disadvantages of the variable power supply using a 3-terminal regulator.

0 - 30v 1-amp Power Supply

The 0.04R RESISTOR

One of the most difficult components to buy or make is a SHUNT RESISTOR. The 0.04 ohm resistor is called a Shunt Resistor because it “shunt” or “bypasses” the current past the meter (actually called a movement).
The meter can only allow a current of 1mA to flow through it for FSD (Full Scale Deflection), so that the remaining 0.999 amp (999mA) must flow through the shunt. The meter in our case has an internal resistance of 40R and if we want 1/1,000th of 1 amp to flow through it, the ratio of the resistance of the shunt to the meter resistance must be 1,000:1. This gives 0.04R for the shunt.

How do you make a 0.04R resistor?

There are two ways. You can buy low-value resistors. By placing them in parallel you can possibly get 0.04R. But the simplest way is to get a short length of fairly fine enamelled wire and place it in the circuit. You will also need a multimeter set to 1amp range. Connect it in series with the positive line and load the supply to 1 amp. If the 1mA movement swings half-way, the length of the shunt-wire must be doubled. If it swings too far, the wire must be shortened.
Simply adjust the length for full scale deflection. The wire can then be coiled on a plastic former and placed under the PC board. You can use resistance wire but is is difficult to solder and is non-insulated. Ordinary enamelled wire works very well. It can be very fine. Feel the coil after 5 minutes of operation. If it is getting too hot to hold, use thicker wire.

The 0 - 30v calibration of the meter (actually called a movement until it is calibrated) can be done via the experimental approach described above. Some movements have a sensitivity of 30uA or 50uA. For these movements, the series resistor (30k in the diagram above) will have to be increased.
The first thing to remember is: 1,000uA = 1mA.
For a 30uA movement, the resistor will have to be: 1M
For a 50uA movement, the resistor will have to be: 600k

When a movement is connected across a shunt and current flows in the shunt, a voltage develops across the shunt. This voltage is passed to the movement.
Suppose 1 amp is flowing in the shunt and the resistance of the shunt is 0.04 ohms. From Ohm’s Law, V=IR.
The voltage across the shunt is 0.04v (40 millivolts). We work in volts but it is handy to realise the voltage is 40 millivolts, as “talk” in millivolts.

Now, take away the shunt and apply 0.04v to the movement. The resistance of the movement in our case is 40 ohms. The current flowing in the circuit will be I=V/R = 0.04/40 = 0.001A = 1 milliamp = FSD of the meter. In other words the meter will swing full scale.
The 0-1mA meter is really a 50uA movement with a built-in shunt. If you pull the 0-1mA meter apart you will find this shunt inside the case.
If you want to use a movement from an old multimeter, you will need to know its sensitivity. The simplest approach is to assume 50uA and use a 600k resistor.
Monitor the voltage with another multimeter and if it does not correspond, use another value resistor. If it is a 30uA movement, you will need 1M.

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0 - 30v 1-amp Power Supply

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