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Build a 40V 3A variable power supply This 1.23-40V adjustable power supply is designed for heavy-duty work. It uses a highefficiency switching regulator circuit & features preset current limiting, full overload protection & an LCD panel meter for precise voltage & current readouts. By JOHN CLARKE By far the biggest advantage that this elegant new power supply has over other designs is its high-efficiency switching regulator circuitry. In this type of circuit, the regulator is either fully on or fully off at any given instant and so it dissipates very little power, even when delivering high current at low output voltage. In practical terms, this means that the regulator generates very little heat and so we don’t need to use large and 16  Silicon Chip expensive heatsinks. And that in turn means that we can greatly simplify the construction and pack the required circuitry into a much smaller case than would otherwise be required for a conventional design employing a linear regulator. In fact, by employing switchmode operation, the regulator in this circuit generates less than 10W under worst case condi­tions. By contrast, a linear regulator in an equivalent 40V supply would need to dissipate around 120W when delivering 1.23V at 3A! This is an enormous amount of heat to extract and would require a large finned heatsink to keep the regulator temperature within specification. This is one power supply that can continuously supply a high output current without suffering from thermal overload problems. By contrast, a linear regulator has inherently high dissipation, especially at very low output voltages (due to the high voltage across the regulator), and this severely limits its output current capability. Another very commendable feature of the circuit is the low level of ripple and hash in the output. Achieving this is not always easy in a switchmode design but we’ve done it using a combination of extra filtering and careful circuit layout. As shown in the specifications panel, the output noise and ripple is just 5mV p-p at 24V, reducing to a minuscule 1mV p-p at 3V. 4 Main Features • Output voltage continuously adjustable from 1.23V to 40V • Greater than 3A output current capability from 1.23-28V • Digital readout of voltage, current or current limit setting • 10-turn pot for precise voltage adjustment • Adjustable current limit setting • Current overload indication • Regulation dropout indication • Output fully floating with respect to earth • Load switch • Low output ripple • Short circuit & thermal overload protection • Minimal heatsinking AMPERES 3 1 0 0 5 10 15 20 VOLTS 25 30 35 40 Fig.1: the voltage vs. current characteristics of the supply. It is capable of supplying a hefty 3.8A over the range from 1.23V to 28V. Beyond that, the available output current decreases due to the transformer regulation. These are excellent figures for a switching design and are comparable to those achieved by linear circuits. The switching hash is also very low. It is far less than in previous designs and, in fact, is below the ripple level. Digital readout Do you need to precisely monitor the output voltage or current, or accurately set the current limit? Well, with this power supply you can because it uses an LCD panel meter to give a digital readout of voltage or current. A single toggle switch selects the measure­ment mode. A 10-turn pot makes it easy to set INPUT VOLTS 2 the output voltage to the exact value required, while the current limit is set by first pressing the Set button and then adjusting the Current Limit pot until the LCD shows the required value. In addition, there are two LEDs on the front panel and these provide current overload and regulation dropout indication. There’s one other control on the front panel that we have­n’t yet mentioned – the Load switch. This simply connects or disconnects the load (ie, the device being powered) from the supply rail and eliminates the need to switch the supply off when making connections to the output terminals. It also allows the output voltage and current limit values to be set before power is applied to the load. Output capabilities Fig.1 plots the performance of the supply. As shown, it is capable of Fig.2: how a switching regulator operates. When S1 is closed & S2 is open, current flows to the load via L1 which stores energy. When S1 subsequently opens & S2 closes, the energy stored in the inductor maintains the load current until S1 closes again. supplying a hefty 3.8A over the range from 1.23V to 28V. Beyond that, the available output current decreases due to the transformer regulation. However, there is still 2.2A avail­able at 30V, 1.4A at 35V and 600mA at 40V. The load regulation is excellent at the higher voltages but is not as good LM2576-ADJ 1 Cin REGULATOR 4 DRIVER 1.23V REF L1 2 OSCILLATOR RESET ON/OFF 5 3A SW THERMAL SHUTDOWN, CURRENT LIMIT D1 Vout C1 R2 3 Vout = 1.23(1 + R2/R1) R1 Fig.3: a basic switchmode voltage regulator based on the LM2576 IC. In this circuit, an internal 3A switching transistor takes the place of S1 in Fig.2, while diode D1 takes the place of S2. The output voltage is set by the ratio of R2 & R1, which feed a sample of the output voltage back to an internal comparator. January 1994  17 REGULATOR DROPOUT INDICATOR IC3c 240VAC INPUT TRANSFORMER T1 AC RECTIFIER AND FILTER 42V SWITCHING REGULATOR IC1 ON/ OFF FILTER L2 R1 CURRENT SENSE The circuit is based on the National Semiconductor LM2576HVT high voltage adjustable switchmode voltage regulator. Fig.2 shows how a switching regulator operates. In operation, S1 and S2 operate at high speed and are alternately closed and opened. These two switches control the current flowing in inductor L1. When S1 is closed and S2 is open, the current flows to the load via inductor L1 which stores up energy. When S1 subsequently opens and S2 closes, the energy stored in the inductor maintains the load current until S1 closes again. The output voltage is set by adjusting the switch duty cycle and is equal to the input voltage multiplied by the ratio of S1’s on time to its off time. Capacitor C1 is used to filter the resulting output voltage before it is applied to the load. Fig.3 shows a complete voltage regulator based on the LM2576 IC. It is a 5-pin device which requires just five extra components to produce a basic working circuit. Its mode of opera­tion 18  Silicon Chip 0V SIGNAL CONDITIONER IC4 DPM-02 LCD VOLTMETER MODULE RANGE AND DECIMAL POINT SWITCH IC3d, IC5 GND Fig.4: this diagram shows all the relevant circuit sections. Switching regulator IC1 forms the heart of the circuit & adjusts its output according to the setting of VR1. IC2 amplifies the voltage across current sense resistor R1 & the amplified voltage is then fed to IC3a where it is compared with the output from VR2 to derive the current limit setting. A 3½-digit LCD panel meter provides precise readout of the voltage & current settings. Basic principle VOLTS OR AMPS S3 OUTPUT VOLTAGE ADJUST VR1 at lower voltages. This is because of higher losses in the circuit due to the higher pulse currents involved at low voltage settings. The line regulation is less than 0.1% for a 10% change in mains voltage – see specifications panel. 0V CURRENT LIMIT VR2 IC2 x200 CURRENT LIMIT INDICATOR IC3b COMPARATOR IC3a SET CURRENT S4 is the same as that described in Fig.2 except that here a 3A switching transistor is used for S1, while an external diode (D1) is used for S2. What happens in this case is that when the transistor is on, the current flows to the load via inductor L1 as before and D1 is reverse biased. When the transistor subsequently turns off, the input to the inductor swings negative (ie, below ground). D1 is now forward biased and so the current now flows via L1, the load and back through D1. The output voltage is set by the ratio of R2 and R1, which form a voltage divider across the output (Vout). The sampled voltage from the divider is fed to pin 4 of the switcher IC and thence to an internal comparator where it is compared with a 1.23V reference. This sets Vout so that the voltage produced by the divider is the same as the reference voltage (ie, 1.23V). Apart from the comparator and the switching transistor, the regulator IC also contains an oscillator, a reset circuit, an on/off circuit and a driver stage with thermal shutdown & current limiting circuitry. The incoming supply rail is applied to pin 1 of the IC and connects to the collector of the 3A switching transistor. It also supplies an internal regulator stage which then supplies power to the rest of the regulator circuit. Basically, the LM2576 uses pulse width modulation (PWM) control to set the output voltage. If the output voltage rises above the preset level, the duty cycle from the driver stage decreases and throttles back the switching transistor to bring the output voltage back to the correct level. Conversely, if the output voltage falls, the duty cycle is increased and the switch­ i ng transistor conducts for longer periods. The internal oscillator operates at 52kHz ±10% and this sets the switching frequency. This frequency is well beyond the limit of audibility although, in practice, a faint ticking noise will occasionally be audible from the unit due to magnetostric­tive effects in the cores of the external inductors. One very useful feature of the LM2576 that we haven’t yet mentioned is the On/Off control input at pin 5. As its name implies, this allows the regula­tor to be switched on or off using an external voltage signal. This feature is put to good use in this circuit to provide the adjustable current limiting feature, as we shall see later on. Block diagram Although the LM2576-ADJ forms the heart of the circuit, quite a few other parts are required to produce a practical working variable supply. Fig.4 shows the full block diagram of the unit. Power for the circuit comes from the 240VAC mains. This feeds power transformer T1 and its output is rectified and fil­tered to provide a 42V DC supply which is then fed to the input of the switching regulator (IC1). VR1 sets the output voltage from the regulator and essentially forms one half of the voltage divider shown in Fig.3. IC3c monitors the input and output voltages from the regu­lator and lights a LED when the difference between them is less than 3.3V. This indicates that the circuit is no longer regulat­ ing correctly. Following the regulator, the current in the nega­tive rail flows through the sensing resistor R1. The voltage across this resistor is then amplified by IC2 and applied to comparator stage IC3a. R1 has a value of just .005Ω, while IC2 operates with a gain of 200. This means that IC2’s output voltage is numerically equivalent to the current (in amps) flowing through R1 (ie, IC2’s output increases by 1V per amp). So, in addition to driving IC3a, IC2 is also used to drive the LCD digital voltmeter (via S4, S3 & IC4) to obtain current readings. IC3a and potentiometer VR2 provide the current limiting feature. In operation, IC3a compares the voltage from IC2 with the voltage set by VR2. This voltage can be anywhere in the range from 0-4V, corresponding to current set limits of 0-4A. The circuit works as follows. If IC2’s output rises above the voltage set by VR2 (ie, the current through R1 rises above the set limit), IC3a’s output goes high and turns off the switching regulator via the On/ Off con­trol. The current through R1 now falls until IC2’s output falls below the voltage from VR2, at which point IC3a’s output goes low and switches the regulator (IC1) back on again. The current now rises until the regulator is switch­ed off again and so the cycle is repeated indefinitely. By this means, IC3a switches the regulator on and off at a rapid rate to limit the current to the value set by VR2. IC3a also drives comparator stage IC3b and this lights an indicator LED when ever current limiting takes place. Switch S4 selects between the outputs of IC2 and VR2, so that either the load current or the current Specifications Minimum no load output voltage ......................................... 1.23V ±13mV Maximum no load output voltage ....................................................... 40V Output current ...........................................................................see graph Current limit range .................................................................. 10mA to 4A Current limit resolution .................................................................... 10mA Line regulation ........................<0.1% for a 10% change in mains voltage Voltmeter resolution........................ 10mV from 1.23V to 16.5V (approx); 100mV from 16.5V to 40V Current meter resolution ................................................................. 10mA Meter accuracy .................................................................1% plus 2 digits Load regulation no load to 3A <at> 24V ......................................................................1.5% no load to 3A <at> 12V .........................................................................2% no load to 3A <at> 6V ........................................................................2.8% no load to 3A <at> 3V ........................................................................4.2% Output ripple and noise 3A <at> 24V ................................................................................ 5mV p-p 3A <at> 12V ................................................................................ 2mV p-p 3A <at> 6V .................................................................................. 1mV p-p 3A <at> 3V .................................................................................. 1mV p-p limit setting is displayed on the LCD panel meter. This makes it easy to set the current limit. All you have to do is press S4 and rotate VR2 (the Current Limit control) until the required value appears on the digital readout. Immediately following R1 is a filter stage which is based mainly on inductor L2. This filter removes most of the ripple and high frequency noise from the positive and negative supply rails. The two supply rails are then applied to the load via S2. Finally, the 3½-digit LCD panel meter is used to display either the output voltage, the output current or the current limit setting, depending on the positions of switches S3 and S4. The selected signal voltage is applied to the panel meter via signal conditioning amplifier IC4, which provides the required level shifting and attenuation. For voltages up to about 18V, the display resolution is 10mV. It is then switched to a higher range with 100mV resolution to prevent over-range for output voltages above 20V. This task is performed using IC3d and IC5. Circuit details Refer now to Fig.5 for the full circuit details. It con­tains all the elements shown in the block diagram of Fig.4. We’ll go through each of the major sections in turn. Transformer T1 is supplied with mains power via fuse F1 and power switch S1. Its 30VAC secondary is full-wave rectified using diodes D1-D4 and filtered using two parallel 4700µF 50VW electrolytic capacitors. The resulting 42V DC supply is applied to the switching regulator (IC1). Note the 100µF capacitor connected between pins 1 & 3 of IC1. This capacitor is necessary to prevent circuit instabili­ty and is mounted as close to the IC as possible. D5, L1, the two parallel 1000µF capacitors and VR1 form the basic switchmode power supply block (see Fig.3). D5 is a Schottky diode which is rated at 10A and 60V. It has been specified in preference to a conventional fast recovery diode because of its low forward voltage drop. As a result, there is very little heat dissipation within the diode and this leads to increased effi­ciency. The output from IC1 feeds directly into L1, a 300µH induc­ tor. This is wound on a Philips ETD29 ferrite core assembly with a 1mm air-gap to prevent core saturation, as can occur when DC currents flow in ungapped core windings. January 1994  19 The 3A-40V Adjustable Power Supply is easy to build since most of the parts are mounted on a single PC board & the LCD panel meter is supplied ready made. No large heatsinks are required in the design because the switching regulator (IC1) dissipates very little power, even at low-voltage high-current settings. VR1 and its associated 1.5kΩ resistor provide voltage feed­back to pin 4 of IC1, to set the output level. When VR1’s resist­ance is at 0Ω, the output from the regulator (pin 2) is equal to 1.23V. This output voltage increases as the resistance of the pot increases. The 680Ω 5W resistor connected across the regulator output discharges the two 1000µF capacitors to the required level when a lower output voltage is selected. Filter circuit 20  Silicon Chip Regulator dropout Comparator IC3c and its associated parts form the regulator dropout indicator depicted on the block diagram. In this circuit, a sample of the output voltage is applied to pin 8 of IC3c and compared with a sample of the regulator input voltage at pin 9. Zener diode ZD2 provides an offset, so that IC3c only switches its output (pin 14) low when the voltage across the regulator drops below 3.3V. In this situation, IC1 is no longer Fig.5 (right): the main switching regulator circuit is based on IC1, L1 & D5, while IC2, IC3a & VR2 control the ON/OFF input of IC1 to provide the current limit feature. IC4 provides signal conditioning for the DVM02 panel meter, with IC3d & IC5 providing automatic range switching. ▲ Inductor L2 and its associated 100µF and 0.1µF capacitors make up the filter circuit shown in the block diagram (Fig.4). This LC network effectively attenuates the switching frequency ripple by a factor of 10. In practice, L2 consists of two separate windings (L2a, L2b) on the same toroidal core. These two windings are phased so that the flux developed by L2a is cancelled by the flux developed by L2b. This type of winding arrangement provides what is known as DC compensation and is done to prevent core saturation. As shown in Fig.5, L2a is used to decouple the positive supply rail, while L2b decouples the negative rail. The inductor thus effectively filters any common mode signals, while the 100µF and 0.1µF capacitors across the output attenuate any remaining spikes. The resulting filtered voltage is then applied to the output terminals via load switch S2. Additional filtering is applied at this point using a 0.33µF capacitor across the termi­nals and a 0.1µF capacitor between the negative terminal and mains ground. Note that this 0.1µF capacitor must be rated at 250VAC to comply with safety standards. January 1994  21 E N ZD1 9V 1W A A 12345 K A K ADJ 100 16VW POWER S1 K VIEWED FROM BELOW 680  5W CASE 240VAC A F1 500mA 10k 47k D 10 8 VR4 5k 3 IC6 LMC7660 0V 15V 0V 15V 6.8k 1k 5 100k 10 D1-D4 4x1N5404 2 3 7 X 1k 4 IC4 OP77GP -9V +9V 4700 50VW +42V 6 0.1 100  100  4700 50VW 2 3 7 100 63VW 10k 22k +9V S4b 11 10 2 4 1 K 1 100k IC3d S3 1 OUT 13 10 MONITOR VOLTAGE 2.2k 4 5 A K IC3a LM339 9 10 11 C B A 680  5W L1 300uH S4: 1: MEASURE CURRENT 2: SET CURRENT LIMIT D5 MBR1060 2 MONITOR CURRENT S4a 2 CURRENT LIMIT VR2 1k 220  680  ON/ GND OFF 3 5 FB IN IC1 LM2576HVT-ADJ REF1 LM336-5 A -9V 1.5k 6 0.1 CURRENT CAL VR3 10k -9V 4 IC2 OP77GP 15k +42V OUTPUT ADJUST VR1 50k 10T 3A-40V CURRENT LIMITED POWER SUPPLY 91k 4 2 T1 M2170 5 cx 3 1000 63VW 4 c 6 1M D6 1N4148 IC5 4053 16 cy 2 2.2k 1000 63VW 7 1 2 2V 200mV +9V 6 7 L2b 8 b 15 RANGE by bx 14 330pF 0.1 R1 . 005  L2a IC3b K  A 0.1 63V +42V 1k 1 X I/P- 10k 47k DP COM DP2 9 8 ~2. 8V COMMON DVM-02 I/P+ 1k 4.7k 0.5W ZD2 3.3V 400mW 12 3 CURRENT LIMIT LED1 100 63VW 0.33 63V DP1 +BAT +9V IC3c REGULATOR DROPOUT LED2 0.1 250VAC LOAD S2 -BAT 14 1k K  A +9V GND OUTPUT 1.23-40V 3A PARTS LIST 1 PC board, code 04202941, 222 x 160mm 1 front panel label, 250 x 75mm 1 plastic instrument case, 260 x 190 x 80mm 2 aluminium front & rear panels for above case 1 M-2170 30V 100VA mains transformer (Altronics) 1 LCD voltmeter module (Altronics Cat. Q-0560) 3 captive head binding posts (1 red, 1 black, 1 green) 1 2AG panel-mount fuseholder 1 500mA 2AG fuse 1 TO-220 heatsink, 26 x 30 x 15mm (Jaycar Cat. HH-8504) 1 SPDT mains rocker switch with neon indicator (S1) 1 DPDT paddle switch (S2) (DSE Cat. P-7693 or equiv.) 1 SPDT toggle switch (S3) 1 DPDT momentary pushbutton switch with common terminal at side (S4) (Altronics S-1394) 1 ETD29 transformer assembly with 3C85 core (Philips: 2 cores 4312 020 3750 2; 1 former 4322 021 3438 1; 2 clips 4322 021 3437 1) 1 RCC32.6/10.7, 2P90 ring core (Philips 4330 030 6035) 2 15mm diameter knobs 1 mains cord & plug 1 cord grip grommet 2 5mm LED bezels 26 PC stakes 5 self-tapping screws to mount PC board 2 4mm screws nuts & washers 4 3mm screws, nuts & star washers 1 3mm countersunk screw, nut & star washer (use a dress screw if the front panel is screen printed) 6 crimp lug eyelets for 3mm screw 2 solder lugs for 9mm thread 1 TO-220 insulating bush & washer 12 cable ties 1 50kΩ 10-turn pot (VR1) 1 1kΩ linear pot (VR2) 1 10kΩ horizontal trimpot (VR3) 1 5kΩ horizontal trimpot (VR4) regulating and IC3c lights LED 2 to provide a warning that the supply has dropped out of regulation. low input offset voltage and input bias current specifications. This is necessary to ensure that IC2’s output will be at 0V when no current is flowing through R1. The OP77GP used here typically has an input offset voltage of just 50µV and an input bias current of just 1.2nA. Because its inputs operate at close to ground potential, IC2 must be powered from both positive and negative supply rails. The positive supply rail for IC2 (and for the remaining ICs) is derived from the output of the bridge Current limiting The current sense resistor (R1) is wired into the negative supply rail before L2b and consists of a short length of 0.4mm enamelled copper wire. As explained previously, the voltage across it is multiplied by 200 using IC2, so that IC2’s output delivers 1V per amp of load current. In this application, IC2 must have 22  Silicon Chip Wire & cable 1 2-metre length of 1.5mm enamelled copper wire 1 3.5-metre length of 0.8mm enamelled copper wire 1 60mm length of 0.4mm enamelled copper wire 1 200mm length of 0.8mm tinned copper wire 1 25mm length of 1.0mm enamelled wire (for use as a feeler gauge) 1 600mm length green/yellow mains wire 1 1.5-metre length of red hook-up wire 1 1.5-metre length of black hookup wire 1 1.5-metre length of green hookup wire 1 1.5-metre length of blue hookup wire 1 200mm length of 3-way rainbow cable 1 200mm length of red 32 x 0.20mm hook-up wire 1 200mm length of black 32 x 0.20mm hook-up wire Semiconductors 1 LM2576HVT-ADJ high voltage adjustable switchmode voltage regulator (IC1) (NSD) 2 OP77GP op amps (IC2,IC4) 1 LM339 quad comparator (IC3) 1 4053 CMOS switch (IC5) 1 LMC7660 switched capacitor voltage converter (IC6) 4 1N5404 3A 400V diodes (D1-D4) 1 MBR1060 Schottky diode (D5) 1 1N4148 signal diode (D6) 1 9V 1W zener diode (ZD1) 1 3.3V 400mW zener diode (ZD2) 1 LM336-5 5V reference (REF1) 2 5mm red LEDs (LED1,LED2) Capacitors 2 4700µF 50VW electrolytic 2 1000µF 63VW electrolytic 2 100µF 63VW electrolytic 1 100µF 16VW electrolytic 3 10µF 16VW electrolytic 1 0.33µF 63VW MKT polyester 4 0.1µF 63VW MKT polyester 1 0.1µF 250VAC polyester 1 330pF MKT polyester Resistors (0.25W, 1%) 1 1MΩ 1 4.7kΩ 0.5W 2 100kΩ 2 2.2kΩ 1 91kΩ 1 1.5kΩ 2 47kΩ 5 1kΩ 1 22kΩ 1 680Ω 1 15kΩ 2 680Ω 5W 3 10kΩ 1 220Ω 1 6.8kΩ 2 100Ω Miscellaneous Insulating tape, solder, heatshrink tubing, heatsink compound, 4.7Ω 5W resistor (for load testing). rectifier via a 680Ω resis­tor and 9V zener diode ZD1. IC6, an LMC7660 switched capacitor voltage converter, generates the -9V rail for IC2. In operation, IC6 first charges the 10µF capacitor between pins 2 & 4 to 9V. It then reverses the connections of the ca­pacitor so that it can charge a second 10µF capacitor at pin 5 with negative polarity. This process is repeated continuously at a rate of about 10kHz so that the resulting output is a relatively smooth DC voltage. Comparator stage IC3a monitors the output voltage from IC2 and compares this with the voltage on its inverting input, as set by current limit control VR2. This potentiometer and its asso­ciated 220Ω resistor form a voltage divider network which is connected across 5V reference REF1. In operation, VR2 sets the voltage on pin 4 of IC2 at between 0V and 4V, corre­ sponding to current limit settings of 0-4A. Because IC3a is an open collector device, its output at pin 2 is connected to the positive supply rail via a 2.2kΩ pull-up resistor. If the voltage at the output of IC2 is greater than that set by VR2, pin 2 of IC3a is pulled high by this resistor. This also pulls pin 5 of IC1 high and switches off the regulator to provide current limiting. At the same time, pin 6 of IC3b is pulled high via D6, and so pin 1 switch­es low and LED 1 lights to indicate current limiting. When the current subsequently falls below the preset limit, pin 2 of IC3a switches low again and the regulator turns back on. Thus, IC3a switches the regulator on and off at a rapid rate to provide current limiting, as described previously. The 1MΩ resistor and 330pF capacitor at pin 6 of IC3b provide a small time delay so that LED 1 is powered continuously during current limiting. Fig.6: this scope photograph shows 100Hz ripple at the output terminals of the power supply when driving a 3A load at 12V. Fig.7: this is the 100Hz ripple for a 3A at 24V. Note the increase in ripple with the higher voltage. Digital panel meter IC4 forms the basis of the signal condi­tioning circuit. This op amp is wired in differential mode and operates with a gain of 0.01, as set by the resistor feedback networks on pins 2 and 3. Its output appears at pin 6 and is applied to the I/P+ input of the digital voltmeter (DVM-02). The DVM-02 is a standard panel meter with differential inputs (I/P+ and I/P-) and requires a 9V power supply between its BAT + and BAT- terminals. Its I/P- input is fixed at 6.2V (ie, 2.8V below the positive supply) and this reference voltage is used to bias pin 3 of IC4 via a 1kΩ resistor. This bias produces an offset at the output of IC4 and ensures that the voltage fed to the digital voltmeter is within its operating range. This signal conditioning is necessary because the DVM-02 cannot be used to directly measure voltages within 1V of either supply rail. The voltage range of the DVM-02 is selected by bridging pads on the volt- Fig.8: this is the high frequency switching noise as seen on a 100MHz oscilloscope using a 10:1 probe. meter PC board. In this case, only the 200mV and 2V ranges are used. The decimal point is selected in a similar manner (ie, by bridging DP1 or DP2 to DP COM). In operation, switch S3 selects either the positive output rail or the output of IC2 to provide voltage or current measure­ ment, respectively. The resulting voltage signal on the wiper of S4b is then applied to pin 3 of IC4 via VR4 and its associated series resistors. Alternatively, pressing S4 applies the voltage on the wiper of VR2, so that the current limit reading will be displayed on the DVM-02. This occurs regardless of the setting of S3. In summary then, IC4 divides the voltage at point D by 100 and adds this to the 6.2V reference signal. Thus, if we are measuring an output voltage of 20V for example, IC4’s output will be at 6.2 + 20/100 = 6.4V. This is 200mV great­er than the reference voltage at I/P- which means that the meter will display 20.0 – assuming suitable range and decimal point switching. Range switching IC3d and IC5 provide the range and decimal point switching, so that this operation is completely automatic. IC3d is wired as a Schmitt trigger and monitors the voltage between point D and the negative output rail (point X) via a voltage divider (47kΩ and 10kΩ). IC3d’s output drives the A, B and C inputs of IC5, a 3-pole 2-way CMOS analog switch. In this application, one switch pole (pole ‘b’) is used for range selection and another (pole ‘c’) for decimal point selection. The third switch pole is left unused. When the voltage at D is less than 18V, IC3d’s output is pulled high and pole ‘b’ connects to the ‘by’ position so that the 200mV range is selected. At the same time, pole ‘c’ connects to the ‘cy’ position so that decimal point DP2 is selected. This allows the display to read from 0.00 to 18.00 volts (approx.) with 10mV resolution. However, if the voltage at point D rises above 18V, the output of IC3d switches low and so the A, B & C inputs of IC5 also go low. Pole ‘b’ now connects to the ‘bx’ position and pole ‘c’ to the ‘cx’ position, so that the 2V range and decimal point DP1 are now selected. The display can now read from 18.0 to 40.0 volts with 100mV resolution (note: the most significant digit is not used in this mode). Because Schmitt trigger IC3d operates with about 3V of hysteresis (as set by the 100kΩ feedback resistor), the voltage at point D must now drop below about 15V before pin 13 switch­ es high again to select the 200mV range on the DVM-02. The voltage at point D must then be increased above 18V again to select the 2V range. This small amount of hysteresis prevents display jitter at settings close to the range changeover point. That completes the circuit description. Next month, we will describe the SC construction. January 1994  23