Nuts and Volts - October 2017

be minimum. If the meter scale is showing a voltage range, it is probably reading correctly.

This can obviously be checked by comparing the power supply voltage with the reading on the meter when the series resistance is low. To make sure that it is not an ammeter, the voltage across the resistor should always be checked and the circuit current calculated.

Once the full-scale meter current and the meter internal resistance have been determined, then external resistance can be added in series to measure voltage or in parallel to measure current.

Full-scale meter values can vary widely. The lowest current (most sensitive meter) that I have has a full-scale range of 20 microamperes. The meter used in the Bird 43 wattmeter, for instance, has a full-scale deflection current of 30 microamperes.

The more sensitive a meter is, the more options it offers when designing it into a circuit. This will be demonstrated later.

Measuring Voltage

To configure a meter to measure voltage, a resistance is usually placed in series with it. If a meter has a full-scale reading of one milliampere, then the series resistance is calculated at 1,000 ohms per volt (minus the internal resistance). (One volt across a 1K ohm resistor equals one milliampere). For instance, if a full-scale voltage reading of 10 volts is required and the internal resistance of the meter is 50 ohms, then the series resistance equals:

10 x 1000 - 50 = 9950 ohms

For the same voltage range, if the meter has a full-scale reading of 50 microamperes, then the series resistance is calculated at 20K ohms per volt (one volt across a 20K ohm resistor equals 50 microamperes). If the internal resistance is 2,500 ohms, then the series resistance equals:

10 x 20K - 2500 = 197,500 ohms

When higher voltage ranges are required — especially when greater than a few hundred volts — it is always a good idea to place the series resistor on the higher voltage side of the meter and connect the opposite terminal to ground as shown in Figure 5.

At voltages greater than 250 volts, then more than one resistor should be used in series — especially if the resistors are rated at 1/4 watt since this is their maximum recommended continuous voltage rating.

A better solution is to use a voltage divider network with the meter connected across an additional small-value series resistor connected to ground (Figure 6).

Obviously, this can only be done if the voltage source can tolerate the additional current load.

It is also important not to exceed the wattage rating of the series resistor. As an example, if a one milliampere movement is being used and the meter range of 300 volts is desired, then the series resistance would be approximately 300K ohms. If a single resistor is used to drop the voltage to the meter, a resistor of at least 300 milliwatts is required.

This would exceed both the voltage rating and the wattage rating of a 1/4 watt resistor.

Most high voltage power supplies used in tube-type amplifiers have a built-in bleeder-resistor network consisting of one or several high wattage resistors. This network is used to discharge the high voltage capacitor bank when the amplifier is turned off to prevent possible electrocution when the covers are removed.

The resistance value of the bleeder-resistor network is usually a compromise between the amount of time that is required to completely discharge the high voltage capacitor (at least five time constants, typically 30 seconds) and the power draw of the bleeder-resistor given off as heat when the amplifier is in operation.