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Do you occasionally need to measure very low resistances accurately but don’t have access to an expensive benchtop Milliohm Meter or DMM? This low-cost adaptor will let you use almost any DMM to make accurate low-resistance measurements. Milliohm Meter Adaptor for DMMs By Jim Rowe 58  Silicon Chip siliconchip.com.au W hen it comes to measuring low resistances (ie, below about 10) with any significant accuracy, very few standard handheld digital multimeters are of much use. Only the top-of-the-range models offer any real performance in this area. And when you want to measure even lower resistances – less than one ohm – even some of these drop out of contention. It’s really only the most expensive benchtop models that will provide milliohm-level measurements as a matter of course. This doesn’t pose much of a problem for most of us, most of the time, because accurate low-value resistance measurements are not needed very often. But sometimes you do: matching the values of low-value resistors used for current sharing in amplifier output stages, for example, or when you need to make up a low resistance current shunt for a panel meter. That’s when you need this Milliohm Meter Adaptor. It’s self-contained and designed to act as a very low resistance measuring ‘front end’ for almost any standard DMM. It works by converting low resistance values into a directly proportional DC voltage (nominally 0-1.000V), so the DMM is simply set for its 1V or 2V DC voltage range, the range where most DMMs have their highest accuracy. So when the adaptor is being used to measure a very low resistance, the resistance value is simply read out on the DMM in millivolts. Actually the adaptor provides two measurement ranges, one a ‘0-1.0’ range where it converts milliohms directly into millivolts (so 125mbecomes 125mV, for example) and the other a ‘0-10’ range where it converts tens of milliohms into millivolts – so 2.2 (ie, 2200m) becomes 220mV. So reading the low value resistances on your DMM doesn’t require much mental arithmetic. If (FORCE CURRENT) Rm (HIGH) ACTUAL RESISTOR TO BE MEASURED RL1 + VOLTMETER RESISTANCE OF LEADS CURRENT SOURCE Rx – RL2 A 2–TERMINAL RESISTANCE MEASUREMENT RL1 F+ Rm (HIGH) If (FORCE CURRENT) S+ RL2 + CURRENT SOURCE Rx VOLTMETER – S– F– RL3 RL4 B 4–TERMINAL RESISTANCE MEASUREMENT The top diagram (Fig.1a) shows the way resistance is measured in “normal” meters (ie, two-terminal). The lower diagram (Fig.1b) shows how higher accuracy is achieved with four-terminal measurement, especially for low resistances. This is the approach taken in this adaptor. Now at this stage you’re probably thinking this: if a lowcost adaptor like the one we’re describing here can make this kind of very low resistance measurement relatively easily, why don’t most DMMs provide such ranges? That’s because there is a catch: in order to measure low resistances accurately, you have to use a four-terminal measurement approach rather than the two-terminal ap- With the exception of the terminals and battery, all components mount on the one PC board. siliconchip.com.au February 2010  59 60  Silicon Chip OUT TO DMM 1k B E A K D3: 1N4004 K SC 6 4 IC2b 7 5 6.2k TP1– 2009 Rx IC2: LM358 K MILLIOHM ADAPTOR FOR DMM'S S1b FORCE– FORCE+ 68 IC2a 3 Vcc – 2.49V D2 A SENSE– INT/EXT SENSING S1a C Q1 BC559 E B 1k 1 2 8 A – ADJ Fig.2: the complete schematic. The circuitry at upper left forms a regulated source of force current, while that at lower right is a DC amplifier with a gain of exactly 100. 47nF 47nF SENSE+ 0 – 10.00 RANGE S2 1k 27k 2.7k K IC1 LM336-2.5V + D1, D2, D4, D5: 1N4148 4 2 1 680 8 CALIBRATE (GAIN) VR4 3 500 7 IC3 AD623AN 5 6 10 F 100nF 1k 0 – 1.000 300 SET 1mA VR2 5k D1 ZERO FORCE CURRENT VR1 TEMPCO 10k A C BC559 100 22k 6.2k SET 10mA VR3 5k SET VR5 ZERO 500 IC4 LM336Z -2.5 TP2+ (+2.49V) 220 F 16V TP2– + – A ADJ D4 K – + ADJ LM336-2.5 – + K A D5 SET ZERO TEMPCO VR6 10k 9V BATTERY S3 POWER A D3 K To understand what we’re talking about here, look first at the upper resistance measurement circuit in Fig.1(A). This shows the kind of twoterminal measurement used by most DMMs to measure resistances. As you can see it’s quite straightforward: a constant current source forces a current, If, through the resistance to be measured (Rx), which is connected to the meter’s test terminals. The voltmeter section of the DMM then measures the voltage drop across the test terminals, which is directly proportional to the resistance between the terminals – because according to Ohm’s law this voltage is given by E = If x Rx. Note that the voltmeter has a very high multiplier resistance (Rm), so it is assumed to draw virtually no current. The drawback with this approach is that as shown, our unknown resistance Rx isn’t the only resistance between the two test terminals – there’s also the resistance of the test leads, RL1 and RL2. These are effectively in series with Rx, so the voltage drop across them as a result of If flowing through them will simply be added to the drop across Rx. The resistance measured by the DMM will therefore be (Rx + RL1 + RL2), rather than just Rx itself. Now from a practical point of view this doesn’t introduce much error when you’re measuring resistances over 10 or so (with fairly short test leads). It’s usually not too difficult to keep the test lead resistances down to a few tens of milliohms (which is less than 1% of the value of Rx). But when you’re trying to measure somewhat lower resistances, the errors can be quite significant. For example, if the resistance you’re measuring is 1, two test leads each with a resistance of 30m will increase the total resistance across the terminals by 60m or 0.06, giving a measurement error of +6%. Now consider what happens when we use the four-terminal measurement approach shown in Fig.1(B). Here we still force a known current through the unknown resistor Rx and measure the voltage drop across it as before, using Vcc = +8.4V Why 4-terminal measurements? TP1+ proach used in the majority of DMMs. Before we look at the new adaptor and the way it works, then, we’d better explain first why it needs to make four-terminal measurements. siliconchip.com.au a high resistance voltmeter. But in this case the force current If is fed to Rx via one pair of terminals F+ and F-, while the voltmeter is connected across Rx via a second set of ‘sensing’ terminals S+ and S-. As you can see the F+ and S+ terminals are connected to one end of Rx via separate leads, while F- and S- terminals are connected to the other end – also via separate leads. So there are now four test leads, with resistances RL1, RL2, RL3 and RL4. But how does this extra complexity help? Look carefully and you’ll see that although the force current If still flows through force lead resistances RL1 and RL4, the voltage drops in these resistances now don’t matter because the voltmeter’s sensing leads are connected directly across Rx itself – ie, we now only measure the voltage drop across Rx alone. And the sensing lead resistances RL2 and RL3 don’t cause any problems either, because they’re simply in series with the very high resistance of the voltmeter circuit (and they carry only its tiny measurement current). So that’s why changing over to fourterminal resistance measurement gives much better accuracy, especially when you’re measuring very low resistances. Circuit description Now that you understand the basic concept of four-terminal resistance measurement, we will look at the circuit of the new Milliohm Measuring Adaptor and the way in works in detail. The schematic diagram (Fig.2), has four measuring terminals just to the left of centre labelled FORCE+, FORCE-, SENSE+ and SENSE-. It will help in understanding the way the circuit operates if you regard all of the circuitry above and to the left of the force terminals as comprising the NON-INVERTING (+) INPUT AMP 1 R3 force current source, while all of the circuitry to the right of the sensing terminals comprises the voltmeter section. (It’s actually a DC amplifier with its output connecting to the voltmeter section of a DMM.) Before we get going, you’ve probably noticed already that the two poles of switch S1 are wired so that the two positive terminals and the two negative terminals can be connected together if desired, for ‘internal sensing’. This switch has been provided purely to allow the adaptor to be used for making ‘quick and dirty’ (ie, less accurate) two-terminal measurements on components which can be connected directly to the force terminals, without any test leads as such. So for the rest of this discussion you should regard both poles of S1 as ‘open’, just as they are shown in the schematic. This ‘external sensing’ position of S1 is the one used for accurate four-terminal measurements, with Rx connected to all four terminals as shown. Let’s turn now to the circuitry used to provide the force current for our measurements. This is the section at upper left of the schematic involving IC1, IC2a and transistor Q1. Although it may look a bit complex, it’s really quite straightforward if you break it into sections. IC1 together with D1, D2, the 6.2k resistor and trimpot VR1 form a regulated voltage source which establishes a voltage difference of 2.490V between test points TP1+ (the adaptor’s supply rail) and TP1-. Why 2.490V? Simply because when the LM336-2.5 reference used for IC1 is adjusted to have this voltage drop, the temperature coefficient or ‘tempco’ of its voltage drop is very close to zero – staying constant over a wide temperature range (0-50°C). IC2a and Q1 are used together with their associated components to generR5 OUTPUT REFERENCE R1 Rg AMP3 OUTPUT R2 INVERTING (–) INPUT siliconchip.com.au AMP2 R4 R6 Fig.3: an instrumentation amp consists of three internal op amps, two used as matched input buffers for the third one (AMP3) connected as a difference amp. ate a constant force current through the adaptor’s force terminals, using the 2.490V voltage drop established by IC1 as its reference. They do this very simply: IC2a increases the base current to Q1 until the voltage level at Q1’s emitter (fed to pin 2 of IC2a) matches the voltage level fed to pin 3 by IC1. The base current is then stabilised at this level and this in turn stabilises the transistor’s emitter and collector Parts List – Milliohm Adaptor for Digital Multimeters 1 PC board, code 04102101, 91x57mm 1 UB3 (130 x 68 x 44mm) utility box 2 8-pin machined pin DIL IC sockets 1 DPDT mini toggle switch (S1) 2 SPDT mini toggle switches (S2, S3) 2 4mm binding posts, red 2 4mm binding posts, black 1 4mm banana jack socket, red, 1 4mm banana jack socket, black 4 15mm long M3 tapped spacers 8 6mm long M3 machine screws 1 9V battery, alkaline or lithium 1 9V battery snap lead 4 self-adhesive rubber feet 12 1mm diam. PC board terminal pins 1 200mm length red insulated light duty hookup wire 1 200mm length black insulated light duty hookup wire Semiconductors 2 LM336Z-2.5 +2.5V regulators (IC1,IC4) 1 LM358 dual op amp (IC2) 1 AD623AN instrumentation amp (IC3) 1 BC559 PNP transistor (Q1) 4 1N4148 100mA diodes (D1,D2,D4,D5) 1 1N4004 1A diode (D3) Capacitors 1 220F 16V RB electrolytic 1 10F 16V RB electrolytic 1 100nF 100V MKT metallised polyester 2 47nF 100V MKT metallised polyester Resistors (0.25W 1% unless specified) 1 27k 1 22k 2 6.2k 1 2.7k 4 1k 1 680 1 300 1 100 1 68 2 10k 25t vertical trimpot (code 103) (VR1,VR6) 2 5k 25t vertical trimpot (code 502) (VR2,VR3) 2 500 25t vertical trimpot (code 501) (VR4,VR5) February 2010  61 VR3 5k 1k CALIBRATE 47nF IC3 AD623 22k 100 D4 4148 47nF IC2 LM358 D1 4148 0102 © 10110140 4148 D3 4004 M H OILLI M R OTPADA OUT TO DMM 9V BATTERY 6.2k VR2 5k BATTERY UPPER LOWER currents as well. SET 10.0mA SET 1mA TP1 SNAP (FORCE) (SENSE) 27k + – Since the voltage level at LEADS BINDING BINDING 1k 220 F POSTS POSTS – the emitter of Q1 is set by the 2.7k FORCE+ D2 VR1 current flowing in the resistF+ S+ – 10k 4148 300 + Q1 + ance between the emitter ZERO IC1 S3 68 FORCE POWER and the positive supply rail, BC559 SENSE+ CURRENT LM336Z 1 S1 1k -2.5 we can set the force current TEMPCO + S2 level by adjusting the emitter RANGE 10 F + resistance. 100nF 6.2k INT/EXT SENSING We provide the adaptor SET – SENSE– SET 1k ZERO with two measuring ranges ZERO TEMPCO S– F– by using switch S2 and the + FORCE– 1 D5 various resistors in Q1’s 680 OUTPUT – 2PT+ IC4 emitter circuit to provide JACKS VR5 VR4 VR6 LM336Z 500 + - TP2 500 10k two different preset emitter TO DMM -2.5 resistances, corresponding Fig.4: follow this component overlay (along with the same-size photo at right) when to two preset force current assembling your Milliohm Adaptor and you shouldn’t have any problems. levels. Because of the balanced nature of For example when S2 is in the po- it before feeding it out to the DMM for the two input buffers their gain (and sition shown, the transistor’s emitter measurement. We use an AD623AN instrumenta- that of the complete instrumentation resistance consists of the fixed 2.7k, 1k and 27k resistors together with tion amp (IC3) for this job, because amp) can be set by varying a single trimpot VR2. By adjusting VR2 we are the requirements are fairly stringent: external resistor, Rg. Note that although the ‘output thus able to set the total effective emit- we need high and stable DC gain ter resistance to 2.490k, which sets (100 times) coupled with high input reference’ terminal of AMP3 in Fig.2 the collector current of Q1 (ie, the force impedance, very low input offset and is shown as earthed, we use this concurrent) to a level of 2.49V/2.49k, or high ‘common mode rejection’. These nection of the AD623AN in the main requirements are most easily met by circuit to allow fine zero adjustment exactly 1.000mA. of IC3. Alternatively if S2 is switched to using an instrumentation amp like the The 680 fixed resistor and trimpot the ‘0-1.000’ position, the 300 AD623AN. By the way if you’re not familiar VR4 connected between pins 1 and 8 and 1k fixed resistors plus trimpot VR3 are connected in parallel with with instrumentation amps, a simpli- of IC3 are used to adjust the gain of the the existing emitter resistances, and fied version of their most common in- amplifier stage to exactly 100 times by adjusting VR3 we are now able to ternal configuration is shown in Fig.3. (ie, they correspond to Rg in Fig.2). As you can see they consist of three As a result VR4 is used to calibrate set the total effective emitter resistance to 249.0. This sets the collector cur- conventional op amps, with the third the adaptor/DMM combination for the rent of Q1 to a level of 2.49V/249, or one (AMP3) operating as a difference most accurate readings. amplifier. As yet we haven’t mentioned IC4 – exactly 10.00mA. The other two amps are configured which as you have probably noticed So switch S2 allows us to set the adaptor’s force current level to either as input buffers, to give each input of already is a second LM336Z-2.5 volt1.000mA or 10.00mA, and that’s how AMP3 a high input impedance. At the age reference, just like IC1. It’s also connected in the same way we provide its two measuring ranges. same time the gain of the two input As mentioned earlier, the section of buffers is carefully matched by laser as IC1, with diodes D4 and D5 plus the circuit to the right of the sensing trimming of their feedback resistors trimpot VR6 used to allow its voltage terminals (SENSE+ and SENSE-) acts R1 and R2. This matching is also done drop to be set to 2.490V – providing as a DC amplifier which takes the small for the resistors around AMP3, and the a near-zero temperature coefficient. voltage drop across our unknown end result is not only very low input So its function is to provide a temresistor Rx (produced by the force cur- offset but very high common mode perature stabilised source of +2.490V (with respect to ground in this case), rent flowing through it) and amplifies rejection as well. Resistor Colour Codes o o o o o o o o o No. 1 1 2 1 4 1 1 1 1 62  Silicon Chip Value 27k 22k 6.2k 2.7k 1k 680 300 100 68 4-Band Code (1%) red violet orange brown red red orange brown blue red red brown red violet red brown brown black red brown blue grey brown brown orange black brown brown brown black brown brown blue grey black brown 5-Band Code (1%) red violet black red brown red red black red brown blue red black brown brown red violet black brown brown brown black black brown brown blue grey black black brown orange black black black brown brown black black black brown blue grey black gold brown siliconchip.com.au the adaptor when operating on the 0-1.000 range is around 14mA, dropping to around 4mA on the 0-10.00 range. The difference is of course due to the change in force current level. Construction The two sets of “horizontal” PC pins at the top centre and bottom left of the PC board are test points, not normally connected. measurable between test points TP2+ and TP2-. Why do we need another source of stabilised DC voltage? Because although the AD623AN instrumentation amp is particularly good in terms of very low input offset, like all components in the real world it isn’t perfect. So in order to set the output to the DMM to exactly 0.000V when IC3 has zero input voltage (ie, when the SENSE+ and SENSE- terminals are shorted together and also connected to ground), we need to vary the DC voltage connected to pin 5 of IC3 over a very small range relative to circuit ground. That’s the purpose of trimpot VR5, which forms the lower leg (together with the 100 resistor across it) of a voltage divider connected across the stabilised 2.490V source provided by IC4. The upper leg of the divider is the 22k resistor, so by adjusting VR5 we are able to vary the voltage level at pin 5 of IC3 between 0V and approximately +10mV. This may seem small, but it’s quite sufficient to allow setting the adaptor’s output to zero – within a tiny fraction of a millivolt. As you can see the complete adaptor circuit operates from a single 9V alkaline battery, with switch S3 used to control power and diode D3 to prevent circuit damage in the event of the battery being connected with reversed polarity. This means that all of the adaptor operates from the unregulated +8.4V (nominal) supply rail. We can do this because IC1 and IC4 stabilise the only critical reference voltages. Incidentally, the battery drain of As you can see from the photos, the adaptor is housed together with its 9V battery in a standard UB3 size jiffy box (130 x 68 x 44mm). Inside the box, all of the components apart from the measurement terminals and output sockets are mounted directly on a small PC board, coded 04102101 and measuring 91 x 57mm. The PC board is supported inside the box using four 15mm long M3 tapped spacers. The four measurement terminals are mounted in one end of the box, while the two output sockets are mounted in the other end. Although there is a reasonable number of components on the board, assembly should be quite easy if you use the overlay diagram and internal photos as a guide. There are no wire links to be fitted but there are 12 PC board terminal pins – four for the two pairs of test points and the other eight for the off-board connections to the measurement terminals, output sockets and battery snap lead wires. Fit these pins first, taking care to fit the test point pins from the component side of the board and the other pins from the copper side. This makes it easier to connect to the latter pins after the board assembly is fitted into The completed PC board mounts upside-down in the utility box so that its switches (and trimpot access holes) emerge through the bottom of the case – which with the addition of a suitable label becomes the front panel. The box lid, with adhesive rubber feet, then becomes the base of the project. (See also Fig.6, overleaf). siliconchip.com.au February 2010  63 the rear of the switches. The tags of each switch need to pass down A A through the board holes as far as they’ll go, before soldering to the pads underneath. 9.5 9.5 With all three switches fitted, 19 the next components to add are A A the fixed resistors. Make sure you fit these in their correct positions as shown in the overlay diagram, 11 because otherwise you adaptor may not work correctly. If neces(MEASUREMENT TERMINAL END) CL ALL DIMENSIONS sary, use your DMM to check the HOLES A: 8mm DIAM IN MILLIMETRES HOLES B: 8.5mm DIAM value of each resistor before it’s fitted in place and soldered. Follow the fixed resistors with the five capacitors. Three are of the unpolarised MKT metallised 9.5 9.5 polyester type and the remaining B B two of the polarised electrolytic type. Make sure you fit these two with the polarity shown in the 17 overlay diagram. Next fit the trimpots, which are all of the miniature multi-turn (OUTPUT SOCKET END) type with their adjustment shaft in one top corner. Be careful in fitting these, not only to fit the Fig.5: drilling detail for the two ends of correct value pot in each position the UB3 utility box. You will also need (there are two 10k pots, two to drill nine holes in the “bottom” of the box – use a photocopy or printout 5k pots and two 500 pots) but of the front panel artwork (Fig.7 also to make sure that each pot is overleaf) as a drilling template. orientated the correct way around as shown in the overlay diagram. VR1, VR2 and VR3 are orientated with the box. After the terminal pins are fitted their adjustment shaft at upper right, and soldered in place, you can fit the while the other three trimpots have the sockets for IC2 and IC3. Follow these opposite orientation with the adjustwith the three mini toggle switches, ment shaft at lower left. If you don’t mount them this way as you may need to use a small needle file to convert the matching holes in you won’t be able to adjust them easily the board into a rectangular shape to when the board assembly is mounted accommodate the connection tags on inside the box. UPPER (FORCE) BINDING POSTS 220F S3 VR6 S2 VR4 S1 VR5 9V BATTERY & SNAP S1 S1 OUTPUT JACKS TO DMM The final components to fit to the board are the semiconductors, starting with five diodes. Take care to fit them the correct way around. Note too that D3 is a 1N4004 diode rated at 1A, while the others are smaller 1N4148 diodes. After the diodes are in place, fit transistor Q1 and the two TO-92 voltage reference ICs, IC1 and IC4, again watching their orientation. Your board assembly will then be complete, apart from the two plug-in ICs. We suggest that you only plug in IC2 at this stage. IC3 is best left out until the initial setting up has been done, because it’s a fairly expensive chip and could possibly be damaged before the force current levels have been set correctly. For the moment just place the nearly completed board assembly aside while you prepare the box by drilling the various holes that are needed. There are no holes to be drilled in the box lid, as this is used purely as a screw-on base for this project. All of the ‘works’ is mounted inside the box proper, as you can see from the photos and the side view assembly diagram. There are several holes to be drilled in the box bottom, as this becomes the Adaptor’s top/front panel. A photocopy of the front panel artwork (or a printout of the panel artwork file from siliconchip.com.au) can be used as a template for locating and drilling these holes. The small holes should all be 3.5mm diameter, while the three larger holes (for the switch ferrules) should all be 7mm diameter. The location and sizes of the holes in the ends of the box are shown in ADAPTOR PC BOARD (ATTACHED TO BOX VIA 4 x 15mm LONG M3 TAPPED SPACERS & 8 x 6mm LONG M3 SCREWS) (BOX LID BECOMES BASE) LOWER (SENSE) BINDING POSTS Fig.6: this “X-ray” view through the utility box side shows how it all goes together. Not seen here are the two red binding posts which, are directly behind the black posts. The 9V battery could be mounted in its own holder or, if you want to save a couple of dollars, do as we did – simply hold it in place with some Gaffer or duct tape! 64  Silicon Chip siliconchip.com.au N CON CO CON ILICONSILIP SILIP S SILIIPCONSIHLIIP IP HI HI DIGITAL I/O 1 +3.3V 100nF CHIP 12 Ya0 Zb 3 5 Yb1 1 Yb0 6 100nF 14 IC1e 10 E S1 S0 9 Vss 8 Vee 7 33pF BFrame EMPH 6 5 3 4 12 DGnd 6 IC5: 74HC14 1F 22k 22k 9 14 5   K 1 2 2 K A 22k 7 9 11 TO - A N A L O G C O N V 1F 1 K 22k 3 K 20 9 AVcc 14 2 8 $ 95* 16 PC4 PC5 9 28 11 26 PC2 13 25 PC1 PC0 RST PB6 7 27 PC3 12 PD6 IC5a 10 2 24 (TO DAC BOARD) all about? 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GPS Clo GPS Tesla Road ift IVy Lo iffe a d and Sunsw id 6... GPS a nt r ? 3 Za 13 14 Ya1 12 e 15 Ya2 8 Vcc S Ch 11 Ya3 U04 11 14 PSCK1 13 PSCK0 26 MT1 F 25 FMT0 20 RXIN 3 24 5 Vdd 16 Vdd 22F 100nF 100nF g if t CAR UTER a COMitP . . or with ! n s own . Use it o oftware mapping s laptop and NOW AVAILABLE: SIX MONTH SUBSCRIPTIONS & AUTO RENEWALS In these tough economic times, we understand that taking out a one or two-year subscription may be difficult. Or perhaps you’d like a trial before committing yourself to a full sub. Either way, we’ve made it easy with our new six-month subscriptions. It’s the easy way to make sure you don’t miss an issue . . . and a six month subscription is STILL CHEAPER than the over-the-counter price AND we pick up the postage tab. Have SILICON CHIP delivered to your door every month, normally a few days BEFORE it goes on sale in newsagents (grab some of the advertised bargains early!). 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CONVENIENT AUTOMATIC q MasterCard SUBSCRIPTION RENEWAL: Your Name____________________________________________________________ q Tick here if you’d like us to automatically renew your subscription Card Expiry: Signature______________________________    as it elapses (ie, 6 month, 12 month or 24 month).    We’ll renew until you tell us to stop! siliconchip.com.au February 2010  65 THIS PAGE MAY BE PHOTOCOPIED WITHOUT INFRINGING COPYRIGHT siliconchip.com.au February 2010  41 *10% DISCOUNT OFFER ONLY APPLIES TO PRINTED EDITION SUBSCRIBERS AND DOES NOT APPLY TO WEBSITE ORDERS. the diagram of Fig.5. Once these holes have been drilled (and if necessary reamed to size), you can fit the measurement terminals and the output jack sockets into them, taking care to tighten their nuts firmly so they won’t come loose in use. Before your Adaptor’s PC board assembly can be fitted into the completed box, it needs to have some of its initial setup adjustments made. These are done with the PC board assembly on the bench, and powered by either its own 9V battery or a suitable 9V mains power supply. board – with the positive lead connected to FORCE+ and the negative lead to FORCE-. Switch S2 with its toggle towards the right (ie, in the 0-10.00 position), and your DMM should give a current reading somewhere in the vicinity of 1mA. Change the DMM’s range if necessary to provide the best possible resolution, and then adjust trimpot VR2 until you get a reading as close as possible to 1.000mA (= 1000A). Once this has been achieved switch S2 to its other position (0-1.0), which should cause the current reading to jump to a higher figure – around 10mA. Again adjust the DMM range if necessary to get optimum reading resolution and then adjust trimpot VR3 to bring the reading as close as possible to 10.00mA. That will complete the initial setup adjustments and you’re almost ready to fit the PC board assembly inside the box. Turn off the power with S3 and then remove the 9V battery from its snap lead. Attach the four 15mm x M3 tapped spacers to the top of the board using four 6mm long M3 screws passing up from underneath. Tighten the screws firmly to make sure they don’t become loose later. Now take IC3 from its protective packaging and plug it carefully into its socket at lower right on the board, making sure that it’s orientated as shown in the overlay diagram. SENSING FORCE CURRENT OUTPUT TO DMM (1.00V = 1.00 / 10.0) and then adjust the lower nuts to bring the lockwasher and flat washer on each ferrule up to a level as close to 15mm above the top of the board as you can – that is, level with the tops of the four board mounting spacers. You might find a small steel rule helpful here. Now, with the upper nuts still off the switch ferrules, the idea is to hold the PC board assembly upright while you lower the main part of the Adaptor’s box down over it (with the correct orientation, of course!) until the switch toggles and then the tops of their threaded ferrules pass up through their matching holes in the box. Initial setup adjustments They should be protruding by about All of the adjustments can be made 1.5-2mm by the time the tops of the using a standard DMM, which can be mounting spacers are up against the the one you’ll be using the Adaptor upper inside of the box, allowing you with later, if you wish. to attach the three remaining switch The first adjustments to be made nuts to each switch ferrule to hold eveare of the two temperature coefficient rything together. Then you’ll be able zero pots VR1 and VR6, and for both to fit the four remaining 6mm long M3 of these adjustments you use the DMM screws to secure the board mounting set to its 0-4V, 0-10V or 0-20V DC spacers to the box as well. range. To adjust VR1, you simply conThe screws should be tightened nect the DMM test leads to test points quite firmly, whereas the switch nuts TP1+ and TP1- and then adjust VR1 need only be ‘finger tight’. with a small screwdriver until you get The final step in assembling your a reading of 2.490V (or as close to this Milliohm Adaptor is to upend the figure as you can get). This done, you box and fit the short connecting wires can transfer the DMM leads to TP2+ which connect the measurement bindand TP2- and now adjust VR6 in the ing posts and output sockets to their same way, to get a reading of 2.490V. corresponding terminal pins on the This completes the first two adjustPC board. The connections for each of ments, and you’ll be ready to make these wires is shown in the overlay/ the next two. For these the DMM is wiring diagram, so if you follow this switched to its low DC current ranges Final assembly methodically you shouldn’t make any and this time its leads are connected To begin the final stage of assembly, mistakes. to the FORCE+ and FORCE- terminal remove the upper mounting nut from By the way there’s no need to use pins on the right-hand end of the each of the three toggle switches S1-S3 heavy-gauge wire for any of these wires – ordinary insulated hookup wire is fine, because of the fourSET 10mA ZERO FORCE SET 1mA terminal measurement system. FORCE CURRENT Once these wires have all been CURRENT TEMPCO fitted, you can mount the Adaptor’s 9V battery on the inside lid/bottom of the box, securing + + – SENSING POWER RANGE it in place with either a small aluminium clamp bracket or a 0–10.00 0–1.000 INT EXT short length of ‘gaffer’ tape. Then the snap lead can be reconnected to the battery after + – – SILICON MILLIOHM ADAPTOR making sure that power switch FOR DIGITAL MULTIMETERS CHIP S3 is in the ‘off’ position and finally the lid/base can be attached to the main part of the CALIBRATE SET ZERO SET ZERO (GAIN) TEMPCO box using the four self-tapping screws provided. Fig.7: same-size front panel artwork. This can be photocopied (or printed out from the file on www.siliconchip.com.au) and preferably laminated before glueing onto the UB3 box base. First, though, drill the three switch holes and six pot access holes. 66  Silicon Chip Final setup Your Milliohm Adaptor is siliconchip.com.au ‘zero’ position is quite easy. After this there will now be only one further setup adjustment to make: the correct setting for gain trimpot VR4, SMALL SMALL so that the Adaptor and DMM ALLIGATOR ALLIGATOR CLIP CLIP combination will give accurate low resistance readings. To prepare for this final adjustment switch off the Adaptor’s power using S3 and then remove the wires that were previously used to connect the S+ and S- binding (FORCE+) (FORCE–) posts to the F- binding post for the zero adjustment. Then take a 1% tolerance (or better) metal film resistor with a known value of close (SENSE–) (SENSE+) to 10.00 (measured with your own DMM, perhaps, or Fig.8: use this test jig to set up your Milliohm ideally with another DMM of Adaptor, as described in the text below higher accuracy), and connect the ends of its leads to the now complete and ready for its final setup adjustments. To prepare for upper binding posts of the Adaptor these connect your DMM’s test leads (F+ and F-). Then use a pair of short to the Adaptor’s output jacks, using clipleads to connect the innermost whatever lead(s) will ultimately be point on each of the resistor’s leads used to connect the two and with the to the corresponding sensing binding post, as shown in Fig.8. correct polarity. Now make sure that switch S1 is in Then switch on power to the DMM and switch it to a low DC voltage range the EXT sensing position and also that – whichever range allows you to read range switch S2 is in the 0-10.0 posivoltage up to a bit over 1.000V with the tion (toggle to the right). Then switch best possible resolution. This will be on the Adaptor’s power switch S3. You should see a reading of around the same range you’ll be using when the Adaptor is ultimately being used 1.000V on the DMM, corresponding to the resistor’s value converted using with the DMM, of course. Before you turn on power to the the factor 1mV/10m. All that you now need to do is adjust Adaptor itself using S3, first connect BOTH of the Adaptor’s S+ and S- bind- trimpot VR4 using a small screwdriver ing posts to the F- binding post, using until the DMM reading corresponds short lengths of tinned copper wire. to the known value of your nominal Next make sure that switch S1 is in 10 resistor. Your Milliohm Adaptor the EXT sensing position (toggle to the will then be set up, calibrated and right) and also that there is NO connec- ready for use. tion to the Adaptor’s F+ binding post because it should be left unconnected Using it Putting the Adaptor to use is quite for this next adjustment. When you switch on power to the easy. It’s simply connected up to the Adaptor using S3, you’ll very likely DMM as it was for the final setup get a very small but significant reading adjustments and with the DMM set on the DMM – a few millivolts, in all for the same low voltage DC range (to give the best measurement resoluprobability. The idea is to reduce this reading to tion). Then you connect the low-value zero (or as close as you can get) using resistor to be measured to all four a small screwdriver to adjust trimpot binding posts, as for the final setting VR5 via its matching adjustment hole up adjustment. You can either connect the resistor in the top of the box (at lower centre). You’ll find that if you adjust VR5 one as shown in Fig.8, or use four sepaway the DMM reading will increase, rate clipleads if the resistor can’t be while if you adjust it the other way it brought up to the force current bindwill decrease. So setting the correct ing posts. NOMINAL 10  1% RESISTOR OF KNOWN VALUE siliconchip.com.au To make the measurement, you simply make sure that S1 is in the EXT sensing position and that S2 is set for the more appropriate measurement range (ie, either 0-1.000 or 0-10.00, depending on the resistor’s value). Then switch on power using S3 and the DMM reading will show the unknown resistor’s measured value – in millivolts, and with a scaling factor of either 1mV/1m or 1mV/10m depending on the range you’re using. So using the Adaptor to make fourterminal measurements of low value resistors is really pretty easy, isn’t it? As mentioned earlier though, it can also be used to make ‘quick and dirty’ (ie, less accurate) two-terminal measurements, if you’re in a hurry and accuracy isn’t all that important. To make two-terminal measurements, all you need to do is switch S1 to the INT sensing position and connect the resistor to be measured only to the F+ and F- binding posts – ideally with the shortest practical lead lengths. Then when you turn on the Adaptor, the DMM will give you a ‘pretty close’ reading of your unknown resistor’s value. SC ANTRIM TRANSFORMERS manufactured in Australia by Harbuch Electronics Pty Ltd harbuch<at>optusnet.com.au Toroidal – Conventional Transformers Power – Audio – Valve – ‘Specials’ Medical – Isolated – Stepup/down Encased Power Supplies Encased Power Supply www.harbuch.com.au Harbuch Electronics Pty Ltd 9/40 Leighton Pl, HORNSBY 2077 Ph (02) 9476 5854 Fax (02) 9476 3231 February 2010  67