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DC offset for digital multimeters Here's a useful accessory for your digital multimeter. Using just two ICs, it provides a precise DC offset so that you can switch your DMM to a lower scale to obtain greater resolution for monitoring voltage drift. It's also handy f ot making relative measurements. By JOHN CLARKE There are many situations where it is desirable to monitor small voltage changes rather than the absolute voltage. However, monitoring these small voltage changes can be difficult if the absolute value is high. That's because you have to switch your DMM to a high range to monitor the voltage and that in turn means low resolution. What's needed in this situation is some means of nulling out the absolute voltage reading on the DMM so that you can switch to a much lower range to obtain greater 66 SILICON CHIP resolution. And that's where this handy project comes in - it can generate an adjustable 0-ZOV voltage offset for your DMM. In use, the device is simply connected to the DMM in series with the voltage to be monitored and its output adjusted to produced a nulled reading (ie, OV). Once this has been done, you can then switch the DMM to a lower range to monitor voltage drifts over time due to temperature and load changes, etc. The idea behind the DC Offset for DMMs is hardly new. Indeed, some top of the range multimeters such as the Fluke 80 series include a relative measurement feature as standard. This allows the user to set the multimeter to read OV at any input voltage. Any subsequent reading on the display will then be the difference between the new input voltage and the preset voltage used for nulling. For example, let's .. say that our original input voltage to the Fluke 85 is 15.00V. If the relative (REL) switch is pressed, the display will then read 00.00V. If the input voltage is now increased to 16.00V, the display will only show 1.00V; ie, the change in voltage. This relative measurement feature is very useful for monitoring changes [or drift) in voltages rather than absolute voltage readings. However, it does have the disadvantage that the display resolution does not increase in the relative measurement mode. In the above example, where we measured a 1.00V change from 15.00V to 16.00V, the resolution remained at lOmV. This is where the SILICON CHIP DC Offset has an advantage, since it allows the maximum resolution of the meter to be obtained. For example, to null out a 15.00V supply, the DC Offset unit is connected with opposite polarity in series with the multimeter and the supply and adjusted so that it also supplies 15.00V. The display on the digital multimeter would then read O.OOV. The multimeter can now be reset to the millivolt range [ie, O.OOOV) which means that we can now read any voltage variations with a resolution of lmV. The output of the DC Offset unit is adjustable from 0-ZOV using a 10-turn potentiometer and, with careful adjustment, can be set to within lmV of the required voltage. The temperature coefficient of the output voltage is better than 50ppm/°C from 25-70°C, while the output impedance is a maximum of 50k0. This is suitable for the lOMO input impedance of digital multimeters. How it works Refer now to Fig.1 which shows the circuit diagram. There are two main components: an LMC7660 [ICl) switched capacitor voltage converter and a TLC431 precision voltage reference [ZDl). The v+ D1 1N4146 +16.6V T C1 4.7 25VW I I 9V ...I.. 1 - IC1 LMC7660 4.7 25VW I c1 - 2.5V A VR1 50k LIN 10T + 0·20V OUTPUT TO METER K0R DC OFFSET FOR DIGITAL MULTIMETERS Fig.1: the circuit uses an LMC7660 voltage converter IC to obtain ± 9V rails from a single 9V battery. D1, D2 & their associated capacitors step the + 9V rail up to + 16.BV and the resulting 25.BV supply is then applied to a TL431 precision voltage reference. S2 and S4 are closed. Let's see how the circuit works. When Sl and S3 are closed, Cl charges to the supply voltage of V + . S1 and S3 are now opened and S2 and S4 are closed. The + side of Cl is now connected to ground and so the opposite side of Cl which is at V - connects to C2 which charges via S4. After a few cycles of this process, C2 charges to V - . In practice, an internal oscillator which nominally operates at about lOkHz is used to drive Sl and S3. This clock signal is also inverted and used to drive S2 and S4. So the pin 5 output of the LMC7660 delivers a - 9V rail and between the + 9V rail and - 9V we get 18V. To increase this voltage further, diodes Dl and DZ plus their associated capacitors are used to double the + 9V rail. Fig.3 shows how this is done. The LMC7660 is used to step-up the battery voltage by about three times and this is then applied to the TL431 which generates a precise output voltage. The reason for stepping up the voltage is so that the reference voltage can be varied all the way up to 20V while operating from a 9V battery. This is a less expensive but more convenient arrangement than using three 9V batteries in series to obtain sufficient voltage. In fact, the cost of the IC and its associated components for tripling the supply is only about that of one battery. Fig.2 shows the internal workings of the LMC7660. It contains four CMOS switches which are shown here as Sl, S2, S3 and S4. Sl and S3 operate together, while S2 and S4 operate together. When Sl and S3 are closed, S2 and S4 are open and when S1 and S3 are open, n-------+-, D1 V+ 9V I 20V VIEWED FROM BELDW = 3 51k 7.5k S2 6 + - POWER S1 = 9V ... + I S1 I I + I 53/ 0 C2r VDUT = 2V+ -(VD1+VD2) 2 5 OVDUT = -v+ = -9V + S2 t Fig.2: inside the LMC7660. S1/S3 & S2/S4 alternately open & close to charge Cl to + 9V & C2 to - 9V. 4.7I Fig.3: how the voltage doubler works. S1 & S2 alternately open & close to charge the 4. 7µF capacitor to almost twice V + . OCT0BER1990 67 first thing to note is that Sl and S2 alternately switch pin 2 of the LMC7660 between the + 9V supply and ground. Initially, when S2 is closed and Sl is open, the 1µ,F capacitor charges to the V + supply via D1. At the same time, the 4.7µ,F capacitor is charged to V + via D1 and D2. When S2 opens and St closes, the negative side of the lµ,F capacitor is pulled to the V + rail and so the positive side goes to almost twice V +, or 18V. This charges the 4.7µ,F capacitor via D2. After a few cycles, the 4. 7µ,F capacitor is charged to almost twice the V + supply. So D1, D2 and their associated capacitors behave as a voltage doubler. Actually, the voltage is slightly less than 2V + due to the voltage drops across diodes D1 and PARTS LIST 1 plastic case , 82 x 54 x 31mm 1 PC board, code SC04209901, 45 x 51 mm 1 Dynamark front panel label, 50 x 79mm 1 50k0 1 0 -turn potentiometer 1 SPOT toggle switch 1 black banana socket 1 red banana socket 1 PC-mounting 9V battery holder 1 216 9V battery 1 knob for potentiometer 4 PC stakes TO TERMINA LS a;) V W IPl Fig.4: wire up the PC board as shown here, then mount the switch & pot. on the case lid & run the wiring. Note the wire links under the battery holder. D2; ie, 18V - 1.2V = 16.8V. This is added to the - 9V r ail from pin 5 of ICl to give a total of 25.8V which is then applied to ZDl via a lkn resistor. The tkn resistor limits the current through ZDl while the 51k0 and 7.5k0 resistors set the voltage at ZDl 's cathode (K). In operation, ZDl provides a nominal 2.5V between its reference (R) and anode (A) terminals and this sets the current through the 7.5k0 resistor to 333µ,A. Since the current into the R terminal of ZD1 is 4µ,A , the total current through the 5 lkO resistor is 337 µ,A and thus the voltage across it is 17.2V. This voltage plus t h e 2.5V developed between the r eference and anode terminals gives us 19. 7V across ZDl. This in turn is applied to a 50k0 10-turn potentiometer which allows the output to be set anywhere between OV and 19.7V to give the required offset voltage. Construction Most of the parts, including the battery holder, are mounted on a small PC board coded SC 04209901. Fig.4 shows the assembly details. Install the three wire links first (these sit under the battery holder), then follow with the resistors and capacitors. Note that the capacitors are all polarised so be sure to install them the right way around. The resistors are all 1 % types check each one for value on your DMM before installing it on the board. ICl, the two diodes and ZD1 can now all be installed. Check the orientation of each component carefully before soldering its leads, then install the battery holder and secure it using screws and nuts. Semiconductors 1 LMC76601N switched capacitor voltage conve rter· (IC1) 1 TL431 CLP programmable precision reference (ZD1) 2 1 N4148 signal diodes (D1 ,D2) Capacitors 3 4 . 7 µ,F 25VW PC electrolytics 1 1 µ,F 1 6VW PC electrolytic Resistors (0.25W, 1 %) 1 51 kO 1 7.5k0 1 1 kO Miscellaneous Tinned copper wire for links, hookup wire, solder, screws and nuts for battery holder. 68 SILICON CHIP The two output sockets are mounted near the bottom of the case to provide clearance for the PC board. The board sits upside down inside the case when the lid is screwed down & can be secured using foam rubber. The PC board'is housed in a small plastic case measuring 82 x 54 x 31mm. As shown in the photographs, the voltage adjust potentiometer (VRl) and the on/off switch are mounted on the lid, while the output banana terminals are mounted on one side. To install the hardware in the case, first drill the holes in the lid using the front panel artwork as a guide, then drill holes in the side for the output sockets. These sockets should be 19.5mm apart and should sit as close to the bottom of the case as possible. This done, attach the front panel artwork, install the potentiometer and switch, and complete the wiring as shown in Fig.4. When installing the wiring, sit the PC board on the back of the lid next to the switch and pot as shown in the wiring diagram. The PC board is then installed upside down in the case when the lid is screwed down and can be held in position using a small piece of foam rubber. Testing is straightforward - just connect the output to your DMM, switch on and check that the output voltage can be varied from 0-ZOV using the 10-turn pot. If you strike trouble, check for 25.8V between the cathode of DZ and pin 5 of ICl. This will tell you whether the fault lies around ZDl or around ICl and the voltage doubler. ~ Burglar Alarm Siren upper and low threshold voltages) will result in the oscillator frequencies being different - you may have to change some resistor values. Second, watch the component polarities, both for the electrolytic capa citors and the semiconductors, particularly the TIP31s and TIP32s. It is all too easy to put these in the wrong way around and then you have a very dud project. Testing Don't be an idiot when you hook this up to your power supply. At the very least, put the horn speaker face down on your workbench when testing it - it is extremely loud and it will just about blow your F• • iI LL LL 0 NI 0 I- en => -, C <( UHF REMOTE CONTROL EA April 89 Our latest UHF transmitter • proven reliable unit • Complete Tx kit and PCB with components for Rx kit. Tx Battery included . 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Better still, do your initial testing with a fair sized resistor connected in series with the speaker. For example, we used an 8200 5 watt resistor when we tested the unit on the bench. However, any value from a few hundred ohms up to say, 2k0 will do the job and protect your ears. What a bout different supply voltages? Yes, you can increase the supply up to 15V which is the limit for the 40106. And the circuit will operate, with reduced power, down to about 9 or 10 volts. Below that, it's not worth bothering and you would have to change resistor values to make the oscillators work correctly. ~ MASTER SLAVE SWITCH EA JANUARY 1990 Remotely switch your non remote TV off an d on via your remote controlled VCR, many oth er uses etc . One switch operation, mains filter and overvoltage protection incl uded .Gom_plete PCB an d all on board components. ONLY $24.90 HALF THE PRICE OF MOST MAINS FIL TEAS . 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