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·1--------------------------------Low ohms tester for digital multimeters 1 Want to accurately measure low value resistors down to 0.010? This simple adaptor circuit will do the job. It plugs directly into your digital multimeter. By JOHN CLARKE Most digital multimeters can accurately measure resistance values down to only about five ohms. Below that figure, you quickly run into resolution problems and you get nonsense readings. There are many situations where accurate low resistance measurements are necessary. These include checking meter shunts, designing loudspeaker crossover networks and amplifier output stages, servic56 SILICON CHIP ing power supplies and, indeed, servicing any circuitry where low value resistors are used. Normally, when measuring a o.rn resistance on a 3-1/2 digit multimeter by itself, you would have to switch to the 2000 range and the reading would be O. rn ± 1 digit. In other words, the resolution of the multimeter would limit the accuracy of the measurement to ± 100% which is ridiculous. Similarly, if you measure a 10 resistor on the 2000 range of a DMM, the best you can expect is a reading of 1.00 ± ldigit; ie, best accuracy is ± 10%. So the meter resolution drastically reduces the accuracy of the measurement even though most DMMs are accuracte to within ± 1 % when measuring values at the top of their respective ranges. This Low Ohms Tester overcomes the limitations of conventional analog and digital multimeters. It plugs straight into the terminals of your DMM (digital multimeter) and can accurately measure resistance values from lkO down to a.am. Below this figure, errors start to become significant due to contact resistance in the test terminals and connecting wires. At the other end of the scale, readings above lkO lose accuracy due to limitations in the constant current source used in the circuit. In any case, digital multimeters by themselves are more than adequate to accurately measure resistance values above 1000 or more. All the circuitry for the Low Ohms Tester is housed in a small plastic case. On the front panel are two binding post terminals to which the resistor to be measured is connected. There is also a rotary 4-position range switch (xl, xlO, xlOO, xlOOO) and a pushbutton test switch. Two banana plugs protrude from the rear of the case at the correct spacing to allow the Low Ohms Tester to plug into virtually any digital multimeter made. The plug spacing, by the way, is 19mm (or PARTS LIST TEST _a, 1 VR1 1k + 16VW+ -4_,-+-~ T 16VWI 9V: ..,. 220pF .J- i OFFSET ADJUST VRJ 100k S2b .,. 10k 1% .,. ~" S2 : 1 : xl 2 : ,10 3 : ,100 4 : ,1000 1001! 1% LOW OHMS TESTER SC4-1-288 Fig.1: the circuit consists of a 5V regulator (78L05), a constant current source (D1, D2 and Ql), and an op amp gain stage (ICl). 1 PCB, code SC4-1-388 , 61 x 95mm 1 front panel artwork, 125 x 63mm 1 plastic case, 130 x 68 x 43mm (Jaycar HB-6013 , Altronics H-0203) 2 banana plugs 2 binding post terminals 1 3-pole 4-position rotary switch 1 momentary pushbutton switch 1 9V battery, Eveready 21 5 or equivalent 1 9V battery clip 1 small knob Semiconductors 1 CA3130 op amp 1 78L05 low power 5V regulator 1 BC559 PNP transistor 2 1 N4148 , 1 N914 s ilicon diodes Capacitors 1 4 .7 µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 220pF polystyrene capacitor Resistors 1 1 1 1 1 1 1 OkO 0.25W 1 % 1 kO 0.25W 5% 1000 0.25W 1 % 1 OOkO multiturn trimpot 1 kO multiturn trimpot 1 000 multiturn trimpot Miscellaneous Hookup wire, solder, tinned copper wire (for wire links), doublesided tape , plastic foam . TO METER Fig.2: wiring details for the PCB. Trim the switch shaft to a length of 10mm before soldering it to the board and don't forget the two wire links. more precisely, 3/4-inch) between plug centres. The output from the Low Ohms Tester is a voltage which is directly proportional to the resistance being measured. In practice, the unit is calibrated so that H2 gives an output of lmV x Range. For example, on the x 1000 range, H2 is equivalent to 1mV x 1000 = 1V. On the xlO range, rn is equivalent to lOmV, and so on. How it works The circuit consists of a 5V regulator, a constant current source (Dl, D2 and Ql), and an op amp gain stage (I Cl). Power for the circuit is provided by a 9V battery which is regulated to + 5V by the 3-terminal regulator. This provides a stable supply for the constant current source and the op amp. Test switch S1 simply switches the supply to the circuit so that current is drawn from the battery only while a measurement is being made. Dl, D2, Ql and the lkO resistor make up the constant current source. Ql is really an emitter follower. It reproduces the voltage fed to its base at its emitter less its 0.6V base-emitter voltage drop. Series diodes Dl and D2 set the base of Ql to a constant 1.2V below the + 5V supply line. This means that the emitter of Ql is always 0.6V below the + 5V line. The lkO resistor sets the current through the diodes to 5mA. The resulting 0.6V across either VRl or VR2, as selected by S2a, sets the current through Ql and the test resistor Rx. When VRl is selected, the test current is lmA; when VR2 is selected, the test current is lOmA. FEBRU ARY1988 57 The PCB should only take you a few minutes to assemble, and clips directly into the plastic case. Note the location of the two jacks on the rear panel. On the lower two ranges (xl and xlO), the voltage across Rx is applied directly to the DMM terminals. On the upper two ranges, the op amp gain stage (ICl) is switched into circuit and the DMM measures the voltage between the op amp output (pin 6) and the test resistor Rx. ICl is connected as a noninverting op amp stage with a fixed gain of 1 + lOkn/1000 = 101. Because we want a gain of precisely 100, we measure the voltage between the output of the op amp and the voltage across Rx. Thus, when S2 is in position 3, the current set by the constant current source is lmA and so the multiplying factor for Rx is xlO0. + r· Rx L. , ,,,.,.J-fJWJ.' When S2 is in position 4, the current is lOmA and the multiplying factor is 100 x 10 = 1000. VR3 adjusts the offset of the op amp so that, with no voltage across Rx, (ie, with the measurement terminals short circuited) the output is zero. Construction Assembly of the unit is a snack. Most of the parts, including the rotary switch, are mounted a small PC board (code SC4-1-388, 61 x 95mm). The board clips into a plastic project box with a plastic lid (Jaycar HB-6013 or Altronics H-0203). Begin construction by installing all the parts on the PCB as shown in Fig.2. Make sure that you don't confuse the transistor and the low power 3-terminal regulator, as they come in identical packages. Check the orientation of the IC before soldering it into circuit. The notched end, adjacent to pin 1, goes towards the centre of the board. Similarly, watch the orientation of the diodes, electrolytic capacitors, transistor and 3-terminal regulator. Trim the switch shaft to a length of 10mm before soldering the switch directly to the PCB. Don't forget the two wire links. You can now affix the front panel label to the case and drill mounting holes for the test terminals and switches. Ream the holes to size, then mount the terminals and test + x10 x100 x1 \ / x1000 ' • / H2 = 1mV x RANGE • TEST LOW OHMS METER Fig.3: this artwork can be used as a drilling template for the front panel. -1- ta --,- ~~ ~ 2mA scale. Set switch S2 to the xl position and adjust VR1 for a reading of lmA. This done, set your DMM to the DC 20mA scale, set S2 to the x10 position and adjust VR2 for a reading of 10mA. Calibration can now be completed by adjusting the offset voltage. To do this, disconnect the meter and set it to the DC 200m V range. This done, set S2 to the xlOO position, short the Rx terminals with the shortest possible length of tinned copper wire and plug the banana plugs of the Tester into the COM and vm terminals of your DMM. Adjust VR3 for an initial reading just above OmV, then adjust back for a reading of exactly Orn V on the DMM. ~~ ~:...t- Fig.4: actual-size etching pattern for the PC board. Final assembly switch and wire them to the PCB. The next step is to trim the plastic shrouds of the two banana plugs to a length of 13mm. The plugs can now be mounted on the lid of the case, adjacent to one edge (see photo). The leads from the plugs are wired directly to the copper pads on the back of the board. Finally, solder the battery con- · nectar leads to the board and clip on the 9V battery. The unit is now ready for calibration. Calibration Apply power by pressing S1 and check that there is + 5V at the output of the regulator and about 3.8V across the lkO resistor in series with the Dl and D2. Now connect your DMM across the Rx terminals and set it to the DC You can now clip the PCB into the plastic case and secure the rotary switch to the front panel. After that, it's simply a matter of covering the copper tracks of the PCB with a layer of plastic foam , securing the battery to the inside of the lid with a piece of double-sided tape, and screwing down the lid. Your Low Ohms Tester is now completed and ready for work. ~ issues? Issue Highlights November 1 98 7: Car Stereo in Your Home; 1 GHz Frequency Meter; Capacitance Adaptor for DMMs; Off-hook Indicator for Phones . Please send me a back issue for D November 1987 D December 1987 D January 1988 Enclosed is my cheque or money order for $ ....... . or please debit my D Bankcard D Visa Name ... ...... ... ._ ................ .. ... ..... ....... ... .... ..... ...... ..... ........ .... ..... ..... . Address .... .... ... .... .. ......... ... .... ... ..... ........ ... .. .. .. ... .. ..... .. .......... ... ... . . Suburb/town ..... ...... .... ........ ...... .. .... .... ... ..... ..... Postcode .... ........... . Card No .. .. ....... ......... ...... ...... ..... ... .... .. .. ... .. ... ......... ....... .. ...... .... ... . Signature .. .... .. ...... .. ....... .. ..... .... ..... .. Card expiry date .. .... ./ .. .. .. ./.... ... L __________ _____________ December 1987: 100W PmNer Amplifier Module; Passive Infrared Sensor for Burglar Alarms; Universal Speed Control and Lamp Dimmer; 24V to 12V DC Converter. I I I ! ! I January 1 988: 4-bay Bowtie UHF Antenna; Dual Tracking Power Supply; Custom Phone Ringer; Subcarrier Adapator for FM tuners. Price: $5.00 each (incl. p&p). Fill out the coupon at left (or a photostat copy) and send it I to: S1ucoN CHIP. PO Box 139, Collaroy Beach, 2097. 1_ _ _ _ _ ___ ___ __ I ~ FEBRUARY 1988 59