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This Low Ohms Tester plugs directly into a digital multimeter and can accurately measure resistances down to 0.01Ω. It’s easy to build and runs off a 9V battery. By JOHN CLARKE Low ohms adaptor for digital multimeters The ability to measure low resistance values is necessary when items such as meter shunts, loudspeaker crossover networks, inductors and contact resistances are to be checked. Unfortunately, a standard digital multimeter can only accu­rately measure resistances down to about 5Ω. Resistors with lower values will give misleading results due to a lack of meter resolution. A couple of examples will serve to illustrate this point. First, let’s assume that a resistance of 0.1Ω is to be checked on a standard 3-1/2 digit multimeter. In this case, you would have to switch down to the 200Ω range (the lowest you can select) and the reading would be 0.1Ω ±1 digit (ie, ±0.1Ω). In other words, 30 Fig.1: block diagram of the Low Ohms Tester. It works by applying a constant current through the test resistor (Rx). The voltage across Rx is then measured using a DMM. Silicon Chip’s Electronics TestBench the resolution of the DMM limits the accuracy of the reading to ±100% which is ridiculous. This situation quickly improves with increasing resistance values. For example, a value of 1Ω will result in a reading of 1.0Ω ±1 digit, assuming that the 200Ω range is used. This represents an accuracy of 10%. For values above 10Ω, the accuracy of the instrument will be 1% or better since the resolution of the reading is considerably improved. This Low Ohms Tester overcomes the limitations of conven­tional digital multimeters for low values of resistance. It does this by applying a constant current through the test resistor Rx. The resulting voltage de- Fig.2: the full circuit for the Low Ohms Tester. REF1, IC1 and Q1 form a constant current source for the test resistor Rx. The resulting voltage across Rx is then either measured directly or amplified by IC2 before being applied to the DMM. veloped across Rx is then amplified and applied to the DMM which is set to read in millivolts. Fig.1 shows the basic scheme. As shown in the photos, all the circuitry is housed in a compact plastic case. This carries a power switch, a 4-position range switch and two binding post terminals for the test resis­tor. The output leads emerge from the top of the instrument and are fitted with banana plugs. These simply plug into the COM and VΩ terminals of the DMM. The output from the Low Ohms Tester is a voltage (in mV) which is directly proportional to the resistance • • • • Main Features Measures from 0.01Ω to 100Ω Four ranges Outputs to a digital multimeter Battery operated being measured. In practice, you simply multiply the reading on the DMM by the range setting on the tester to get the correct value. For exam­ple, a DMM reading of 5.6mV when the 0.1Ω range is selected is equivalent to 5.6 x 0.1 = 0.56Ω. From this, it follows that if the 1Ω range is selected, the reading on the DMM is directly equivalent to the value in ohms. Values from 100Ω down to 0.01Ω can be measured via the tester. Below this, errors start to be significant due to contact and lead resistance. Values above 100Ω can also be measured via the tester but this is rather pointless. That’s because the DMM alone can be used to accurately measure values above this figure. Circuit details Refer now to Fig.2 for the complete circuit of the Low Ohms Tester. It Silicon Chip’s Electronics TestBench  31 Fig.4: this is the full-size etching pattern for the PC board. its “+” and “-” terminals. This device is connected between the positive supply rail and ground via a 5.6kΩ current limiting resistor. VR1 allows the reference voltage to be adjusted slightly and is used for calibration. Op amp IC1 and transistor Q1 function as a buffer stage for REF1. Because this stage is simply a voltage follower, the vol­tage on Q1’s emitter will be the same as the voltage on pin 3 of IC1. This means, in turn, that the voltage across the resistance Fig.3: install the parts on the PC board selected by S2b is equal to the and complete the wiring as shown here. REF1 voltage. As a result, a constant current consists of a constant current source flows through the selected resistance (which supplies the current through and this current also flows through test resistor Rx) plus an amplifier stage Q1, test resistor Rx and diodes D1 & to drive the DMM. D2 to ground. IC1, REF1 and Q1 are the basis of In greater detail, when S2b selects the constant current source. REF1 positions 1, 2 or 3, the 2.4kΩ resistor is a precision voltage source which is in circuit and so has the REF1 voltprovides a nominal 2.490V between age across it. If REF1 is adjusted to 2.4V, then 1mA will flow through the resistor and thus through Q1 and Rx. Conversely, when S2b selects position 4, the constant current source delivers 10mA to Rx (assuming that VR2 is correctly set). IC2 functions as the amplifier stage. This operates with a gain of either x10 or x100, as set by switch S2a. Switch S2c selects between the collector of Q1 and the amplifier output at pin 6. Thus, when position 1 is selected, the amplifier is by­passed and the DMM directly monitors the voltage across Rx. Because the constant current source supplies 1mA through Rx in this position, the reading in millivolts is directly equivalent to the value of Rx in ohms. Conversely, when positions 2, 3 or 4 are selected, IC2 amplifies the voltage across Rx and drives the DMM via its pin 6 output. IC2 operates with a gain of 10 when position 2 is select­ed and a gain of 100 when positions 3 or 4 are selected. These gain values are set by RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ 32 No. 1 1 1 1 1 1 1 1 1 1 Value 1MΩ 91kΩ 10kΩ 5.6kΩ 2.4kΩ 2.2kΩ 1kΩ 200Ω 100Ω 91Ω 4-Band Code (1%) brown black green brown white brown orange brown brown black orange brown green blue red brown red yellow red brown red red red brown brown black red brown red black brown brown brown black brown brown white brown black brown Silicon Chip’s Electronics TestBench 5-Band Code (1%) brown black black yellow brown white brown black red brown brown black black red brown green blue black brown brown red yellow black brown brown red red black brown brown brown black black brown brown red black black black brown brown black black black brown white brown black gold brown the impedance seen by this input to that seen by the pin 2 input. This ensures that equal currents flow in the two op amp inputs and this in turn minimises the output offset voltage. VR3 nulls out any remaining offset voltage and is adjusted so that the DMM reads 0mV when Rx is 0Ω (ie, when the test terminals are shorted together). One interesting point is that the lower end of Rx is two diode drops above ground, due to series diodes D1 and D2. This ensures that IC2 operates correctly when the output is only 1mV above the lower Rx connection point. Power for the circuit is derived from a 9V battery via power switch S1. Two 47µF capacitors across the supply provide decoupling and lower the impedance of the 9V rail, while LED1 provides power on/off indication. Construction The PC board carries nearly all the parts and is mounted by clipping it into the guide notches of a standard plastic case. Note that the locking collar of the rotary switch (under the mounting nut) must be set to position 4, as described in the text. the 1MΩ, 10kΩ, 1kΩ & 91kΩ resistors in the feedback network. In position 2, all four resistors are connected in parallel to give a feedback resistance of 900Ω. IC2 thus operates with a gain of 1 + 900/100 = 10. In the other three positions, only the 1MΩ and 10kΩ resistors are connected and these give a feedback resistance of 9.9kΩ. The gain is now 1 + 9900/100 = 100. Note that the 0.1µF capacitor is always connected across the feedback path, to reduce any high frequency noise. The 91Ω resistor at pin 3 matches Most of the parts are mounted onto a small PC board coded 04305961 and measuring 60 x 100mm. The board clips into the inte­gral side pillars of a plastic case measuring 130 x 66 x 43mm. Begin construction by checking the PC board for shorted tracks or small breaks. Check also that it clips neatly into the case. Some filing of the PC board sides may be necessary to allow a good fit without bowing the case sides. Begin the board assembly by installing the PC stakes. These are located at the three external wiring points and at the con­ nections for switch S1. This done, insert the single wire link (it sits immediately beneath VR3). Next, install the resistors (see table for colour codes), then install the diodes and ICs, taking care to ensure that they are oriented correctly. The capacitors can go in next – note the polarity of the two 47µF electrolytic types. Silicon Chip’s Electronics TestBench  33 PARTS LIST 1 PC board, code 04305961, 60 x 100mm 1 front panel label, 62 x 125mm 1 plastic case, 130 x 66 x 43mm 1 9V battery holder 1 9V battery 1 SPDT toggle switch (S1) 1 3-pole 4-way PC mount rotary switch (S2) 2 10kΩ horizontal trimpots (VR1,VR3) 1 100Ω horizontal trimpot (VR2) 1 12mm knob 2 banana plugs 2 banana panel sockets 6 PC stakes 1 6mm ID rubber grommet 1 20mm length of 0.8mm tinned copper wire 1 300mm length of hook-up wire 3 2.5mm screws and nuts Semiconductors 2 CA3140E Mosfet input op amps (IC1,IC2) 1 BC328 PNP transistor (Q1) 1 LM336Z-2.5 reference (REF1) 2 1N914, 1N4148 signal diodes (D1,D2) 1 5mm red LED (LED1) Capacitors 2 47µF 16VW PC electrolytic 1 0.1µF MKT polyester or monolithic ceramic Resistors (0.25W, 1%) 1 1MΩ 1 2.2kΩ 1 91kΩ 1 1kΩ 1 10kΩ 1 200Ω 1 5.6kΩ 1 100Ω 1 2.4kΩ 1 91Ω 1 1Ω 1% (for calibration) Miscellaneous Hook-up wire, tinned copper wire. REF1 and Q1 can now both be installed. Note that these two devices look the same so make sure that you don’t get them mixed up. LED1 is mounted on the end of its leads so that it will later protrude through a matching hole in the front panel. For the same reason, switch S1 is soldered to the top of the previously in­stalled PC stakes. Rotary switch S2 is mounted directly on the PC board. Ensure that it 34 has been pushed fully home and sits flat on the PC board before soldering its pins. This done, loosen the switch mounting nut, lift up the star washer and rotate the locking collar to position 4. This turns what was a 12-position rotary switch into a 4-position rotary switch. Check that the switch operates correctly, then do the nut up tight again so that the locking collar is secured. The board assembly can now be completed by mounting the trimpots and fitting the battery holder. Note that VR2 is a 100Ω trimpot, while VR1 and VR3 are both 10kΩ types so be careful with the values here. The battery holder is secured to the PC board using the 2.5mm mounting screws supplied with it. Final assembly It’s now just a matter of installing the board and the ancillary bits and pieces in the case. First, attach the front panel label, then drill holes for the LED, switches S1 & S2, and the two test terminals. A hole will also have to be drilled in the top of the case to accept a small grommet. The PC board can now be clipped into the case, the test terminals mounted in position and the wiring completed as shown in Fig.3. This done, check that the switches and the LED line up with the front panel holes. Adjust the height of the LED and switch S1 if necessary, so that they fit correctly. The leads to the meter run through the grommetted hole in the top of the case. Keep these leads reasonably short and termi­nate them with banana plugs. It will be necessary to trim the shaft of switch S2, so that the knob sits close to the front panel. Test & calibration Now for the smoke test. Apply power and check that the LED lights (if it doesn’t, check that the LED has been oriented correctly). Now check the supply voltages on IC1 and IC2 using a multimeter. In each case, there should be about 9V between pins 7 and 4. If everything is OK so far, check the voltage between pin 3 of IC1 and the positive supply rail (ie, the voltage across REF1). Assuming VR1 is centred, you should get a reading of 2.4-2.5V. Pin 2 of IC1 should be at the same voltage as pin 3. Silicon Chip’s Electronics TestBench + + Rx 0.1Ω 0.01Ω 1Ω 1mΩ + VALUE per mV + + POWER LOW OHMS TESTER Fig.5: this full-size artwork can be used as a drilling template for the front panel. To calibrate the unit, follow this step-by-step procedure: (1) Monitor the voltage across REF1 and adjust VR1 for a reading of 2.4V (this sets the constant current. (2) Plug the Low Ohms Tester into the DMM and short the Rx test terminals using a short length of 1mm tinned copper wire. (3) Select the 0.01Ω range and adjust VR3 for a reading of 0mV on the DMM. Check for a similar reading when the 1mΩ range is selected. (4) Connect a 1Ω 1% resistor between the test terminals, select the 0.01Ω range and adjust VR1 again for a reading of 100mV. (5) Select the 1mΩ range and adjust VR2 for a reading of 1V. (6) Short the test terminals again and verify that the DMM reads close to 0mV for all ranges. That completes the calibration procedure. The lid can now be attached to the case, the knob fitted to S2 and the unit pressed into service. SC