Transistor tester circuit

Posted on Apr 30, 2012

The design objective was to produce an hFE tester with switched collector currents for the DUT (Device Under Test) covering a range suitable for the selection and matching of output transistors for amplifiers such as the JLH Class-A, ESP DoZ etc. The tester should provide a range of collector test currents from 0.05A to 3A in (roughly) logarithmic steps. It is important to avoid the need for high power resistors and (rotary) switches with high current rated contacts, especially since the latter can be very difficult to obtain. It is also important to minimise the cost. Although the circuit may appear complex, it isn't really, but it does test devices at a specified (and fixed) collector current - this is the way that it should be done, but most circuits don't.

Transistor tester circuit
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Refer to Figure 1 (below). D1 (or U2 shown in Figure 2), R3 and VR1 create an adjustable voltage reference. Rc1-Rc7 resistor values have been selected so that the design collector current flows in the DUT when the voltage across the resistor bank is 1V. Q1 monitors the voltage across the resistor bank, compares it with the preset voltage reference and drives sufficient current into the base of the DUT to maintain 1V across the current setting resistors. Note that the supply is 20V DC. The connection points shown indicate positive and negative, and do not mean or imply a dual supply - only the polarity of the connections. Unmarked resistors are 0.25W. Q2, a power Darlington, has been included as a buffer to minimise voltage (and therefore current) variations when DUTs with a low hFE are being tested. SW1 to SW7 should be rated at a minimum of 2A DC. Fuse F1 provides protection from high collector currents and has been fitted where shown, rather than in the supply rail, so that a 3.15A fuse can be used. If fitted in the supply rail, the next higher standard value (4A) would be needed, thus providing reduced protection. This is because the base current would also be passing through the fuse if it were in the supply rail. Fuse F2 has been provided to prevent excess base current in the event of the DUT being faulty or incorrectly connected. A resistor in the collector of Q2 could achieve a similar effect but, if large...

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