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Build this useful test accessory A zener diode tester for your DMM Plug this simple adaptor into your DMM and you can di­rectly read the values of zener diodes. It covers the range from about 2.2V right up to 100V. By JOHN CLARKE 96 Silicon Chip’s Electronics TestBench H OW MANY ZENER DIODES do you have stashed away which cannot be used simply because their value is unknown? In many cases, the type number will be missing (rubbed off) or will be very diffi­cult to read because the print is so small. And even if it can be read, the type number will not directly give you the value you anyway – instead, you have to look it up in a data book. This Zener Tester is the answer to this problem. It plugs directly into your DMM, so that you can directly read the break­down voltage of the zener being tested. The unit can measure all the common types from very low values of around 2.2V right up to 100V. It’s best for 400mW and 1W power devices, although it will also provide a reasonably accurate measurement for 3W zeners. Testing zener diodes Testing zener diodes has always been difficult. This is because the current needed to test a low-voltage zener is vastly different to that required for a higher voltage type. In the past, many zener testers tried to circumvent this problem by applying a constant 5mA and then reading off the value of breakdown voltage. Thus, for a 5V zener, the power dissipated would be 25mW and for a 30V zener, 150mW. While these values may appear OK, let’s see why the constant current idea does not work in practice. Fig.1 shows the typical zener characteristic. In the forward direction, the zener behaves as a diode and begins to conduct at about 0.7V. Conversely, in the reverse direction, there is very little current flow (as in a normal diode), until the “knee” is reached. At this point, the zener breaks down and the voltage remains essentially constant over a wide range of currents. Note the maximum power position (the power rating of the zener) and the 10% maximum power location. These two power limits set the operating range of the zener. If the current is taken below the 10% maximum power posi­tion, the zener voltage will drop markedly as it follows the knee in the curve. This means that if we read the zener voltage below the 10% position, the reading will be well under the correct zener voltage which can only be obtained Fig.1: the typical zener characteristic. In the reverse direction, there is very little current flow until the “knee” is reached, at which point the zener breaks down and the voltage remains virtually constant over a wide range of currents. at higher currents. Note: some zener diode types have a very sharp knee, which enables the diode to operate at very low currents Features • Tests 400mW and 1W zener diodes • • Test range from 2.2V to 100V • Connects to a multimeter for zener voltage reading • Battery powered Constant power testing at 200mW while maintain­ing its rated breakdown voltage. Fig.2 shows the curves for both 1W and 400mW zener diodes for voltages from 3-100V. The lower two traces show the 40mW (10% of 400mW) and the 100mW (10% of 1W) power curves, while the upper two traces show the maximum power curves for 400mW and 1W. To properly test 400mW and 1W diodes, we must have the zeners operate between the 100mW and 400mW curves. In this way, we will be above the 10% power point for both types and below their maximum limits. The trace (dotted) for a zener tester using a constant 5mA current shows Specifications Zener diode test power �������������������� 200mW Test power linearity �������������������������� within 10% of 200mW for zener diodes from 4V to 100V; less than 3.5% change for battery supply variation from 6-9V Battery current drain ������������������������ 35mA <at> 9V; 47mA <at> 6V Open circuit output voltage �������������� 112V nominal Overall efficiency ������������������������������ 63% Converter efficiency ������������������������� >90% Silicon Chip’s Electronics TestBench  97 Fig.2: voltage vs. current curves for both 1W and 400mW zener diodes, for voltages from 3-100V. The lower two traces show the 40mW (10% of 400mW) and the 100mW (10% of 1W) power curves, while the upper two traces show the maximum power curves for 400mW and 1W. that while zeners from 20-80V fit between these limits, the maximum dissipation is exceeded for 400mW diodes above 80V. At the other end, the 10% limit prevents 1W diodes from giving accurate readings below 20V (for 400mW diodes, the limit is extended to below 8V). One way around this is to use a fixed resistor tester oper­ating from a 110V supply. This will enable all 400mW and 1W zener diodes to be 98 tested down to about 3V. Note, however, that this type of tester will go close to the 400mW limit at about 66V. At the same time, the tester will also need to provide up to 1.42W of power to dissipate 40mW in a 3V zener. This repre­sents an efficiency of just 3%. While efficiency may not appear to be a problem, it does present a strain on a small 9V battery when it is called upon to deliver 160mA. Silicon Chip’s Electronics TestBench The final trace shows the 200mW power curve and this fits neatly between the limits specified. The SILICON CHIP Zener Tester follows this curve closely. It always provides the same power to the zener diode, regardless of voltage. And, as a bonus, battery drain is much lower at 35mA. Block diagram The Zener Tester is based on a high voltage supply, pro­duced by stepping Fig.3: block diagram of the Zener Tester. It uses a converter to step up the voltage from a 9V battery so that high-voltage zeners can be tested. The error amplifier and pulse controller ensure that the power delivered to the zener diode remains constant. up from 9V using a converter – see Fig.3. This converter produces up to about 112V, so that high-voltage zeners can be tested. The current supplied to the converter is monitored by error amplifier IC1b which in turn drives a pulse controller (IC2). This maintains a constant current to the converter from the 9V battery. Since the battery voltage is also constant, the power delivered to the converter and thus to the zener is also constant. In practice, this means that the converter alters its cur­rent output depending on the zener voltage. At high zener voltag­es, the current is low and at low voltages, the current is high. A LED reference is used to provide a fixed voltage for the error amplifier, so that current can be maintained. Note that this reference is also compensated for input voltage, so that as the battery voltage falls, the reference voltage rises and allows more current flow through the converter. This maintains the constant power to the converter, regardless of variations in the supply voltage. A standard digital or analog mul- timeter is used to read the value of zener voltage. How it works The full circuit for the Zener Tester is shown in Fig.4. It consists of just a few low-cost components and a stepup trans­former. The step-up circuit uses the two windings of transformer T1 to produce up to 112V. Mosfet transistor (Q1) is used as a switch to charge the primary winding via the 9V supply. When Q1 is switched off, the charge is transferred to the secondary and delivered to a 0.1µF capacitor via diode D1. The advantage of using a 2:1 stepup transformer is that the voltage developed across Q1 is only half that developed across the secondary winding. This means that a 60V Mosfet can be used rather than a 200V type. Q1 is driven by an oscillator formed by 7555 timer IC2. This operates by successively charging and discharging a .0039µF capacitor via a 6.8kΩ timing resistor connected to the output (pin 3). When power is first applied, the .0039µF capacitor is dis­charged and the pin 3 output is high. The capacitor then charges to the threshold voltage at pin 6, at which point pin 3 goes low and the capacitor discharges to the lower threshold voltage at pin 2. Pin 3 then switches high again and so the process is repeated indefinitely while ever power is applied. The current through Q1 is monitored by measuring the vol­tage across the 1Ω source resistor. This voltage is filtered using a 120Ω resistor and a 0.1µF capacitor and applied to error amplifier IC1b. Its output (pin 7) directly drives the threshold pin (pin 5) of IC5. If the current is too high, IC1b pulls pin 5 of IC2 slight­ly lower, so that the pulse width duty cycle to Q1 Fig.4 (below): the circuit diagram of the Zener Tester. IC1b is the error amplifier and this controls the duty cycle of oscillator IC2. IC2 in turn drives Q1 which switches the primary of step-up transformer T1. The secondary output of T1 is then rectified via D1 and applied to the zener diode. Silicon Chip’s Electronics TestBench  99 The PC board fits neatly into a standard plastic case, with room for the battery at one end. Take care to ensure that the test terminals are correctly wired. is reduced. This in turn reduces the current. Conversely, if the current is too low, IC1b pulls pin 5 of IC2 higher. This increases the duty cycle of the drive to Q1’s gate and thus increases the current. IC1b compares the average current value with a reference at its pin 5 (non-inverting) input. This reference is derived from the power supply and LED1 via IC1a. In operation, pin 2 of IC1a monitors a voltage dependant reference derived from a voltage divider (100kΩ & 560Ω) across the supply rails. This reference is fed to pin 2 via a 100kΩ resistor, while a 100kΩ feedback resistor gives the amplifier a gain of -1 for this signal path. Similarly, the 1.8V that appears across LED1 is divided using 100kΩ and 2.4kΩ resistors to give about 42mV at pin 3 of IC1a. IC1a then amplifies this signal by a factor of 2 (1 + 100kΩ/100kΩ) to give 84mV. To understand how this all works in practice, let’s assume that the power supply is at 9V. In this case, the voltage across the 560Ω resistor will be 50mV and so the output (pin 1) of IC1a will be at 84 - 50 = 34mV. However, if the power supply falls to 7.5V (for example), then the voltage across the 560Ω resistor will be 42mV. The pin Fig.5: this diagram shows the winding details for the stepup transformer (T1) – see text. Note that both windings are wound in the same direction. 100 Silicon Chip’s Electronics TestBench 1 output of IC1a will now be at 84 42mV = 42mV. Thus, as the supply voltage goes down, the reference vol­tage applied to pin 5 of IC1b goes up. This ensures that greater current is supplied at lower voltages, to maintain the constant power. As the accompanying specifications panel shows, this scheme works well, with the power varying by only 3.5% for bat­tery voltage ranging from 6-9V. Power supply Power for the circuit is derived from the 9V battery via switch S1. Note that the battery condition is indicated by the brightness of the LED. If LED1 is dim, then it is time to change the battery. The fact that the circuit will work down to below 6V means that battery life is quite good. Construction Construction of the SILICON CHIP Zener Tester is straight­forward, with most of the parts mounted on a PC board coded 04302961 (56 x 104mm). Begin construction by checking the PC board for shorted tracks or small breaks. In addition, the corners of the PC board will need filing out so that it will fit inside the case. The actual shape is shown on the copper side of the PC board. This done, install PC stakes at the Fig.6 (right): make sure that transformer T1 is correctly oriented when installing the parts on the PC board (ie, pin 1 to bottom left). Fig.7 (far right) shows the full-size PC pattern. external wiring points – see Fig.6. These are located at the positive (+) and negative (-) battery wiring points, at the positive and negative terminal connection positions, and at the switch (S1) and LED1 positions. Once these are in, in­stall the two wire links (next to IC1 and next to IC2). Next, install the resistors, followed by the diodes and ICs. Table 1 lists the resistor colour codes but it is also a good idea to check them using a digital multimeter. Make sure that the diodes and ICs are correctly oriented. The capacitors can now be installed, taking care to ensure that the 100µF electrolytic is oriented correctly. This done, install Mosfet Q1 on the board (metal tab towards IC2). LED1 is mounted on the end of its leads, so that it will later pro­trude through the front panel. Similarly, switch S1 is soldered on the top of its corresponding PC stakes. end on pin 6; (2) wind on 20 turns side-by-side in the direction shown and terminate the free end on pin 3; (4) wrap a layer of insulating tape around this winding. The secondary is wound on in similar fashion, starting at pin 5 and winding in the direction shown. Note that the 40 turns are wound on in two layers (20 turns in each), with a layer of insulating tape between them. Terminate the free end of the winding on pin 4. The transformer is now assembled by sliding the cores into each side of the former and then securing them Transformer winding Transformer T1 is wound using 0.25mm enamelled copper wire – see Fig.5. The primary is wound first, as follows: (1) remove the insulation from one end of the wire using a hot soldering iron tip and terminate this TABLE 1: RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  4 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 Value 10MΩ 470kΩ 100kΩ 6.8kΩ 2.4kΩ 1kΩ 560Ω 120Ω 10Ω 1Ω 4-Band Code (1%) brown black blue brown yellow violet yellow brown brown black yellow brown blue grey red brown red yellow red brown brown black red brown green blue brown brown brown red brown brown brown black black brown brown black gold gold 5-Band Code (1%) brown black black green brown yellow violet black orange brown brown black black orange brown blue grey black brown brown red yellow black brown brown brown black black brown brown green blue black black brown brown red black black brown brown black black gold brown brown black black silver brown Silicon Chip’s Electronics TestBench  101 + + - + Ζ + ENER TESTER POWER + Fig.8: this full-size artwork can be used as a drilling template for the front panel. The test leads are fitted with banana plugs (red for positive, black for negative), so that they can be plugged into standard multimeter terminals. The zener breakdown voltage is the read directly off the multimeter display. with the clips. This done, insert the transformer into the PC board, making sure that it is oriented correctly, and solder the pins. Final assembly A plastic case measuring 64 x 114 x 42mm is used to house the assembled PC board. This is fitted with a self-adhesive label measuring 55 x 103mm. Begin the final assembly by affixing the label to the front panel (lid), then drill out mounting holes for the LED bezel, switch S1 and the two banana plug terminals. You will also need to drill a hole in one end of the base to accept a small grommet. This done, mount the two test terminals (red for positive, black for negative) and fit the grommet and LED bezel in place. Next, fit the board inside the case (it 102 sits on four inte­gral mounting pillars) and secure it using four small self-tapping screws. The lid can now be test fitted to check that the switch and LED line up correctly with the front panel. Adjust them for height as necessary, then solder the battery clip leads to their respective PC stakes. Finally, run short lengths of hookup wire from the PC board to the test terminals. Additional leads are then attached to the test terminals and brought out via the grommet fitted to one end of the case. Terminate these leads using banana plugs (red for positive, black for negative). This lets you plug the leads directly into a standard DMM or analog multimeter. Testing You are now ready to test the unit. Silicon Chip’s Electronics TestBench Apply power and check that the LED lights. If is doesn’t, check that the LED is orient­ed correctly. Now measure the voltages on IC1 using a multi­meter. There should be about 9V DC across pins 4 & 8 and a similar voltage between pins 1 & 8 of IC2. If these voltage checks are correct, plug the output leads into your multimeter and press the Power button. Check that the meter reads 112V. If it doesn’t, switch off immediately and check for wiring errors. If everything is OK so far, connect a 1kΩ resistor across the test terminals and check the voltage again (press the Power button). This time, you should get a reading of about 14V across the resistor, which means that the resistor is dissipating about 200mW. If this reading is quite different, check that the voltage across LED1 is 1.7-1.8V and that about 42mV at present on pin 3 of IC1. Assuming a fresh battery, you should also get about 50mV across the 560Ω resistor. If the latter two reading are incor­ rect, check the associated voltage divider resistors. If all is working correctly, you are now ready to measure zener diodes. PARTS LIST 1 PC board, code 04302961, 104 x 56mm 1 plastic case, 64 x 114 x 42mm 1 front panel label, 55 x 103mm 1 pushbutton momentary contact switch (S1) 1 9V battery and battery clip 1 red banana socket 1 black banana socket 1 red banana plug 1 black banana plug 1 EFD20 transformer assembly (Philips 2 x 4312 020 4108 1 cores, 1 x 4322 021 3522 1 former, 2 x 4322 021 3515 1 clips) (T1) 1 2-metre length of 0.25mm enamelled copper wire 1 100mm length of red hook-up wire 1 100mm length of black hookup wire 1 30mm length of 0.8mm tinned copper wire 8 PC stakes 4 3mm screws 1 small grommet 1 3mm LED bezel Semiconductors 1 LM358 dual op amp (IC1) 1 7555, TLC555, LMC555CN CMOS timer (IC2) 1 MTP3055E or A version N-channel Mosfet (Q1) 1 3mm red LED (LED1) 1 1N4936 fast recovery diode (D1) 1 56V 3W zener diode (ZD1) Capacitors 1 100µF 16VW PC electrolytic 2 0.1µF MKT polyester 1 0.1µF 400VDC polyester 1 .0039µF MKT polyester Resistors (0.25W, 1%) 1 10MΩ 2 1kΩ 1 470kΩ 1 560Ω 4 100kΩ 1 120Ω 1 6.8kΩ 1 10Ω 1 2.4kΩ 1 1Ω There’s just one important thing to watch out for here – be sure to connect the zener diode to the test terminals with the correct polarity; ie, cathode (banded end) to positive, anode to SC negative. Silicon Chip’s Electronics TestBench  103