The Johnson Viking Ranger
by Greg Latta, AA8V

Meter Circuit Diagram and Circuit Description

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How A Shunted Current Meter Works

A shunted current meter places a resistor in parallel with the main current meter to shunt current around the meter. It allows the meter to measure currents larger than the normal full scale rating of the meter. For instance, in the Johnson Ranger, the front meter is rated at 5mA full scale, yet the meter can be used to read currents up to 200mA, as when it is used to read plate or modulator current.

The shunt also allows the circuit to operate even when the meter is disconnected from the circuit, since the shunt can carry the current in the absence of the meter. By placing shunts in various circuits throughout the transmitter, a single meter can be switched across the appropriate shunt to measure the current in that particular circuit.

When the desired full scale current Ifs is flowing into the system, the desired full scale current is split into two parts at point A. Part of the current travels through the shunt (Ish) and part flows through the meter (Im), as shown in the diagram below:

Conservation of current requires that:

Ifs=Ish + Im (1)

Thus, the shunt current is the desired full scale current less the full scale meter current:

Ish=Ifs - Im (2)

In the case of the Ranger, Im=5 mA so:

Ish=Ifs - 5 mA (3)

Now, the meter movement has internal resistance, Rm. By Ohms law, this means that the full scale voltage Vfs across the meter terminals (points A and B) must be Im x Rm:

Vfs=Im x Rm (4)

In the case of the Ranger:

Vsf=5 mA x 20 ohms=0.1V (5)

Since the shunt and the meter are in parallel, the voltage across them is always the same so that:

Full Scale Voltage Across Shunt=Vsf (6)

By Ohm's law, the shunt current Ish times the shunt resistance Rsh must equal the voltage across the shunt Vfs so that:

Ish x Rsh=Vfs (7)

By combining equations (2), (4), and (7) we obtain the equation for the shunt resistance:

Rsh=(Im x Rm) / (Ifs - Im) (8)

In the case of the Ranger:

Rsh=0.1V / (Ifs - 5 mA) (9)

In the table below I have used equation (9) to calculate the various shunt resistances in the Ranger and compared the actual values to the calculated values:

As you can see, the actual shunt values are very close to the calculated values, especially when you consider the tolerances of the shunt resistors used.

Meter Circuit
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Meter Circuit:

Meter Switch: Five different meter shunts are scattered throughout the Viking Ranger, as listed in the table below. The meter switch switches the appropriate shunt resistor across the meter.

L Network: The meter is susceptible to RF from any of the many circuits in the Ranger. It is covered with a metal shield, but it is still possible for RF to get into the meter through the connecting leads. In addition, it is possible for harmonics capable of producing television interference (TVI) to enter through the connecting leads and to escape through the front of the meter. To prevent any RF from entering via the connecting leads, L networks are installed at the entrances to the meter shield. These consist of an RF choke which blocks any RF, and a capacitor that shorts any remaining RF to ground. Such L networks are used extensively throughout the Ranger to prevent any RF from getting where it shouldn't.

Meter Movement: The meter movement in the Johnson Viking Ranger has a movement with a full scale sensitivity of 5 mA and a resistance of 20 ohms, or 200 ohm/V. Though a more sensitive meter could have been used, a less sensitive meter is less susceptible to RF and is more rugged.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu