Nuts and Volts - November 2014

are discussed later in this article.

Circuit Description and Operation

The schematic of the tester is shown in Figure 1. The key to operation and building ease is the use of a transformer, T1, which is readily available from multiple distributors and designed for use in small switching power supplies. T1 has six independent and identical polarized windings of which four are used in the tester: T1-L1 is the “primary” driving the “feedback” winding, T1-L4, and storing energy in the transformer’s magnetic field; then T1-L2 and T1-L3 are connected in series to form the “secondary,” discharging the stored magnetic energy into the diode under test (DUT). Some of you will recognize this basic circuit as a simple “blocking oscillator,” widely used with vacuum tubes for radar applications in WWII, and later adopted as a transistor configuration for the earliest generation of solid-state switching power supplies. In the power supply realm, it’s now termed a “flyback” circuit, harking back to CRT-based/TV horizontal-output circuits using this topology, or often referred to as a “boost” circuit. It’s simple, works with many different transistors, and doesn’t require specialized integrated circuits.

S1 is an SPDT momentary pushbutton switch. Until S1 is pushed, the 9V battery is disconnected from the circuit, and output filter capacitor, C4, is discharged through R8. Pushing S1 to the ON or TEST position removes R8 from the output, and applies 9V to the primary, T1-L1, and to the start-up resistor, R2. It also turns on LED1 — a flashing red LED that indicates to the user that the circuit is

26 November 2014

■ FIGURE 2. Timing diagram showing transformer voltages.

NAME DESCRIPTION PART SOURCE R1 1K R2 10K R3 1.5K R4 150

R5 5. 6

R6 15K C1 100 µF/16V electrolytic C2 0.1 µF ceramic C3 .001 µF ceramic C4, C5 0.22 µF film 871-B32671P5224K000 Mouser EPCOS B32671P5224K000 (See construction text)

D1 1N4148

D2 1N4937 (fast recovery) (See construction text)

LED1 Flashing red 276-312 RadioShack LED2 Yellow 276-350 RadioShack Q1, Q2 2N3904

T1 VPH2-1600-R 283-3671-ND Digi-Key S1 SPDT momentary PB switch Switchcraft 903X (pictured) 89K7683 Newark -or- 28063 Jameco -or- 275-1549 RadioShack PCB 276-150 RadioShack Enclosure SERPAC 231 (Almond) SR231A-ND Digi-Key