Vermes
Advanced Member level 4
This tool may be useful when you want to meassure beta of a transistor. Measurement of beta (in principle for the silicon transistors the zero currents can be omitted) is based on the measurement of the base current at a fixed Ic current:
h21E = Ic/Ib
In fact, there are three voltage stabilizers – one of them is the right meter built on IC1 and transistors BD135-136, which maintains a constant voltage across the resistor in the collector, what „programs” the collector current (resistors switched by ranges switch), comparing them to a voltage of IC2 stabilizer (Ur= +3,00V) – for pnp transistors, or to a voltage of stabilizer built on IC3 and BC308 (Ur= -3,00V) – for npn transistors.
The system requires symmetrical supply ±(5,5÷15)V, when Uce voltage of tested transistor is:
Uce=(|U|-3)V; - for example for supply ±5,5V, Uce=2,5V.
Those voltages do not need to be stabilized (although in this case stabilized power supply was used), but they cannot drop below 5,5V because of the systems 741 and Uce of tested transistor, at the current efficiency depending on the set range.
The smallest range Ic=2uA (all buttons pressed out), the largest is limited to 1A (1,222A at all buttons pressed), because at that static method of measurement, heating the transistor is caused by power (Uce, Ic current) falsifies the measurement.
The measurement is temporary and takes place when you press the P button, which includes the supply voltages to the system.
The smallest ranges 2uA-200uA require a measurement amplifier of small currents that extend the capabilities of a typical multimeter (200uA) to a range of 20nA, something like that: LINK or a simple amplifier on a operational amplifier.
Resistor 15ohm was made by serial-parallel connecting 4 pieces of AT resistors 15ohm/1%/0,25W, 3ohm from simple wires, they were tested with Wheatstone bridge.
Pictures:
Link to original thread (useful attachment) – Betametr półautomatyczny