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SemTest: A Discrete Semiconductor Test Set; Pt.2

Check all those semiconductors in your collection with this easy-to-build test set!

By Jim Rowe

THERE’S QUITE A LOT of circuitry in the new SemTest, despite the fact that most of its operation is managed by a microcontroller. For that reason, the circuitry is accommodated on two PCBs which are stacked inside the case.

To begin the circuit description we will start with the lower or main PCB which carries the micro, the power supplies and metering. This section of the circuit is shown in Fig.5.

Microcontroller IC4 forms the brain
of the SemTest. We have used a PIC16F877A because it has five I/O ports, including three that are eight bits wide. It also includes a 10-bit A-D (analog-to-digital) converter with a choice of eight input channels.

All eight bits of both ports B and D are used to control the various relays which provide the test configurations. The two ports drive IC5 and IC6, which are ULN2803A octal Darlington arrays which in turn drive a total of 16 relays.

Most of these are on the upper PCB but four relays are on the main board:

• Relay 1, which is used to switch the device test voltage on and off;

• Relay 2, which switches the test voltage between its “BV” or “OPV” modes;

• Relay 7, which controls the value of the current shunt resistor used to measure device current (and hence switch current ranges); and

• Relay 8, which controls the voltage divider ratio for device voltage measurement (ie, to switch voltage measurement ranges).

The connections to the relays on the upper PCB are made via CON3 and CON4.

Bit lines RC0-RC3 of port C, together with RE0 and RE1 of port E, are used to control the LCD module, which is again on the upper PCB. These connections are made via CON2, which links to CON5 on the upper board via a 10-way IDC ribbon cable.

The remaining bit lines RC4-RC7 of port C plus bit line RE2 of port E are used to monitor activity on the five pushbutton switches S3-S7. These mount on the instrument’s front panel and are connected using a 16-way IDC ribbon cable from CON7 on the upper PCB to CON4 on the main board.

The same connectors and cable are used to make the connections for relays 3-6, 9, 15 & 16, plus the connection for LED1 (the “Test Volts Present” indicator).

Features & Specifications

Main Features

A compact yet flexible test set for most common discrete semiconductor devices, including diodes (junction and Schottky), LEDs, zeners, diacs, bipolar junction transistors (BJTs), Mosfets, SCRs and thyristors (including Triacs). Based on a PIC16F877A microcontroller, with device and test selection, plus the test results, displayed via a 16x2 alphanumeric LCD readout. Devices to be tested are connected to the test set via an 18-way ZIF socket.

Five test voltages are available: a 600V source for avalanche breakdown (BV) testing plus a choice of either 10V, 25V, 50V or 100V for operating voltage (OPV) tests. All test voltages are applied to the DUT via current limiting resistors – 100kΩ in the case of BV tests or 2kΩ in the case of OPV tests.

Maximum avalanche current which can flow during BV tests is 6mA (short circuit current).

Maximum device/leakage current which can flow with OPV = 100V is 30mA; with OPV = 50V is 25mA; with OPV = 25V is 12.5mA; and with OPV = 10V is 6mA.

Minimum leakage current which can be measured = 1μA.

Diode tests

(1) Reverse avalanche current IR (BV)

(2) Reverse leakage current IR (OPV)

(3) Forward voltage drop VF (OPV)

(4) Zener/avalanche voltage VR (BV)

LED tests

(1) Reverse leakage current IR (OPV = 10V)

(2) Forward voltage drop VF (OPV)

BJT tests

(1) Breakdown voltage C-B with emitter o/c V(BR)CBO

(2) Breakdown voltage C-E with base o/c V(BR)CEO

(3) Leakage current C-B with emitter o/c ICBO (OPV)

(4) Leakage current C-E with base o/c ICEO (OPV)

(5) Forward current gain hFE with a choice of three base current levels: 20μA, 100μA or 500μA

Maximum hFE which can be measured with IB = 20μA is 1500 (OPV = 100V)

Maximum hFE which can be measured with IB = 100μA is 300 (OPV = 100V)

Maximum hFE which can be measured with IB = 500μA is 60 (OPV = 100V)

Mosfet tests

(1) Breakdown voltage D-S with G-S shorted V(BR)DSS

(2) Leakage current D-S with G-S shorted IDSS (OPV)

(3) D-S current IDS versus G-S bias voltage VGS (ie, gm)

SCR, PUT & Triac tests

(1) Breakdown voltage with G-K (SCR) or G-A (PUT) shorted V(BR)AKS

(2) Leakage current with G-K (SCR) or G-A (PUT) shorted IAKS (OPV)

(3) Current IAKS with gate current applied (20μA, 100μA or 500μA) and OPV applied

(4) Voltage drop A-K when conducting VAK (OPV)

Note: the test set operates from an external power source of 12V DC. Current drain varies from around 65mA when a test is being set up, to a maximum of approximately 900mA during testing. It can therefore be powered from either a 12V SLA battery or a 12V/1A mains power supply or regulated DC plugpack.

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