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Build this: High-Voltage Megohm Tester This high-voltage insulation tester can measure resist­ances from 1-2200MΩ. It is battery powered and displays the read­out on a 10step LED bargraph display. By JOHN CLARKE Y OU CAN USE this Megohm Tester to check the insulation of your 240VAC mains appliances, high voltage capacitors and high value resistors. As well, it can be used as a Go/No Go Tester for testing Voltage Dependent Resistors (VDRs, also known as MOVs or metal-oxide varistors). In a pinch, it could also be used to check SCRs and Triacs (high voltage blocking test). It uses an inbuilt inverter to generate a high voltage which can be selected as 100V, 250V, 500V, 600V & 1000V and the insulation reading is indicated on a bargraph display (dot mode) using an LM3914 display driver. These days, virtually all appliances are operated from the 240VAC mains supply, either directly or stepped down to a lower voltage using a transformer. However, for all its 18  Silicon Chip Fig.1: this block diagram shows the basic building blocks of the Megohm Meter. It uses a step-up converter to generate the test voltage. A voltmeter with a LED bargraph display shows the results. advantages there is a downside to electricity and that is its potential to kill. Under normal circumstances, if appliances are well-insulat­ed and correctly earthed, there should not be any cause for concern about safety. However, should there be an insulation breakdown within an appliance, there is the possibility that the appliance can become dangerous. This is particularly true for earthed items where this connection has failed, which is why safety switches are a good idea. There is, however, no substitute for an appliance which has excellent insulation between Active and Earth and between Neutral and Earth. This is where the SILICON CHIP Megohm Tester comes into play because it allows you to check the integrity of the ap­pliance insulation under high voltage conditions. It operation, the tester applies a high voltage (up to 1000V) between the terminals being checked and accurately displays the insulation resistance up to 2200MΩ. You could of course use an ordinary multimeter to check the insulation but this isn’t a valid test. This is because a multimeter only produces a very low test voltage (around 1.5V) and most insulation breakdown occurs at much higher voltages. By applying a high voltage between the test points, the Megohm Tester overcomes this problem. Another problem with a multimeter is that it will only show overrange for “good” insulation measurements rather than an actual value of the resistance. This is because insulation re­sistance measurements usually result in readings of hundreds of Meg­ ohms rather than the nominal 20MΩ maximum value for a multi­meter. So an ordinary multimeter cannot really tell you how good the insulation is and nor can it test under high voltage condi­tions. Naturally, the appliance being tested must be unplugged from the mains socket during the test procedure. Note, however, that the on/off switch on the appliance itself may have to be switched to the ON position, in order to get a valid reading. If this isn’t done, the mains switch will effectively isolate the Active and Neutral wiring inside the device from the Main Features • • • • • • • • • LED bargraph display Five test voltages from 1001000V Measures from 1MΩ to 2200MΩ (2.2GΩ) Can test VDRs and MetalOxide Varistors (MOV) Battery operated Overrange indication External voltage indication Discharge path for charged capacitors Overcurrent trip-out at 10mA test leads and give misleading results. Note also that the Megohm Tester only checks the integrity of the insulation between Active and Earth and between Neutral and Earth. It doesn’t check the integrity of the Earth connection itself. This means that if the Earth connection has failed (eg, there’s a break in the Earth lead), the unit will usually in­ dicate the overrange (OR) condition. The point here is that if you are checking a mains ap­ pliance, you should always independently check the integrity of the Earth connection itself by some other means, either a multi­meter switched to a low Ohms range or a continuity tester. Main features As can be seen from the photos, the SILICON CHIP Megohm Tester is a self-contained unit with just a few self-explanatory controls. It can measure high values of leakage resistance for six different DC test voltages: 100V, 250V, 500V, 600V and 1000V. In addition to checking mains insulation, it can also test capacitors for leakage. A 10-LED bargraph display is used to indicate the leakage resistance, while a 3-position range switch selects either x1, x10 or x100 scale readings, thereby allowing measurements from 1-2200MΩ. The measurements are made via insu­lated external test leads. The front panel also includes an indicator which shows whether there is an external voltage present between the two test points. The output impedance is low enough to discharge any capacitors which may pose a nasty shock hazard after the test procedure – see panel. The Megohm Meter is also fitted with an overcurrent trip circuit. This immediately shuts down the high voltage supply if the current through the probes exceeds 10mA. This current setting is sufficiently high to prevent nuisance tripping when measuring insulation resistance but low enough to prevent the probes caus­ing a bad shock if you accidentally get across them. Block diagram Refer now to Fig.1 for the block diagram of the Megohm Tester. It is powered by a 9V battery and this is stepped up to the required high voltage using a transformer in a January 1999  19 Fig.2: this is the full circuit diagram for the Megohm Meter. IC1, a TL494 switchmode converter IC, is used to drive step-up transformer T1 via Q1, Q2 and Q3 to produce the test voltage. IC2a provides the 10mA overcurrent trip feature, while IC3 functions as a high-impedance buffer amplifier stage for the LED bargraph display driver (IC4). 20  Silicon Chip switch­mode configuration. The high voltage output is then applied to the Test switch (S4) and is also monitored via resistors on switch S2a to derive a feedback voltage. This feedback voltage controls the switchmode supply so that it automatically maintains the selected output voltage. Test switch S4 (a pushbutton type) is wired so that it normally selects the external voltage indicator circuit. This means that LED13 lights if an external voltage is present across the test points. This indicator circuit will also discharge the external voltage if it has been stored in a capacitor. When S4 is pressed, the high voltage supply is switched through to the positive test terminal instead. Any leakage cur­rent between the positive and negative test leads is then fed to a current-to-voltage converter which is simply a resistance selected via Range switch S3. The resulting voltage is then fed to a high input impedance voltmeter circuit which is calibrated to display the resistance across the test terminals. This voltmeter circuit consists of an amplifier stage based on op amp IC3 plus a LED bargraph display based on IC4. Note that this is no ordinary voltmeter since it cannot draw any significant current via the test terminals, otherwise false readings will occur. In fact, a simple calculation will tell us just how small the currents flowing between the test terminals are. Let’s assume a 1000V test voltage and a 2000MΩ (2GΩ) re­sistance between the test terminals. In that case, the current will be only 500nA (500 x 10-9). The same resistance at 100V will give a current of just 50nA. Op amp IC3 provides the high input impedance for the volt­meter circuit, while IC4 drives the LED bargraph display. This display uses 10 LEDs to form the bargraph plus an overrange LED which indicates that the next range should be selected. Finally, the 10mA trip circuit monitors the current through the current-to-voltage converter. If the current exceeds 10mA, the trip circuit shuts down the high voltage supply. Pressing Reset switch S5 restores the supply to normal operation. Circuit details Fig.2 shows the full circuit of the Megohm Tester. It uses four ICs, a Fig.3: the top trace of this scope readout shows the gate drive to Mosfet Q1 when the 100V test voltage range is selected, while the lower trace shows the waveform for the 1000V range. Note how the pulse width increases for the higher test voltage. small transformer, Mosfet Q3 and a handful of other components. The high voltage output is produced by using IC1 to switch step-up transformer T1. It does this via Mosfet Q3 and buffer transistors Q1 & Q2. IC1 is a TL494 switchmode controller which incorporates a nominal 5V WARNING! This Megohm Meter is capable of charging capacitors to very high voltages (up to 1000V). Depending on their value, such capacitors are capable of providing a severe electric shock which, in some circumstances, could even prove fatal. For this reason, always allow the capacitor to fully discharge via the External Voltage LED after releasing the Test switch. This involves leaving the test leads connected to the capacitor until the LED has fully extinguished. Finally, use your multimeter to confirm that the capacitor has fully discharged before disconnecting the test leads. reference, an internal oscillator, two op amp error amplifiers and two output drivers. The outputs can be used in either push-pull or single-ended mode but in our application, we have used the latter configuration. The RC components at pins 5 & 6 set the oscillator frequen­cy to around 22kHz. The outputs appear at pins 9 & 10 and drive buffer transistors Q1 & Q2 which in turn drive Mosfet Q3 to switch T1. The step-up converter uses the two windings in transformer T1 to produce up to 1000VDC. When Q3 is turned on, current flows through the primary winding via the 9V supply. When Q3 is subse­quently switched off, the voltage across the primary is stepped up in the secondary winding and delivered to a .0056µF 3kV ca­pacitor via diodes D1-D3. These diodes are rated at 500V each and so together provide greater than the required 1000V breakdown voltage. The voltage across the .0056µF 3kV capacitor is sampled via a voltage divider consisting of two series 4.7MΩ resistors and a resistor selected by S2a. The sampled voltage is then fed to pin 16 of IC1. This pin is the non-inverting input (+IN2) of an internal error amplifier which monitors the sampled voltage. The gain of this op amp stage is set January 1999  21 Parts List 1 PC board, code 04301991, 87 x 135mm 1 front panel label, 90 x 152mm (note: two versions available – see text) 1 plastic case, 158 x 85 x 52mm 1 SPDT toggle switch (S1) 1 2P6W rotary switch (S2) 1 2P3W slider switch or 1P3W (S3) 1 SPDT momentary pushbutton switch (S4) (Altronics S1393) 1 SPDT momentary pushbutton or SPST push-to-close switch (S5) 1 red banana panel mount socket 1 black banana panel mount socket 2 insulated test leads with banana plugs and insulated probes 1 10kΩ horizontal trimpot (VR1) 1 9V alkaline battery 1 9V battery holder 1 EFD30 transformer assembly (T1) 1 150mm length of red hookup wire 1 150mm length of blue hookup wire 1 150mm length of yellow hookup wire 1 150mm length of green hookup wire 1 400mm length of mains rated wire 1 5m length of 0.25mm ENCW 1 100mm length of 0.8mm tinned copper wire 1 19mm knob 16 PC stakes Semiconductors 1 TL494 switchmode controller (IC1) 1 LM358 dual op amp (IC2) 1 TL071, LF351 op amp (IC3) by the 4.7kΩ resistor between pins 15 & 14 (the +5V reference) and by the 4.7kΩ and 1MΩ resistor in series between pins 15 & 3. The asso­ciated 0.1µF capacitor rolls off the response above about 1.5Hz, while the unfiltered 4.7kΩ resistor allows the op amp to respond quickly to sudden changes. The op amp output is at pin 3 and is 22  Silicon Chip 1 LM3915 log bargraph driver (IC4) 2 BC337 NPN transistors (Q1,Q4) 1 BC327 PNP transistor (Q2) 1 MTP6N60E 600V N-channel Mosfet (Q3) 1 BC557 PNP transistor (Q5) 2 3mm red LEDs (LED11,LED12) 1 10-LED bargraph (LED1LED10) (Jaycar ZD-1700 or 2 x Altronics Z 0179) 1 bi-colour LED (LED13) 3 1N4936 fast recovery diodes (D1-D3) 4 1N4148, 1N914 switching diodes (D4-D7) Capacitors 3 100µF 16VW PC electrolytic 5 10µF 16VW PC electrolytic 1 0.22µF MKT polyester 1 0.1µF MKT polyester 1 .001µF MKT polyester 1 .0056µF 3KV ceramic Resistors (0.25W 1%) 2 4.7MΩ 1 W 1 15kΩ 1 1MΩ 1 12kΩ 1 820kΩ 4 10kΩ 1 430kΩ 1 9.1kΩ 1 180kΩ 3 4.7kΩ 2 100kΩ 2 2.2kΩ 1 91kΩ 1 1.8kΩ 1 82kΩ 1 1.2kΩ 1 75kΩ 3 1kΩ 1 56kΩ 1 680Ω 3 47kΩ 1 180Ω 1 43kΩ 3 100Ω 1 39kΩ 1 27Ω 2 33kΩ 1W 1 1Ω 1 22kΩ Test resistors 2 10MΩ 1 x 3.9MΩ 1W (see text) 1 15kΩ also compared inter­nally with a sawtooth waveform which operates at the oscillator frequency. This frequency is set by the 47kΩ resistor on pin 6 and by the .001µF capacitor on pin 5. The resulting pulse width modulated signal appears at pins 9 & 10 (E1 & E2) of IC1. This drives pushpull pair Q1 & Q2, which in turn drive the Mosfet (Q3). If the voltage on pin 16 of IC1 rises above the +5V reference, the duty cycle of the pulse width waveform reduces to lower the output voltage across the .0056µF capacitor. Conversely, if the voltage on pin 16 goes below 5V, the duty cycle increases to increase the output voltage. As a result, the high voltage output is regulated so that the voltage on pin 16 of IC1 equals the voltage on pin 14 (ie, +5V nominal). Thus, when S2a is in position 1, the division ratio is 43kΩ/(4.7MΩ + 4.7MΩ + 43kΩ) = .00455. So if the reference voltage is 4.75V (minimum value) the output voltage will be regulated to 4.75/.00455 = 1043V. Note that we offer a method of reducing this value later on in the article should the voltages be more than 10% high. Similarly, the other four switch positions give regulated output voltages of (nominally) 600V, 500V, 250V and 100V. The 10µF capacitor at pin 4 of IC1 provides a “soft” start for the high voltage converter circuit. When power is first applied to the circuit, pin 4 is initially pulled to the +5V reference via the capacitor. This prevents any pulses from ap­pearing at pins 9 & 10. The pulses then begin to appear and gradually widen as the capacitor charges via the 4.7kΩ resistor to ground. Full regulation of the output voltage occurs once the capacitor has fully charged. 3V supply A +3V reference is required for the remainder of the cir­cuit and this is derived from the +5V reference via a voltage divider consisting of 10kΩ and 15kΩ resistors. The resulting +3V rail is filtered using a 10µF capacitor and applied to pin 3 of op amp IC2b which is wired as a voltage follower. Its output appears at pin 1 and is decoupled using a 100Ω resistor. A 100µF capacitor provides further filtering for the resulting +3V refer­ence. When Test switch S4 is pressed, the test voltage is applied to the positive (+) test terminal. As a result, a leakage current will flow between the positive and negative test terminals (ie, between the test points) and through one of three pairs of resis­tors selected by Range switch S3. This leakage current also flows through the 100Ω resistor between the wiper of S3 and the +3V refer- The PC board can accommodate either two 5-LED bargraph displays (as shown here) or a single 10-LED display. Make sure that all parts are correctly oriented and note that Mosfet Q3 (near transformer) is bent over so that it will clear the front panel. ence. The voltage devel­oped across this resistor (and thus the current through it) is monitored by pin 5 of op amp IC2a (via the associated 47kΩ and 2.2kΩ series resistors). Pin 6 of IC2a is biased to +4V by the 10kΩ and 39kΩ voltage divider network between the +5V rail and ground. If the current through the 100Ω resistor rises above 10mA, the voltage across it will be greater than 1V. When added to the 3V reference, this means that the voltage on pin 5 of IC2a rises above +4V. IC2a is wired as a comparator and so its pin 7 output now switches high. This does three things. First, it turns on transistor Q4 which in turn lights LED11, the overcurrent indicator. Second, it pulls pin 16 of IC1 high via diode D4, which shuts down the high voltage supply. And third, it pulls pin 5 of IC2a high via D5 so that the comparator (IC2a) is latched with its output high. Normal circuit operation can now only be restored by press­ing the Reset switch (S5). This pulls the voltage on pin 5 of IC2a below the voltage on pin 6 and so pin 7 switches low and the switchmode converter starts working again. Voltmeter circuit As indicated previously, IC3 and IC4 form a high-impedance voltmeter. IC3 (TL071) functions as a buffer amplifier which monitors the voltage across the resistors selected by S3. This op amp offers a very high input impedance of about 1,000,000MΩ (1TΩ) and has a nominal 200pA input current. The gain of IC3 is x10 for the 1000V position of S2b and x100 for the 100V setting. The remaining test voltage positions (250V, 500V & 600V) give gains between these two figures. These gain adjustments are necessary to compensate for the different currents that flow through the selected detector resistors when different ranges are selected. The 0.22µF capacitor between pins 2 & 6 rolls off the fre­quency response above about 0.8Hz, thereby filtering out any hum pickup. The 100kΩ input resistor at pin 3 protects the input from damage if the test terminals are shorted (even at the 1000V setting), Specifications Test voltages................................................... 100, 250, 500, 600 & 1000V Test voltage accuracy after adjustment...............................................<10% Display readings......................................1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 Reading ranges...................................................x1MΩ, x10MΩ & x100MΩ Current drain................................................................ 50mA <at> 1000V out January 1999  23 Fig.4: install the parts on the PC board as shown on this wiring diagram. Note that the leads from the test terminals are terminated on the copper side of the PC board. Fig.5: (left) the wiring details for the step-up transformer. The 10turn primary is wound on first, followed by the 70-turn secondary – see text. 24  Silicon Chip while diodes D6 & D7 limit the input voltage swing to 0.7V above and below the 3V supply. VR1 is the offset adjustment. It allows the output at pin 6 to be trimmed to 3V under no-signal conditions. S3 is used to switch one of three series resistor pairs in series with the 100Ω resistor on its wiper, to give the x1, x10 and x100 ranges. Position 1 selects a total of 128Ω, position 2 selects 1.28kΩ and position 3 selects 12.78kΩ. At first glance, these may appear to be unusual values. However, they have been selected to correspond to the 1.28V full-scale reading for the LM3915 LED bargraph driver (IC4). IC3’s output is applied to the pin 5 input of IC4 via a 1kΩ resistor. IC4 is a logarithmic LED bargraph display driver, connected here to drive LEDs 1-10 in the dot mode. Each step represents 3dB (ie, a 1.41 ratio), giving a total range of 30dB. The internal reference is 1.28V and this sets the maximum sen­sitivity of the display. The overrange indicator circuit relies on the fact that when IC4 overranges, all the LEDs are off. By including a 100Ω resistor in series with the commoned LED anodes, the voltage across it can be monitored using PNP transistor Q5. When a LED is on, the voltage across the 100Ω resistor is greater than 0.7V and so Q2 is biased on. This shorts out LED12 and so the overrange indicator is off. However, if all the LEDs are off (ie, when IC4 overranges), the voltage across the 100Ω resistor is zero and Q5 is off. This removes the short from across LED12, which now lights via its 1.8kΩ current limiting resistor. LED13 is the “External Voltage” indicator. This bicoloured LED is wired in series with two 33kΩ 1W resistors between Test switch S4 and the +3V reference. Normally, one side of LED13 is directly connected via S4 to the positive test terminal. If there is an external voltage at the test terminals, current can flow from the positive test terminal, through LED13 and the 33kΩ resistors, and back to the negative test terminal via the resistors selected by switch S3. The LED glows red for DC voltages of one polarity and green for DC voltages of the opposite polarity. If an AC voltage is present, both colours will come on together to display orange. Note that the LED will begin to glow SMART FASTCHARGERS® 2 NEW MODELS WITH OPTIONS TO SUIT YOUR NEEDS & BUDGET Now with 240V AC + 12V DC operation PLUS fully automatic voltage detection Use these REFLEX® chargers for all your Nicads and NIMH batteries: Power tools  Torches  Radio equip.  Mobile phones  Video cameras  Field test instruments  RC models incl. indoor flight  Laptops  Photographic equip.  Toys  Others  Rugged, compact and very portable. Designed for maximum battery capacity and longest battery life. AVOIDS THE WELL KNOWN MEMORY EFFECT. SAVES MONEY & TIME: Restore most Nicads with memory effect to capacity. Recover batteries with very low remaining voltage. CHARGES VERY FAST plus ELIMINATES THE NEED TO DISCHARGE: charge standard batteries in minimum 3 min., max. 1 to 4 hrs, depending on mA/h rating. Partially empty batteries are just topped up. Batteries always remain cool; this increases the total battery life and also the battery’s reliability. DESIGNED AND MADE IN AUSTRALIA For a FREE, detailed technical description please Ph: (03) 6492 1368 or Fax: (03) 6492 1329 2567 Wilmot Rd., Devonport, TAS 7310 Fig.6: check your PC board for defects before mounting the parts by comparing it with this full-size etching pattern. for external voltages of about 30V and will be fully lit at 240V. Basically, this circuit is intended to discharge any residual voltages that may be left following the test procedure. This can commonly occur when testing capacitors for leakage or if an internal capacitor in the appliance being tested is charged to the test voltage. Power for the circuit comes from a 9V battery via switch S1. There are several 100µF and 10µF capacitors across the supply and these are used to decouple the 9V rail. Construction The SILICON CHIP Megohm Tester is built on a PC board coded 04301991 and measuring 87 x 135mm. Fig.4 shows the assembly details. Begin construction by checking the PC board for any defects by comparing it with Fig.6. This done, install PC stakes at the external wiring positions. These are located at the (+) and (-) battery wiring points, the wiring points for S3 and the (+) and (-) output terminal points. Next, install the links and resistors. Table 2 shows the resistor colour codes but we recommend that you check each value on your digital multi­meter just to make sure. The ICs and diodes can now be installed, taking care to ensure that each part is correctly oriented and that it is in the correct location. This done, install trimpot VR1 and the capaci­tors (the electrolytics must be correctly oriented), followed by the transistors. Note that transistors Q1, Q2, Q4 January 1999  25 Use medium-duty hook-up wire for the leads to the test terminals and keep them separated as shown here. The leads from the 9V battery holder are also terminated on the underside of the board. and Q5 should all be mounted close to the PC board. Just push them down onto the PC board as far as they will comfortably go before soldering their leads. Don’t get these transistors mixed up – there are three different types involved here. Mosfet Q3 is mounted using its full lead length so that it can be bent horizontally over transistors Q1 and Q2, to allow clearance for the case lid (see photo). Note that its metal tab faces trans­former T1. Now for the switches. First, cut the shaft for S2 so that the knob can be pushed down close to the threaded collar. This done, lift the locking tab located under the nut and star washer and rotate it to position 5. Finally, solder the switch to the PC board and check that there are now only five positions available for this switch. Switches S1, S4 & S5 are also directly mounted on the PC board. Note particularly that S4 and S5 Table 1: Capacitor Codes ❏ ❏ ❏ ❏ ❏ Value 0.22µF 0.1µF .0056µF .001µF IEC 220n 100n 5n6 1n 26  Silicon Chip EIA 224 104 562 102 must be oriented correct­ l y, with their common (COM) pins located as shown (S4 goes in with its COM terminal towards the bottom edge of the board, S5 with its COM terminal towards the top). If you are using a 2-pin push-toclose switch for S5, then solder it in with its pins in the COM and NO positions. Transformer winding Fig.5 shows the winding details for transformer T1. It is wound on an EFD30 former using 0.25mm enamelled copper wire (ENCU). The primary winding goes on first. Strip back the insula­tion on one end of the wire using a hot soldering iron and termi­nate this end on pin 1 of the former. Now wind on 10 turns side-by-side in the direction shown on Fig.5 and terminate the free end on pin 5. Cover the primary winding with a layer of insulat­ing tape. The secondary begins at pin 9 and must also be wound in the direction shown. You will need to wind on the 70 turns in several layers. Cover each layer with insulating tape before winding on the next and terminate the winding on pin 6. The transformer is now completed by sliding the cores into each side of the former and securing the assembly with metal clips. Finally, install the completed transformer on the PC board, making sure that it is oriented correctly; ie, pin 1 to top left. The LEDs can now all be installed at their appropriate locations but don’t solder them just yet – that step comes later. Once again, these must be oriented correctly (the anode lead is the longer of the two). The exception is LED13 which can really be installed either way around. Two different bargraph displays can be used in this cir­ cuit: (1) a single 10-LED bargraph from Jaycar; or (2) two 5-LED bargraphs from Altronics. Which ever type you use, be sure to in­stall the bargraph with its LED anodes to the left, as shown on Fig.4. Splay the leads slightly so that the bargraph remains in position but again leave the leads unsoldered. Final assembly The Altronics bargraph is slightly longer than the Jaycar bargraph, so we have designed two different front panel labels to suit. Just choose the appropriate front panel for your bargraph. Affix this panel to the lid of the case, then drill and file the holes for the LED bargraph, LEDs11-13 and switches S1-S5. You will also have to drill two holes in one end of the case for the output terminals. These should be positioned near the bottom of the case, to provide clearance for the PC board. Table 2: Resistor Colour Codes ❏ No. ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  1 ❏  3 ❏  1 ❏  1 ❏  2 ❏  1 ❏  1 ❏  1 ❏  4 ❏  1 ❏  3 ❏  2 ❏  1 ❏  1 ❏  3 ❏  1 ❏  1 ❏  3 ❏  1 ❏  1 Value 4.7MΩ 1MΩ 820kΩ 430kΩ 180kΩ 100kΩ 91kΩ 82kΩ 75kΩ 56kΩ 47kΩ 43kΩ 39kΩ 33kΩ 22kΩ 15kΩ 12kΩ 10kΩ 9.1kΩ 4.7kΩ 2.2kΩ 1.8kΩ 1.2kΩ 1kΩ 680Ω 180Ω 100Ω 27Ω 1Ω As shown in the photos, the PC board is mounted on the lid of the case and is secured by nuts on the switch collars. Before mounting the board, it will be necessary to first run short lengths of hookup wire to slide switch S3. This done, secure S3 to the lid using its mounting screws and install one nut on each of S1, S4 & S5. The lid can now be fitted over the switches and secured by installing the nut on rotary switch S2 and by fitting extra nuts to S1, S4 & S5. If necessary, adjust the nuts on the underside of the lid so that the lid is parallel to the PC board. Once the lid has been secured, push the LED bargraph and the separate LEDs into their respective holes, then solder their leads. All that remains now is to fit the battery holder and the test terminals to the case and complete the wiring. The battery holder can either be glued to the base of the case using epoxy ad- 4-Band Code (1%) yellow violet green brown brown black green brown grey red yellow brown yellow orange yellow brown brown grey yellow brown brown black yellow brown white brown orange brown grey red orange brown violet green orange brown green blue orange brown yellow violet orange brown yellow orange orange brown orange white orange brown orange orange orange brown red red orange brown brown green orange brown brown red orange brown brown black orange brown white brown red brown yellow violet red brown red red red brown brown grey red brown brown red red brown brown black red brown blue grey brown brown brown grey brown brown brown black brown brown red violet black brown brown black gold gold hesive or secured with small screws. Use 250VAC-rated cable for the leads to the positive and negative test terminals and keep the leads separate to eliminate leakage between them. Note that the leads from the test terminal and from the battery holder terminate on the underside of the PC board. Testing It will probably be easier to check voltages on the PC board if it is detached from the lid. A word of warning here – don’t touch any part of the circuit during the test procedure otherwise you could get a nasty shock from the high-voltage converter. To test the unit, install the battery, apply power and check that either a bargraph LED or the overrange (OR) LED lights. If this doesn’t happen, check that the LEDs are oriented correctly. Now check the supply voltages. 5-Band Code (1%) yellow violet black yellow brown brown black black yellow brown grey red black orange brown yellow orange black orange brown brown grey black orange brown brown black black orange brown white brown black red brown grey red black red brown violet green black red brown green blue black red brown yellow violet black red brown yellow orange black red brown orange white black red brown orange orange black red brown red red black red brown brown green black red brown brown red black red brown brown black black red brown white brown black brown brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown red black brown brown brown black black brown brown blue grey black black brown brown grey black black brown brown black black black brown red violet black gold brown brown black black silver brown There should be about 9V across pins 1 & 8 of IC1, pins 4 & 8 of IC2, pins 4 & 7 of IC3 and pins 2 & 4 of IC4. In addition, check for about 3V between TP2 and the negative side of the battery. Now switch the unit off, select the 1000V or higher range on your multimeter and connect the positive lead of the meter to the cathode of D3. Reapply power and check for the correct test voltages as selected by rotary switch S2. If the voltages are all high by about 10% or more of the correct value, substitute a 3.9MΩ 1W resistor for one of the 4.7MΩ resistors. Assuming that all is correct so far, switch off again, connect your multimeter between test points TP1 & TP2 and select the DC mV scale. Set the Range switch on the Megohm Meter in the x1 position and slowly adjust VR1 until you obtain a 0mV reading. You can now check the calibration January 1999  27 Fig.7: here are the full-size front panel artworks for the Megohm Meter. The panel at left suits the Altronics 5-LED bargraph displays, while the panel at right suits the Jaycar 10-LED display. by connecting the test terminals to a 20MΩ resistor (ie, two 10MΩ resistors in series). Select the x1 Range and press the Test switch. The display should indicate either 16MΩ or 22MΩ. Check that you get the same reading for all the test voltages, as selected by S2. The current trip circuit can be tested by connecting a 15kΩ resistor across the test terminals. Select the 100V position and press the Test switch; the display should read below 1MΩ. Now select the 250V position and press the Test switch again. This time, the overcurrent trip LED should light. The display should also show a reading but this should be ignored. 28  Silicon Chip Pressing the Reset switch (Test switch released) should extinguish the over­current LED and restore normal operation. Once all the tests have been completed, attach the lid and install the unit in the case. Testing capacitors If a capacitor is being checked for leakage, be sure to select the correct test voltage (ie, do not exceed the capaci­tor’s voltage rating) and always wait until the capacitor charges before taking the reading. If necessary, hold down the Reset switch if the overcurrent trip LED lights, to override this feature. Note that the lowest test voltage is 100V. This means that the Megohm Meter is not suitable for testing low-voltage electrolytic capac­itors. Take care with fully charged capacitors since they can give a nasty electric shock. Always discharge the capacitor after testing by releasing the Test switch with the probes still con­nected. When you initially release the Test switch, the External Voltage LED will light to indicate that the capacitor is charged. Wait until this LED has extinguished before removing the test probes. When checking appliances, always check that the earth is intact by measuring with your multimeter between the earth pin on the mains plug and the metal body of the appliance. You SC should measure zero ohms.