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Ilifferential input buffer for oscilloscopes This simple circuit will allow you to measure mains waveforms on your oscilloscope and to observe signals which cannot be connected to earth. It has a differential input and it provides 7 .5kV isolation between the input & output by means of linear optocouplers. By JOHN CLARKE Measuring mains voltages on most general-purpose oscilloscopes is a dangerous and inaccurate procedure. Unfortunately, it's not simply a matter of connecting the CRO probe to the mains, selecting the appropriate voltage division ratio and observing the waveform on the screen. Instead, there are at least two problems that must be overcome. First, you must connect the probe earth clip to Neutral. Connecting the probe the wrong way around, with the earth clip to Active, will not only blow a fuse but could also give you a nasty shock. In addition, the Neutral is not necessarily at earth potential which means that a large current may flow in the earth lead, even if the probe is connected correctly. The second problem you will probably run into is that the CRO lacks sufficient voltage division ratios. Most general purpose oscilloscopes can only be switched to a maximum of 5V/division, which corresponds to 50V/division when the probe is set to a 10:1 ratio. Because there are only eight divisions on the screen, it follows that only waveforms up to 400V p-p can be displayed in their entirety. A 240V mains waveform is about 680V p-p which means that it is much too big to fit on the screen. Of course, the variable VOLT/DIV control can be used to reduce the size of the waveform so that it does fit. However, the waveform then becomes uncalibrated and no worthwhile voltage measurements can be made. Difficulties also occur when measurements are to be made on Triac and SCR phase control circuits, or where you wish to monitor the current through a mains appliance (via a suitable shunt). Because the earth lead of the probe can no longer be used in these cases, the oscilloscope must be used in differential mode. This involves using both inputs of the CRO to measure a single waveform (ie, we must use a dual-trace unit). However, the waveform will again be too large to fit on the screen unless we are prepared to settle for an uncalibrated display. Differential input buffer The Differential Input Buffer divides the input signal by 1000, which means that you can now monitor calibrated mains waveforms. The unit also makes it possible to measure signals which cannot be referenced to earth. 76 SILICON CHIP This Oscilloscope Differential Input Buffer solves all these problems. Because it has a differential input, no earth lead is required on the probes and so it is impossible to incorrectly connect the earth to Active. In addition, the circuit divides the mains voltage by 1000, so that the vertical attenuator can be set to 1V/DIV to provide a fully calibrated waveform which easily fits on the screen. The input buffer also allows measur.ements of mains AC waveforms and __ •• --"-'- .,,~ The assembly of the unit is straightforward since most of the parts are mounted on a single PC board. Power is derived from two internal 9V batteries & this, together with the use of linear optoisolators in the signal paths, provides a nominal 7.5kV of isolation between the input & output sockets. other signal waveforms which cannot be referenced to earth. These include measurements at the outputs of bridged amplifier circuits and across motors driven by H-pack output circuits. Fig.1 shows how the Differential Input Buffer is used in a typical situation, in this case to monitor the output of a bridged audio amplifier. The unit has two inputs, one inverting(-) and the other non-inverting (+), and these are simply connected across the output'. The output of the input buffer is then connected to the CRO input in the conventional manner, leaving the second input of a dual-trace CRO free for other measurements. To ensure operator safety, the metal case of the input buffer is earthed back to the oscilloscope case earth and the rated isolation between the inputs and the output is 7.5kV. This isolation is provided by using optocouplers in the signal paths and by running the circuit from 9V batteries. Note that this figure is based on the rated isolation of the optocouplers and is a nominal value only. In practice, the true isolation is likely to be somewhat less than this figure due to the BNC input and output sockets. The complete circuit is housed in a metal case measuring 95 x 52 x 151mm. The front panel carries the OSCILLOSCOPE + DIFFERENTIAL INPUT OSCILLOSCOPE BUFFER • OUTPUT FRAME Fig.1: this diagram shows how the Differential Input Buffer can be used to monitor the output of a bridged audio amplifier, where no earth can be connected. two BNC input sockets and an on/off switch, while the rear panel carries the BNC output socket. Also on the back panel is an earth socket and this is connected to the earth terminal on the CRO via a banana plug lead (see Fig.1). Note that conventional CRO probes are connected to the two BNC input sockets and these should be set for 1:1 division. However, the earth clips on the probes are no longer used. In fact, depending on your application, it would be better to remove the clips altogether (or at least tape them up) to avoid accidental contact with the mains. How it works We've used a very interesting new device in this circuit - an 11300 linear optoisolator from Siemens. In fact, the circuit uses two such devices, one for each input. In addition, there are five op amps, two transistors and a handful of minor parts - see Fig.2. Before we get fully immersed in the circuit description though, we'll first take a closer look at the 11300. There's a very interesting twist to this device, as we shall explain. In the past, optocouplers have been used mainly to isolate digital control circuits from Triac circuits operating APRIL 1992 77 ~ - - - - - 4 t - - - - + - - - V 1 +, V+ TP5 INVERTING' "itlPUf T 0.1 TP4 VR3, VR4 CALIBRATE V- 4.7k 0.1T LINEAR OPTOISOLATOR IC2 IL300 ;t"'" V1- GAIN VR3 10k 8.2k 6" 1.1k TO OSCILLOSCOPE INPUT 5- ! +1000' VR1' 2oon- v- .,. IC5 LM334 CURRENT SOURCE 0.1! 10k V1- 680Q V+ V- V+ TP7 0.1T l NONINVERTINf INPUT "j_ V- TP6 VR3, VR4 CALIBRATE 10k V+ B1 9V. l"™ 6· -T 1 :!:.L.. v10 + 16VW• - 10~ 1.1k V- 5 1/2 SUPPLY AMPLIFIER ), +1000 VR2. 200n v- 'f" *PHILIPS VR37 0.5W 1500VAC Sl 8 v-<at>.v. ELJc l ,. B2 9V VIEWEO FROM BELOW 16VW -T : V1+ + 10k 10k :!:.L.. V1- DIFFERENTIAL INPUT BUFFER FOR OSCILLOSCOPES Fig.2: the differential input signals are first divided by 1000 & then applied to buffer stages ICla & IClb. These op amps in turn drive linear optoisolators IC2 & IC3 via current amplifiers Ql & Q2. Finally, the outputs of the optoisolators drive differential amplifier IC6a which produces the output signal. at mains potential. These optocouplers usually contained an internal LED and a photodiode detector. To activate the device, the LED was simply driven at a level sufficient to fully saturate the detector, which meant that the device was either on or off. We cannot use this type of optocoupler in an analog circuit however - at least, not if we expect good performance. That's because their output characteristics are far from linear and are subject to wide variations with changes in temperature. We cannot apply feedback either, since that would defeat the purpose of the optoisolator. And so we come to the 11300. Here's the twist: it overcomes the above problem because it has not one photodiode but two. One photodiode (on the iso78 SILICON CHIP lated side) is used to provide the output as before, while the second is used to provide the feedback to the LED driver circuit that's so necessary to ensure good linearity. In fact, the output sensor can be driven with a linearity of .01 %. Now let's get down to the circuit details. Op amps ICla and IClb are used to buffer the differential input signals. In each case, the incoming signal is.attenuated by a factor of 1000 using a voltage divider network consisting of a 1.ZMQ input resistor, a 1. lkQ resistor and a ZO0Q calibration trimpot. ICla and IClb drive optocouplers ICZ and IC3 via buffer transistors Ql and QZ. These transistors are wired as emitter followers and ensure sufficient current drive for the optocou- 1/2 SUPPLY AMPLIFIER pler LEDs via series ZZ0Q resistors. Feedback from the optocouplers is derived from the non-isolated detectors and is applied to the inverting input of each driver op amp. Since we want each optocoupler to operate with an AC signal, it is necessary to bias them so that their internal LEDs are at about "half-brightness" when no signal is applied to the inputs. This bias is derived from a constant current sink (IC5) which pulls 50µA via each of the 10kQ resistors connected to the inverting inputs of ICla and IClb. Essentially, ICla and Ql operate as a class-A amplifier, with the emitter of Ql sitting at close to half-supply (ie, 0V) under no-signal conditions. As the signal increases, Ql turns on harder and increases the drive to the LED. Conversely, as the signal decreases, Ql throttles back and the LED output decreases. IClb and QZ operate in exactly the same fashion. This CRO screen photograph shows the waveform across a 'Ii:iac 'in a typical phase control circuit. The vertical attenuator was set to 0.lV/ division and this, combined with the 1000:1 attenuation of the Differential Input Buffer, gave a display calibrated to lO0V/division. IC5 (the constant current sink) is an LM334 3-terminal device. Its operating current is set by the 680Q resistor between its "R" input and V-. In this circuit, the current is set at lO0µA and is equally divided between the inputs of ICla & IClb. IC4 is used to derive a half-supply rail from 9V battery BATT1. This op amp is wired as a voltage follower and derives its input from a voltage divider consisting of two 10kQ resistors. The resulting mid-point voltage is then decoupled by a l0µF capacitor and buffered by IC4, the output of which connects to the signal ground. The positive terminal of the battery thus becomes the +4.5V supply (V+), while the negative terminal becomes the -4.5V supply (V-). These rails power all the circuitry on the non-isolated side of the 11300 optocouplers. The outputs from the isolated detectors in IC2 and IC3 are used to drive differential amplifier IC6a. VR3 and its series 8.2kQ resistor set the voltage between the inverting input (pin 2) and the output (pin 1), while VR4 and its series 8.2kQ resistor set the voltage on the non-inverting input (pin 3). In operation, IC6a acts as an inverting amplifier for signals from IC2 and as a non-inverting amplifier for signals from IC3 . VR3 sets the overall Specifications Frequency response: DC to 100kHz Input impedance: 1.2MQ Input attenuation: 1000:1 Isolation: 7.SkV Power supply: 2 x 9V batteries PARTS LIST gain of this stage, while VR4 is an offset adjustment that's used to compensate for the extra gain in the noninverting path. IC6 is powered from 9V battery BATT2. In this case, voltage follower stage IC6b and the 10kQ voltage divider resistors on its input are used to derive a half-supply rail. This stage works in exactly the same manner as IC4. The output of IC6b connects to the signal ground and also to the case. The positive terminal of BATT2 thus becomes the +4.5V rail (Vl+) for IC6, while the negative terminal becomes the -4.5V supply rail (Vl-). Double-pole switch S1 provides power on/ off switching. S la switches the BATT1 supply while Slb switches the BATT2 supply. Construction The circuit is assembled on a PC board coded SC04204921 and measuring 121 x 84mm. This fits neatly into a standard metal case (see parts list) that's also large enough to accommodate the various input/output terminals and the on/off switch. Fig.3 shows the parts layout on the PC board. Start construction by installing PC stakes at the six test points and all external wiring points. Once this has been done, install the passive components (links, resistors, capacitors & trimpots), then the transistors and ICs. The LM334 can also be installed at this stage. Make sure that you don't confuse this device with the two transistors. The board assembly can now be completed by installing the two battery holders. Secure them using 2mm screws and nuts, then check the board assembly carefully against Fig.3 be- 1 PC board, code SC04204921, 121 x84mm 1 Dynamark front panel label, 92 x51mm 1 K&W instrument case, 95 x 52 x 151mm 2 216 9V PCB-mount battery holders 2 216 9V batteries 1 double pole rotary mains switch (must be mains-rated) 1 knob for switch 3 BNC panel sockets 1 banana earth terminal 4 6mm standoffs 5 3mm x 9mm screws plus nuts & shakeproof washers 6 2mm x 6mm screws plus nuts 1 solder lug 15 PC stakes 1 150mm-length 0.6mm tinned copper wire 1 250mm-length blue mains wire 1 250mm-length brown mains wire 1 100mm-length green/yellow mains wire 2 10kQ miniature horizontal trimpots (VR3,VR4) 2 220Q miniature horizontal trimpots (VR1 ,VR2) Accessory leads 2 oscilloscope probes 1 BNC to BNC plug lead 1 banana plug to banana plug earth lead Semiconductors 2 TL072 dual op amps (IC1 ,IC6) 2 IL300 linear optocouplers (IC2,IC3 - Siemens) 1 TL071 op amp (IC4) 1 LM334 3-terminal current source (IC5) 2 BC548 transistors (Q1 ,Q2) Capacitors 2 10µF 16VW PC electrolytic 4 0.1µF monolithic 2 10OpF ceramic Resistors (0.5W, 1%) 2 1.2MQ Philips VR37 0.5W 610kQ 21 .1kQ 2 8.2kQ 1 6800 2 4.7kQ 2 220Q Note: do not substitute for the Philips VR37 resistors as these are necessary to ensure an adequate voltage rating. APRIL 1992 79 SOLDER LUG FRONT RTH ~TERMINAL REAR Fig.3: install the parts on the PC board as shown in this wiring diagram & be sure to use the specified 1.2MQ input resistors (see parts list). All external wiring from the board must be run using mains-rated cable. fore moving on to the next stage. Once you are satisfied with the board assembly, the adhesive label can be attached to the case and holes drilled to accept the two BNC input sockets and the on/off switch. This job is best done by drilling small pilot holes first and then carefully enlarging them to the correct size with a tapered reamer. You will also have to drill holes in the rear of the case to accept the output BNC socket, earth terminal and earth screw, plus four mounting holes in the base for the PC board. Position the PC board towards the back of the case as shown in the photograph, to provide clearance for the OSCILLOSCOPE DIFFERENTIAL INPUT BUFFER ·- . ~N= ·.·c_ __ + (+) OFF + + (-) on/ off switch on the front panel. The various items can now all be mounted in position and the wiring completed as shown in Fig.3. Note that the earth lug is secured to the case using a screw, nut and shakeproof washer. The PC board is mounted on 6mm spacers and secured using screws, nuts and washers. Use mains-rated cable for all the internal wiring. Checkout & calibration Before applying power, doublecheck your work to make sure that there are no wiring errors. When you are satisfied that all is well, install the two 9V batteries and apply power. Use your multimeter to check that there is 9V between pins 4 & 8 of both IC1 & IC6 and between pins 7 & 4 of ON OUTPUT AT REAR 1V:1000V; 7.SkV ISOLATION CAPACITOR CODES o Value IEC Code EIACode D D 0.1µF 100pF 100n 100p 104 101 Fig.4: this artwork can be used as a drilling template for the front panel. RESISTOR COLOUR CODES D D D D D D D D 80 No. Value 4-Band Code 5-Band Code 2 6 2 2 2 1 2 1.2MO 10kO 8.2kO 4.7kO 1.1kO 6800 2200 brown red green yellow brown black orange brown grey red red brown yellow purple red brown brown brown red brown blue grey brown brown red red brown brown not applicable brown black black red brown grey red black brown brown yellow purple black brown brown brown brown black brown brown blue grey black black brown red red black black brown SILICON CHIP r. \ I -• el N 0, 0 N • 0 u en • 0 Fig.5: check your PC board against this full-size pattern before mounting any of the parts. IC4. The output ofIC4 at pin 6 should be at half-supply. You can check this by measuring between the positive terminal of BATTl and TP3, and between the negative terminal ofBATT1 and TP3. You should get readings of +4.5V and -4.5V respectively. The output of IC6b should also be at half supply. This can be verified by checking for +4.5V between the positive terminal of BATT2 and chassis and for -4.5V between the negative terminal of BATT2 and ground. To calibrate the instrument, first connect TP4 to TP5 so that a voltage is applied to the input of IC1a. Measure this voltage by connecting your multimeter between TP1 & TP3. Now connect your multimeter across the output BNC socket. Adjust VR3 so that the reading is the same as that just measured between TP1 & TP3. Next, disconnect TP4 from TP5 and connect TP6 to TP7 so that a voltage is applied to the input of IC1b. Check this voltage by measuring between TP2 & TP3, then connect your multimeter to the output BNC socket again. Adjust VR4 until the output voltage matches the previous reading (ie, the voltage between TPZ & TP3). VR1 and VR2 are now be adjusted to provide an exact 1000:1 division ratio for the input signals. These adjustments must be done with the power switched off. To make these adjustments, first use your digital multimeter to check the resistance of the 1.2MQ resistor between the inverting input and TP1. Note the value. This done, connect the multimeter between TP1 & TP3 and adjust VR1 for a reading which is 111000th the previous value. For example, if the 1.2MQ resistor measures 1.195MQ, adjust VR1 for a reading of 1.195kQ. This procedure is now repeated for the non-inverting input. Measure the value of the 1.2MQ resistor connected to this input, then connect your multimeter between TP2 & TP3 and adjust VR2 to the correct value (ie, 1/ 1000th the previous reading). The circuit is now calibrated and ready for use. As a final c}:ieck, connect the unit to an oscilloscope as shown in Fig.1 and connect the test probes to mains Active and mains Neutral. Check that the mains waveform can now be displayed on the oscilloscope screen with the vertical attenuator switched to the 1V/DIV setting. If you are not going to be checking mains voltages, you can modify the input attenuator circuit so that you can observe low-level signals in differential mode. For example, to divide the input signal by 100, increase the 1.1kQ resistors at TP1 & TP2 to 10kQ and increase VR1 & VR2 to 5kQ. Finally, exercise great care when measuring mains voltages. Make sure that the input probes & leads are in good condition and are rated for mains operation. Remember - contact with the mains can be fatal! SC .1.~~.r..1;ti:.~: RCS Radio Pty Ltd is the only company which manufactures and sells every PCB [, front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. APRIL 1992 81