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A Low Capacitance Adaptor for DMMs This neat little adaptor allows a standard digital multimeter to measure low values of capacitance – from less than one picofarad to over 10nF. It will allow you to measure tiny capacitors or stray capacitances in switches, connectors and wiring. By JIM ROWE A lthough some modern digital multimeters do provide capacitance measuring ranges, these are generally not particularly useful when it comes to measuring low value capacitors or the stray capacitance associated with connectors, switches and other components. For most of these small capacitance measurements you normally need to use a dedicated low-value capacitance meter and these can be a bit pricey. The Adaptor is easy to build, with all of the components mounted on a small PC board. The board fits into a box which is small enough to be used as a dedicated ‘low capacitance probe’ for the DMM, making it well suited for measuring stray capacitances. Just about any modern DMM is suitable for the Capacitance Adaptor, provided it has an input resistance of 10M or 20M. How it works Essentially the Adaptor works as a capacitance-toDC-voltage converter, as shown in Fig.1. First we generate a square wave ‘clock’ signal with a frequency of between 110kHz and 1.1kHz (depending on the measuring range) using a simple relaxation oscillator based on capacitor C1, resistor R1, trimpot VR1 70  Silicon Chip siliconchip.com.au R2 SQUARE WAVE OSCILLATOR VR1 R1 SCHMITT BUFFER EXCLUSIVE-OR GATE TRUTH TABLE VC1 (NULL STRAY C) EX-OR GATE BUFFER INTEGRATOR R4 1.000 DC VOLTS R3 (=R2) T1 C1 T2 Cx (CAPACITOR UNDER TEST) and a Schmitt trigger inverter. This square wave signal is then passed though a Schmitt buffer stage to ‘square it up’ and produce a waveform with very fast rise and fall times. The output from the Schmitt buffer is then split two ways and passed through identical resistors R2 and R3. Then they are fed to the two inputs of an exclusive-OR (XOR) gate. The signal which passes through R2 has a small trimmer capacitor VC1 connected from the ‘output end’ of R2 to ground, while the signal which passes through R3 has the capacitance which is to be measured connected from the output end of R3 to ground (ie, between terminals T1 and T2). So each signal is fed to the inputs of the XOR gate via an RC delay circuit. The combination of these two RC delay circuits and the XOR gate form a simple ‘time delay comparator’. Remember that when both inputs of a XOR gate are at the same logic level (either high or low), its output is low. And whenever the two inputs are at different logic levels, its output switches high. This is summarised in the truth table associated with Fig.1. Now consider the situation where there is no discrete C2 + – DMM (SET TO DC V) INPUT A INPUT B L L L L H H H L H H H L OUTPUT Fig.1: it’s essentially a capacitanceto-DC-voltage converter, as this block diagram shows. The truth table for the exclusive-OR gate is shown above. capacitor connected between the test terminals, so there will only be a small ‘stray’ capacitance between them. As a result, there will only be a very short delay in the signal passing through R3 to the lower input of the XOR gate. If trimmer VC1 is set to provide the same low capacitance for the signal passing through R2, the two signals applied to the inputs of the XOR gate will be delayed by the same amount of time, and so will arrive at the gate inputs ‘in sync’ – rising and falling at exactly the same times. In this situation the output of the XOR gate will remain low at all times, because both inputs of the gate are always high or low, both switching together between the two levels. But when we connect an unknown capacitor (Cx) between terminals T1 and T2 the signal passing through R3 will be delayed more than the signal passing through R2. So now the lower gate input will switch high and low a short time after the upper input and as a result, the logic levels of the two gate inputs will be different for short periods of time following each H-L or L-H transition of the square wave signal. The output of the XOR gate will switch high during these transition delays, generating a series of positive-going puls- Here’s a view inside the open low capacitance adaptor, looking towards the unknown capacitor terminals. The jacks on the right-hand end connect via banana leads to the digital multimeter – although elsewhere in this article we give a possible “plug-in” alternative which saves you using leads at all. siliconchip.com.au March 2010  71 es with their width as those for lower directly proporvalues because of Specifications tional to the extra the increasing curThree measuring ranges –   delay time caused vature of the R4-C2 Range A:  0.1pF = 1mV, [gives a range from below 0.3pF to above 100pF.] by the unknown   Range B:  1pF = 1mV, [gives a range from below 1pF to above 1000pF (1nF)] charging/dischargcapacitor Cx. ing exponential.   Range C:  10pF = 1mV, [gives a range from below 10pF to above 10.0nF.] In fact the width Accuracy: Within approximately 2% of nominal full scale reading, Circuit details of the pulses will (assuming you can calibrate ranges using capacitors of known value). be directly proThe full circuit Power: 9V alkaline or lithium battery. portional to the of the Capacitance Current drain: less than 5mA. value of unknown Adaptor is shown capacitor Cx, bein Fig.2. Schmitt cause we deliberately limit the delay time to a relatively inverter IC1a operates as the square wave clock oscillator. small proportion of the half-wave period of the square The only difference from Fig.1 is that switches S1b and wave ‘clock’ signal. S1c allow three different C1/VR1 combinations to be used, The rest of the circuit is used as a simple integrator, to for oscillation at three different frequencies, to provide the convert the positive-going pulses into a DC voltage. We three measurement ranges. feed the pulses through a non-inverting buffer, to ensure The remaining inverters in IC1 (a 74HC14 device) are the pulses are all of constant peak-to-peak amplitude and used to form the non-inverting Schmitt buffer following then through the integrator formed by series resistor R4 the oscillator. IC1b squares up the signal initially and then and shunt capacitor C2. drives IC1c-f in parallel to re-invert the signal and square The average DC voltage developed across C2 is directly it up even further. proportional to the width of the pulses and it is this DC The paralleled outputs of the clock buffer drive the upvoltage that is measured by the DMM. per and lower arms of the ‘time delay comparator’. Here Although we are only using a simple RC combination the two 10k 1% resistors correspond to R2 and R3 in to perform this integration, the relationship between the Fig.1. However, the signals from the two delay circuits R2/ pulse width and the output DC voltage is reasonably linear VC1 and R3/Cx now pass through another pair of Schmitt because we have deliberately limited the integration to the inverters, IC2c & IC2a, which are part of a second 74HC14. initial 20% of the exponential RC charging and dischargThis has been done to ‘square up’ both signals, to ensure ing curve. that the width of the output pulses from IC3a maintain That’s why the nominal full-scale reading on each of our their linear relationship to the value of the capacitor becapacitance ranges is only 1.000V, even though all of the ing measured. Adaptor circuitry operates from a 5V supply rail. Although this squaring up is only necessary for the lower In fact, you can use the Capacitance Adaptor to measure (Cx) signal, because of its longer delay and hence greater capacitors with a value of more than the nominal full scale ‘rounding’, we also pass the upper (VC1) signal through an value on each range but the readings won’t be as accurate identical inverter to ensure that it is inverted in the same D1 1N4004 A 100nF 2 1 4 3 IC1b 10k VR3 4 5 14 IC1c-f 8 9 11 VR2 VR1 IC1: 74HC14 VR1-VR3: 5k x 25T 6 10 12 13 7 2 10nF 10 F 1nF 100nF 3 2 1 S1c RANGE FUNCTION 1 (POWER OFF) 100pF (0.1pF/mV) 1nF (1pF/mV) 2 3 4 2009 GND 47 F IC1a 3 SC  +5V OUT S1: RANGE /POWER S1b 100nF IN 3 1 3 REG1 78L05 2 4 9V BATTERY 4 1 S1a K 4 IC2a-f 10k 1% VC1 3-10pF NULL STRAYS 5 6 9 8 10 11 IC3: 74HC86 1 IC3a 3 2 IC3b 14 4 5 10 10k 1% Cx (CAP UNDER TEST) 14 100nF + – 1 2 13 12 9 12 13 7 6 IC3c 1k 8 IC3d 11 10 F 7 IC2: 74HC14 – 78L05 10nF (10pF/mV) DMM CAPACITANCE ADAPTOR + OUT TO DMM GND IN4004 A K IN OUT Fig. 2: the complete circuit diagram. The three active switch positions give a range of about 0.3pF to 10nF. 72  Silicon Chip siliconchip.com.au way as the lower signal. Thus both signals have the same nominal phase and both signals have the same propagation delay, ie, via IC2a & IC2c. IC3a is the XOR gate of the time delay comparator, while the remaining three gates in IC3, a 74HC86 device, are used as a non-inverting buffer to drive the RC integrator. Here the 1k resistor corresponds to R4 in Fig.1, while the 10F tantalum capacitor across the output jacks corresponds to C2. Gates IC3b-d are used simply as non-inverting buffers by tying the second input of each to ground logic low. 90 x 50.5mm and coded 04103101. This fits snugly inside a plastic instrument box measuring 120 x 60 x 30mm. The only components which are not mounted directly on the PC board are the binding posts and the output ‘banana’ jack sockets (or banana jacks themselves) for connection to the DMM. The former mount on one end of the box while the latter mount on the other end. In each case the posts and jacks connect to PC board pins. Note that the binding posts and jacks are both spaced apart by the standard 19mm (3/4”), to make them compatible with double-plug connectors etc. Before you begin fitting the components to the PC board, it’s a good idea to open up the box and check that the board will slip inside the lower half (the half with the countersunk holes for the final assembly screws). You may need to file off a small amount from all four sides of the board so that it will slip down to rest on the support pillars moulded in the inside of the box. You may also need to file small shallow rounded recesses in the two ends to clear the larger pillars around the box assembly screw holes. It’s much easier to do this before any components have been mounted on the board. Begin board assembly by fitting the three wire links, followed by the six PC pins: two each for the input terminals and output jack connections and two for the battery clip lead connections (just below the positions for D1 and REG1, at lower centre). Next, fit the three 14-pin IC sockets for the three ICs, noting that the socket for IC1 should have its notched end to the right while those for IC2 and IC3 are to the left, as on the overlay diagram of Fig.3. Then fit the four fixed resistors, followed by the three 5k 25-turn trimpots. Make Power supply Power is supplied by a 9V alkaline or lithium battery, with diode D1 used to prevent any possibility of reversepolarity damage. Switch S1 acts as a combined power and range switch, with S1a is used to switch off the Adaptor in the fourth (fully anticlockwise) position. The Adaptor circuit needs to run from a regulated DC supply rail, so that the measurements don’t vary as the battery voltage droops with age. Regulator REG1 is therefore used to provide a regulated +5V supply rail, provided the battery voltage remains above 7.5V. Since the current drain of the circuit is below 5mA, we are able to use a 78L05 regulator (TO-92 package) for REG1. The 47F, 10F and 100nF capacitors are used to filter any noise and switching transients which may appear on the +5V supply line. Construction As you can see from the photos and the PC board overlay diagram of Fig.3, virtually all of the components used in the Adaptor are mounted on a small PC board, measuring CAPACITANCE MEASURING BINDING POSTS 2x 100nF IC3 74HC14 100nF Cx+ + 5k VR1 VR2 VR3 REG1 D1 5k 100nF 5k IC1 74HC14 + DMM TEST LEAD JACKS + 1nF 10k BOX END PANEL OUT+ 10 F + 4004 VC1 3-10pF BOX END PANEL 9002 © 19021140 9V BATTERY 47 F – OUT– 78L05 9V + 10k 10 F – 10k Cx– S1 RANGE 10nF ZERO NULL - + 74HC86 1k IC2 E C NATI CAPA C RETE M R OTPADA S M M D R OF Fig.3 (above): life-size component overlay diagram, with posts and jacks, plus a slightly enlarged photograph of the same thing. The only thing not shown here is a small cable tie which should be used to secure the battery snap leads to the PC pins – flexing of the leads when the battery is changed is a sure-fire recipe for them to break off at the solder joints. siliconchip.com.au March 2010  73 Connecting to your DMM: another approach While this project was being prepared for publication, it occurred to us that there was another, perhaps even more logical way to connect the adaptor to a DMM – particularly if you would like a more “hands free” operation. This takes into account the fact that the overwhelming majority of DMMs which use 4mm sockets (and we would have to say ALL pro-quality units) have a standard 19mm spacing between those sockets. Therefore, we reasoned that it would be quite sensible to replace the banana jack sockets on the “output” end with banana jacks – thus allowing the unit to be plugged directly into the DMM. At the expense of some flexibility, this would mean that there would be no need to make up a set of Adaptor-to-DMM leads. Try as we might, we could not easily find a set of these already made up. You can get banana to probe, banana to alligator clip, banana to multiple adaptors, even banana to blade fuse fittings (for automotive use) but banana to banana? Nada. Zilch. Nyet! So the only alternative would have been to buy some figure-8 red and black lead (believe it or not, also getting hard to find in lightweight, flexible type!), two pairs of red and black banana plugs and solder them onto the lead. The alternative approach, as shown above and below, is to fit a pair of red and black banana plugs through the end of the case. We used a scrap of PC board, cut and shaped the same as the end panels, with a strip of copper removed down the middle. Drilled appropriately, this gave us a handy “platform” to which we soldered the two banana plugs (inside) without their plastic shrouds. The plugs were then soldered back to their respective PC pins using short lengths of tinned copper wire (eg, resistor/capacitor lead offcuts). Presto – a plug-in adaptor. And if you want to use it off the DMM? Simply use a banana-to-alligator clip lead set. sure you place the latter with their screwdriver-adjustment screw heads at lower left. Now add the fixed capacitors, taking care to place the polarised 47F and 10F caps with the correct orientation, as shown in the overlay diagram. Then fit the mini trimcap (VC1) in position, with its ‘flat’ end to the left as shown. Rotary switch S1 is fitted next, after cutting its spindle to about 10mm long and filing off any cutting burrs with a small file. The switch mounts on the board with its moulded locating spigot at approximately the ‘7:30’ position, viewed from above and with the board orientated as shown in the overlay diagram (ie, with IC1 at lower left). S1 is a “universal” type of switch offering a number of switch positions so after it is installed, it needs to be set for the four positions we require. Remove the nut and lockwasher from its threaded bush and then lift up the stopwasher as well. Then turn the spindle anticlockwise by hand as far as it will go and refit the stopwasher with its ‘stop pin’ passing down through the hole between the digits ‘4’ and ‘5’ moulded into the switch body. Then replace the lockwasher and the nut, threading the latter down until it’s holding down both washers firmly. You should now find that if you try turning the spindle by hand, it will have a total of four positions – no more and no less. Don’t be caught out by the old trap of thinking you only have three positions because it only clicks three times. Remember it clicks to three more positions from its end position. Then you can fit REG1 and D1 to the board, noting their correct polarity. Plug IC1-3 into their respective sockets and your board assembly will be complete. You put it aside while you drill the various holes which need to be cut in Another way of measuring “C” – using a small length of 4mm brazing rod with a point and slot, (shown below) you can fashion a “probe” to get into tight spots. 74  Silicon Chip siliconchip.com.au the top, bottom and end panels of the box. Preparing the box Two holes need to be drilled in each of the end panels and five holes in the top of the box. You will also need to cut away a small amount from the sides of the assembly screw surround pillars on both the top and bottom of the box, to provide clearance for the ‘rear ends’ of the capacitance measuring binding posts and DMM test lead jacks, when the box is assembled. This cutting away can be done with a small milling cutter in a high speed rotary tool or done manually with a sharp hobby knife (careful!). Both pairs of holes in the end panels need to have a diameter to suit the binding posts and banana jacks you are using. They are located on the centre line of their panel but 9.5mm away from the centre-line in each case - so the binding posts and jacks both end up spaced apart by the standard figure of 19mm (3/4”). The five holes in the top of the box can be located quite accurately using a photocopy of the front panel artwork (or a printout from siliconchip.com.au) as a template, because you’ll see that this includes a dashed outer rectangle to show the outline of the box itself. The central hole for the power/range switch is 10mm in diameter, while the other four holes are 3.5mm in diameter, which allow adjustment of the zero null trimcap and calibration trimpots when fully assembled. The exact location and amount of material which must be removed to clear the binding posts and banana jacks will depend very much on the actual posts and jacks that you use. You can see from the internal photos where material needed to be cut away for the posts and jacks used in the prototype. (By the way, the binding posts used were the PT-0453 & PT-0454 from Jaycar, while the banana jacks were the PS0406 & PS-0408 – also from Jaycar. Other posts and jacks may need the removal of either less or more material but you should be able to fit in most types that are currently available.) The last step in preparing the box is to make another photocopy or printout of the front panel artwork on either an adhesive-backed label sheet with a piece of clear selfadhesive film over the top or, for really long life and best protection, plain paper laminated in a plastic sleeve. The label is then cut out and applied to the front of the upper half of the box, lining up the holes of course. First assembly steps The first step in assembling the Adaptor is to mount the binding posts and banana jacks on their respective end panels, tightening their mounting nuts to make sure they won’t be able to rotate and work loose. Note that in the case of the banana jacks, you also need to mount them with their solder tags orientated vertically downwards so that after the nuts are tightened, the tags can be bent up by 90°. This is to allow the holes in the tags to be shortly slipped down over the terminal pins in the PC board. Next lower the PC board assembly into the lower half of the box, and fix it in place using four very small self-tapping screws (no longer than about 5mm). Then you should be able to lower the end panel with the output jacks down into the slot at that end of the case, with the tags on the rear of the jacks passing down over the terminal pins of siliconchip.com.au Parts List – DMM Low Capacitance Adaptor 1 PC board, code 04103101, 90 x 50.5mm 1 Utility box, 120 x 60 x 30mm (eg Jaycar HB6032, Altronics H0216) 1 3 pole 4 position rotary switch (S1) (eg Altronics S-3024, Jaycar SR-1214) 1 Instrument knob, 16mm diameter 1 Binding post, red 1 Binding post, black 1 Banana jack socket, red 1 Banana jack socket, black 1 9V alkaline or lithium battery 1 9V battery snap lead 3 14-pin DIL IC sockets 6 1mm diameter PC board terminal pins 1 small cable tie 4 Small self tapping screws, max 5mm long Semiconductors 2 74HC14 hex Schmitt inverter (IC1,IC2) 1 74HC86 quad XOR gate (IC3) 1 78L05 low power +5V regulator (REG1) 1 1N4004 1A diode (D1) Capacitors 1 47F 16V PC electrolytic 1 10F 16V PC electrolytic 1 10F 25V TAG tantalum 3 100nF multilayer monolithic ceramic 1 100nF MKT metallised polyester 1 10nF MKT metallised polyester 1 1nF MKT metallised polyester 1 3-10pF mini trimcap (VC1) 3 known value reference capacitors (see text) Resistors (0.25W 1% unless specified) 3 10k 1 1k 3 5k 25T cermet trimpots (VR1,VR2,VR3) the board. When the panel is down as far as it will go, you can solder the jack tags to the terminal pins to make the connections permanent. The other end panel (with the binding posts) is then fitted in much the same way, except that in this case there are no solder tags at the rear of the posts. Instead you may need to bend over the terminal pins on the PC board so that they clear the rear spigots of the binding posts and are alongside them, ready for soldering. Then when this panel is down as far as it will go, the binding posts can be soldered to the board pins. The next step is to cut the battery snap lead wires fairly short -- about 20mm from the snap sleeve - then strip off about 5mm of insulation from the end of each wire, tin them and solder them to the PC board pins just below REG1 and D1. The positive (red) wire goes to the pin immediately below D1, as you can see from the overlay diagram and pics. Ideally, these wires should be secured to the PC board pins with a very small cable tie. After checking that everything looks correct, connect the battery to the battery snap and your Low Capacitance March 2010  75 Adaptor should be just about ready for its initial set-up. All that remains is to fit the operating knob to the spindle of switch S1 temporarily, to make things easier during the set-up operation. Initial set-up & calibration NULL B 1mV = 1pF – Fig.4 same-size front panel artwork which can also be used as a template for drilling the five holes required. This can also be downloaded from siliconchip.com.au 76  Silicon Chip OUTPUT TO DMM (DCV) UNKNOWN CAPACITANCE Select the DMM that you are going to use with the Adaptor and make up a connecting lead to connect the output of the Adaptor to its DC voltage inputs. In most cases the lead will need standard banana plugs at each end. Then connect the Adaptor and DMM together using this lead and turn on the DMM, switching it to a fairly low DC voltage range, eg the range Arguably the wrong way to with a full-scale reading of 1.999V or 1999mV. measure a small capacitor – Turn S1 to the first position (‘Range A’) for the there is too much lead on it present. You should find that the DMM will give so stray capacitance could distort the reading. However, a relatively low reading - less than 10-15mV. This reading is due to the fact that the ‘stray’ we got away with it in this case – as you can see, the capacitor capacitance of the Adaptor’s input binding posts is labelled “6” (6pF) and the is not as yet being nulled by trimpot VC1. So DMM is reading 6.08pF the next step is to use a small plastic or ceramic alignment tool to adjust VC1 very carefully to get a minimum or ‘null’ in the DMM’s reading. You should actual value in picofarads. For example, if the capacitor be able to bring the reading down to below 1mV. has a known value of 1.013nF or 1013pF, adjust VR2 until If you are able to achieve this null, your Adaptor is very the DMM reading is 1.013V. likely to be working correctly and the next step is to caliFinally repeat the process again for Range C, this time brate each of the three ranges. using the 10nF reference capacitor and trimpot VR1 to For the three calibration steps you’re going to need three make the adjustment. The correct setting for this range is polystyrene, polyester or silvered mica capacitors whose where the DMM reading in millivolts corresponds to the values are accurately known, because the accuracy of your capacitor’s actual value in tens of picofarads. For example Adaptor will depend upon them. The three capacitors if the capacitor has a value of 9.998nF, the DMM reading should have values close to 100pF, 1nF and 10nF respec- should be 999.8mV or 0.9998V. tively, because these are the nominal full-scale readings of That’s all there is to it. Once you have calibrated each the Adaptor’s three ranges. range in this way, you can turn off the Adaptor using S1, They needn’t have these exact values but ideally you remove the knob from its spindle and then fit the top of the should know their actual values, measured using a cali- box carefully - making sure you don’t catch the battery snap brated digital capacitance meter or LCR meter. wires under the side. Then turn the complete box over and Once you have these three known-value or ‘reference’ fit the four countersunk head screws used to fasten the top capacitors the calibration of your Adaptor is relatively to the bottom. After this all that should remain is to refit straightforward. the knob to the spindle of S1. With the Adaptor still switched on and set to Range A, Just before you declare your Adaptor ready for use, first connect the 100pF capacitor to the Adaptor’s binding though, it’s a good idea to check the setting of null trimcap posts using the shortest possible lead lengths. Then adjust VC1, because the stray capacitance associated with the intrimpot VR3 until the DMM reading in tens of millivolts put binding posts does tend to change very slightly when corresponds to the capacitor’s actual value in tenths of a the box is fully assembled. picofarad (pF). For example, if your capacitor has a known value of 101.5pF, adjust VR3 until the DMM reading becomes 1015mV or 1.015V. Once this is done you repeat this process on Range B, this time using the 1nF reference capacitor and trimpot VR2 + + OFF to make the adjustment. VR2 should ZERO be adjusted until the DMM reading in A 1mV = 0.1pF millivolts corresponds to the capacitor’s – C 1mV = 10pF C B A CALIBRATE siliconchip.com.au So switch the Adaptor on again, in Range A but with nothing connected to the binding posts and if necessary adjust VC1 using the alignment tool (passing down through the ZERO NULL hole in the front panel) to see if you can improve the null reading on the DMM. Using the Adaptor Putting the Adaptor to use is also quite straightforward. Basically it’s just a matter of hooking it up to your DMM, setting the DMM to the 0-2V DC range and then turning on the Adaptor to the appropriate range and connecting the capacitor to be measured to its binding posts. Then you read the voltage on the DMM and convert this to find the capacitance, using the legends printed on the Adaptor’s front panel. But there are a few things to bear in mind if you want to achieve the best measurement accuracy. For example when you are measuring really low value capacitors in particular (ie, below 100pF), try to connect them to the binding posts with the shortest possible lead length. This is because any excess lead length will add extra stray capacitance, as well as a tiny amount of lead inductance. Both of these will degrade reading accuracy, because measurements on Range A are done at a frequency of about 110kHz. If you can’t connect a capacitor directly to the binding posts with minimum lead lengths, an alternative is to make up a pair of short but stiff (ie, heavy gauge) test leads, each with a banana plug at one end and a small crocodile clip at the other. The leads should then be plugged into the binding posts, and zero null trimcap VC1 then adjusted with an alignment tool (on Range A) to null out the additional stray capacitance. Then you can connect the capacitor to the test lead clips and measure its capacitance as before. You can follow a similar procedure to use the Adaptor as a handheld ‘probe’ to measure stray capacitance, as opposed to measuring the value of discrete capacitors. Here it’s a good idea to make up a small ‘probe tip’ out of a 30mm length of 4mm (5/32”) diameter brass rod (eg, brazing rod), with a fairly sharp point ground or filed at one end and the other end slit down the centre with a fine hacksaw for about 8-10mm. The slit end can then be expanded slightly with a small screwdriver, so that it will just slip inside the socket on the front of the Adaptor’s positive (red) binding post and stay in position. You also need to make up a short but stiff test lead for the ‘earth return’, with a spade lug at one end (to be clamped under the negative binding post) and a small crocodile clip on the other end to connect to the reference metalwork for the stray capacitance to be measured. The probe tip and earth return lead I made up are visible in one of the photos. Here again you need to null out the additional stray capacitance associated with the added probe tip and earth return lead, before making the actual measurement. But this is again easy to do: simply fit the probe tip and earth return lead, turn on the Adaptor to Range A and adjust VC1 with an alignment tool for the deepest null in the DMM reading. Then you can proceed to make your measurements of stray capacitance. Get the idea? It’s quite in order to use test leads and/or measuring jig attachments to connect whatever capacitance you want to measure to the Adaptor’s binding posts, providing you null out the added stray capacitance using VC1 (on Range A) before making the actual measurements. SC Custom Battery Packs, Power Electronics & Chargers For more information, contact SIOMAR BATTERY ENGINEERING Phone (08) 9302 5444 or email mark<at>siomar.com www.batterybook.com siliconchip.com.au March 2010  77