المقياس المللي فلطي - القياسات في الدارات الالكترونية .. الإلكترونيات

Millivoltmeter - Measurements in electronic circuits .. Electronics

Millivoltmeter

It is known that the current of the thermal ion valve anode changes according to the change in voltage across the network, so measuring devices that use ion valves are calibrated according to the voltage on their network, as the voltages we want to know are transmitted to the anode via a high-value resistor M1 (259), and the measurement range can be changed by adding other resistors, M1, M2, M3, etc.

259 - Valve voltage meter.

The valve of the electrical measuring device allows the measurement of very small voltages. This valve is very sensitive to the network voltage in it, and it is equipped with a cap to facilitate the connection processes and prevent leakage. Special circuits are used with such valves to measure very small currents and voltages, and to measure currents that are not sensed by measuring devices with a moving coil. Amplifiers are used to increase the sensitivity of such devices (260) where the electric valve is represented by the symbol S1 0
In addition, we note that any change in the anode current leads to a change in the voltage across the resistance, which is taken to the valve S2 through a second resistance M2, and this results in a change in the anode current across the resistance M4. It is worth noting that adjusting the variable resistance is done so that the measuring device does not give any reading and the current intensity passing through each of the two resistances M4 and M5 is equal.

Also, the low current and low voltage present in the "check points" lead to moving the device indicator. The variable resistance M2 is useful in adjusting the sensitivity of the device. It must be noted that amplification by transistors leads to the manufacture of very sensitive measuring devices (261). The low current leads to a change in the current of the transistor T1, which is observed through the resistance M. This in turn leads to a change in the current intensity of the transistor T2 through the resistance M2, which leads to a change in the current of the transistor T3 and the voltage across the resistance M3. The variable resistance is adjusted so that it does not give any current at first. As for the "Zener" diodes, they help in stabilizing the emitter voltages of T2 and T3 because they maintain a certain voltage across their terminals. In this form, it is noted that each of the three transistors performs the amplification process in series.

Often, balance circuits are used in such devices to avoid errors resulting from the transistor context process (slow change in some of its characteristics).

260 - An electrical meter with a balanced amplifier.

261 - A transistor millimeter error meter.