Efficient Testing: Simplifying Procedures
Testing most 3-terminal active components using just an ohmmeter is common, but it can quickly become tedious, especially when dealing with a large number of devices. The idea to consolidate fast and easy testing for these components into a single instrument emerged as a solution. The described unit offers a streamlined approach, allowing you to test a variety of components including NPN and PNP bipolar transistors, N- or P-channel FETs or MOSFETs, UJTs, triacs, and thyristors. Crucially, the testing procedure is non-destructive, ensuring the safety of the components being tested. Universal connectors provide the flexibility to test various package types, including SMDs up to a certain extent. Moreover, the unit’s design allows for seamless transitions between different types of devices without the need for complex and expensive multi-pole switches.
Building an Affordable Versatile Instrument
To create a versatile instrument at an exceptionally low cost, IC1, a 4066 quad CMOS switch, is utilized to switch between bipolar transistors and FETs. LEDs D1-D4 provide visual feedback about the test device’s condition when the ‘Test’ button is pressed. However, the 4066 can handle only a few milliamps, insufficient for testing other component types. This limitation necessitates the use of relay RE1, a 12 V relay with two normally open (NO) contacts. The first contact supplies power to the UJT test circuit, while the second applies power to the triac and thyristor test circuit.
Dynamic UJT Testing and Triggering Thyristors
Dynamic testing of UJT transistors is achieved using a relaxation oscillator, with network R11/C1 setting the oscillator frequency to around 2 Hz. Pin B2 of the UJT produces short pulses, which are lengthened by IC2 configured as a monostable, making them visible through LED D5. The relay’s second pole drives the thyristor’s or triac’s trigger pin. Resistor R18’s value is chosen as a compromise to accommodate varying trigger currents for these devices. R17 plays a critical role, ensuring the holding current is sufficiently high for a triac; a suitable compromise is 250 mA. LED D6 indicates the device’s condition, but it’s essential to confirm the test result by briefly cutting the power to reset the triac.
Construction Details and Testing SMD Devices
On the author’s webpage, CAD files including PCB layout and front panel designs are available, along with project photos. During prototyping, LEDs and the ‘Test’ button were wired onto the copper side of the PCB. Salvaged female connectors for tested devices were used, although numerous models with standard pitch are available in the market. For testing surface-mount devices (SMD), it’s crucial to use small crocodile clips for the test cable.