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Battery discharge pacer for electric vehicles I Based on a handful of low-cost ICs, this project can indicate the percentage amperehour capacity used or remaining in a rechargeable battery. Alternatively, it can be used as a "fuel" pacer to obtain maximum performance from an electric racing car. By DIETER KUENNE A common problem with rechargeable batteries is determining how much of their charge capacity has been used. This project is designed to give you that sort of information and can be wired to give readings in one of three modes. Of course, it does require careful calibration to match the battery being used but we'll give you 24 SILICON CHIP all those details later. The circuit to be described is similar to one installed in a battery-powered racing car called the "Rocket", developed by Ian Sims of Ferntree Gully, Victoria. A flat battery is a common problem with electric racing cars, due to over-zealous use of the "throttle". However, with the Battery Dis- charge Pacer on board, the state of charge can be monitored to obtain the best performance while conserving battery charge in order to finish the race. In fact, the first time that the Pacer was used in the "Rocket", it won the race. As used in the "Rocket", the unit operates in "Pacer" mode. 1!1 this mode, the unit integrates the current drawn from the battery and compares this value with the integral of the energy (average discharge current) that should have been used to that time. The difference between these two values is shown on the IPeter, which is set to give a centre-zero reading. If the resultant is zero, then you will get the maximum performance from your available energy source. A deflection to the right indicates surplus energy, while a deflection to the left indicates that the discharge is faster than the desired rate. Thus, depending on how the unit is calibrated, the "Pacer" mode can be used to optimise vehicle range or to Although shown here with a small sealed lead acid battery, the Discharge Pacer can be used to monitor virtually any rechargeable battery. The unit can be wired to show percentage charge used or charge remaining, or it can be used as a "fuel" pacer for an electric car. Power for the unit is supplied from the battery being monitored. obtain maximum speed over a given distance. The other two operational modes are similar to each other and indicate the degree of battery discharge. You can wire the unit to show either the percentage ampere-hour capacity used or the percentage ampere-hour capacity remaining (ie, the unit operates just like a fuel gauge). As well as monitoring conventional 12V car batteries , you can also use the unit to monitor nicad battery packs. In fact, you can calibrate the unit to monitor virtually any rechargeable battery. Note that the unit always assumes that you are starting off with a fullycharged battery. Note also that the unit cannot be used in reverse; ie, it cannot be used as a charging indicator. Presentation The circuitry for the Battery Discharge Pacer is housed in a small plastic instrument case. In addition to the meter, there are just two switches on the front panel. One switch powers up the device, while the second resets the reading to zero. In use, the unit can be powered from the battery being monitored. Because it draws only 11.SmA, it will have negligible effect on the ampere hour capacity of a car battery and only a small effect on a large nicad battery pack. Block diagram Fig. 1 shows the block diagram of the Battery Discharge Pacer circuit and shows how it is connected to a load consisting of a motor and its associated controller. However, you can use the circuit to monitor a battery driving virtually any kind of load; the principle is exactly the same. As can be seen from Fig.1 , the battery supplies power to both the motor via its controller and to the circuit via S1. The power supply circuit regulates the battery supply to +5V and also generates a -5V rail for the circuit. If the battery voltage is above 30V, the input for the circuit power supply must be derived from a battery tapping lower than 30V. In most cases , this simply involves tapping into a nominal 12V or 24V point above the ground reference. As shown on Fig.1, a shunt resistor is added in series with the motor (or load). At switch on, current flows through the motor and also through the shunt resistor which produces a voltage proportional to that current. This voltage is then amplified and filtered to prevent noise upsetting the circuit. The following stage consists of VCO1 which is a voltage controlled oscillator. Its output frequency (F1) is determined by the voltage applied to it by the amplifier/filter stage. The output from the VCO is then applied either to the count-up input or countdown input of an UP/DOWN counter via an input selector. The counter outputs are in turn applied to an 8-bit digital-to-analog converter (DAC) which produces a voltage that 's proportional to the digital count applied to it. This analog voltage is then amplified and used to drive the meter which has its negative terminal connected to VREFZ. This voltage reference allows the meter to deflect fully left for a count of hex 00, fully right for a count of FF and to mid-scale for a count of 80. Initially, at power up, the counter is preset to either hexadecimal 00 for the "Charge Used" mode, FF for the "Charge Remaining" mode or 80 for the "Pacer" mode. In addition, VCO1 's output is connected to the UP input of ]ULY1991 25 AD- MOTOR AMPLIFIER MOTOR CONTROLLER VC01 SHUNT UP/DOWN INPUT SELECTOR VREF1 +1.2V + BATTERY VC02 UP/DOWN COUNTER F2 OV AMPLIF_IER '!' I I S1 OAC ..L. i CIRCUIT POWER SUPPLY -sv VAEF2 .,. .,. Fig.I: the block diagram of the Battery Discharge Pacer. In operation, the circuit monitors the voltage developed across a shunt resistor in series with the load & uses this voltage to control the frequency ofVCOI. This VCO then drives a counter, the output of which is fed to a digital-to-analog converter (DAC) to derive a voltage that's proportional to the digital count. The DAC then drives a meter movement to show either the charge remaining or the charge used. In the "Pacer" mode, VCOI drives the DOWN input of the counter, while VC02 (which operates at a fixed frequency) drives the UP input. the counter for the "Charge Used" mode and to the DOWN input for the "Ch arge Remaining" and "Pacer" modes. It is now simply a matter of counting the pulses from VCOl to obtain either the "Charge Used" or "Charge Remaining''. Pacer mode For the Pacer mode, it is necessary to subtract the A.h (ampere-hour) value that should have been used up to a certain point from the value actually used. This is achieved by using a second voltage controlled oscillator, VCOZ, to drive the UP/DOWN counter. Its output frequency is set to a fixed value by the voltage app lied to it from VREFl. In operation, VCOl drives the DOWN input of the counter while VCOZ drives the UP input. The frequency of VCOl is then adjusted so that Fl equals F2 at the discharge rate required to just flatten the battery at the desired time. This is indicated by a zero reading on the centre-zero reading meter. However, if the discharge rate is greater than the required rate, Fl will be greater than FZ and so the counter will count down and the meter will deflect to the left. Conversely, if the discharge rate is too low, the counter 26 SILICON CHIP will count up and the meter will deflect to the right. Circuit details Refer now to Fig. 2 which shows the circuit details. Before getting down to the nitty-gritty, let's quickly relate the various sections to the block diagram. The two VCO circuits are easy to spot and cons ist of op amps IC2d & IC2c (VCOl) and IC2a & IC2b (VCOZ) . Of the remaining sections, IClc is the amplifier/filter section; ICs 5-7 the UP/DOWN counter; IC8 the DAC; D5 & D6, VREFl; and ICla, VREF2. As already mentioned, power is derived from the battery being monitored and this is applied to the circuit via a lOQ resistor and 30V zener diode (ZDl). ZDl is there to clip voltage transients or noise spikes (eg, from the motor) to prevent damage to the following circuitry. The supply rail is then fed via Sl to REGl which is an LP2950CZ-5 3-terminal regulator. This particular regulator was chosen for a number ofreasons but mainly for its low quiescent current. Typically, it can supply lOOmA while drawing a quiescent current of just 75µA . The output is also very accurate at 5V ±50mV and it can remain in regulation with an input voltage that's only 138mV greater .than the output voltage. In addition , the temp erature coefficient of the output voltage is just 15Dppm/°C, which m eans that the regulator can be used as a voltage reference. A standard 3-terminal regulator (such as the 7805) should not be substituted for REGl. Its output is nowhere near as accurate and it would introduce an extra l0mA of current drain . The negative supply rail for the circuit is obtained using an LMC7660 Switched Capacitor Voltage Converter (IC9). Fig.3 shows the internal workings of the LMC7660. It contains four CMOS switches which are shown here as Sl, SZ, S3 and S4. Sl and S3 operate together, while SZ and S4 operate together. When Sl and S3 are closed, SZ and S4 are open and when Sl and S3 are open, SZ and S4 are closed. Now lets see how it works. When Sl and S3 are closed, Cl charges to the V + supply vo ltage which in our case is +5V. Sl and S3 are then opened and SZ and S4 are closed. The positive side of Cl is now connected to ground and the opposite Fig.2 (right): the main circuit contains all the elements shown in the block diagram (Fig.I). The two VCO circuits are easy to spot and consist of op amps IC2d & IC2c (VCOI) and IC2a & IC2b (VC02). Of the remaining sections, ICic is the amplifier/filter section; ICs 5-7 the UP/DOWN counter; IC8 the DAC; D5 & D6, VREFI; and ICia, VREF2. N '-I -< ..... (Cl co ..... C: ---... 1 ~-' +5V A e VREF1 0 LK3 , , LK6 11 01! 16 10 C jB 021 IC5 40193 ~ 131 CARRY 12' oowN eoRRO 15,A 4 s u~OAO LK2 LK1 -1 . 7 2x1N414~ ~ - 12 0.11 I 8 . . 00 7 OC 6 "~ QA UPi OOWN COUNTERS 910 1olc 11 IC6 40193 ) oowN eonnowl 13 16 I •5V - 5V 220k 220k ~ ·:-~ ·-- I~ I - 5V OlT ') 1 VC02 -i 12k I I - 5V 390k "Vf' J'~ I CARRYl12 15,A 4 I i 5 u~oAo J. ,5V 0.1 -II- !- VR1 ,h 330kl BATTERY DISCHARGE PACER 16VWI 1 1N~i4BJ; LKs ' ' LK4 RWTll B2ki 06 1N4 14B +5V 1 ~ BATTERY : T MOTOR +5V 01 !.!!. .,. A -2.lo 1QJC, 11.e ,sl 1 VC01 IC7 40193 .,. QC 6 QB 2 Q 3 :l ·,loow I IUP ·,1 LOA 1~ 03 1N4148 0.1 47k l 471 VR(Fl 100k 88 81 82 83 -5V 9 · 85 10 86 11 87 12 5 6 7 B 84 ~ 'fj v- ICB OACDBOO VLCl1 011 82k or .,. 10 16VW 10 35VWI ~ q 0.1 09 1N4148'f 08 1N4148 4.7k 4. 7k IC9 LMC7660 ,- 5V GNO REG 1 POWER LP2950CZ5 • ,u..---,0UT S1~ ~ , - - - - - - • 5V I N ~.OUT VIEWED FROM BELOW 5-30V(MAX) 1:0!? +5V---<I>------, I" • 2.2k 1mA ..W, 27k 100!1 _J. • 1 2V -5 V -• 5V PARTS LIST 1 PC board, code SC11108911, 123 x 135mm 1 front panel label, 140 x 55mm 1 meter scale label, 51 x 40mm 1 plastic instrument case, 154 x 65 x 158mm 1 MU45 1mA meter 1 SPDT toggle switch 1 push-on momentary switch 1 5mm ID grommet 1 300mm-length twin hookup wire 1 500mm-length 0.8mm tinned copper wire (for links) 10 PC stakes 4 self-tapping screws 1 100kQ miniature horizontal trimpot (VR1) 1 10kQ miniature horizontal trimpot (VR2) 1 1kQ trimpot (required for testing only) Semiconductors 2 LM324 quad op amps (IC1 ,IC2) 1 4066 quad CMOS analog switch (IC3) 1 4011 quad NANO gate (IC4) 3 40193 4-bit binary up/down counters (IC5,IC6,IC7) 1 DAC0800 digital to analog converter {IC8) 1 LMC7660 negative voltage generator (IC9) 1 LP2950CZ5.0V 5V regulator (REG1) 1 30V 1W zener diode (ZD1) 91N4148, 1N914 switching diodes (D1 -D9) Capacitors 1 10µF 35VW PC electrolytic 2 10µF 16VW PC electrolytic 2 1µF 16VW PC electrolytic 8 0.1 µF metallised polyester 3 .01 µF metallised polyester Resistors (0.25W, 1%) 1 390kQ 1 27kQ 1 330kQ 1 22kQ (for testing) 10 220kQ 1 12kQ 8 100kQ 5 4.7kQ 3 82kQ 1 2.2kQ 2 47kQ 1 100Q 1 36kQ 1 10Q side of Cl, which is now at V- or -5V, connected to CZ via S4. After a few cycles of this process, CZ charges to V- to provide the -5V rail. In practice, an internal oscillator which normally operates at about 10kHz provides a clock signal to drive Sl and S3. At the same time, an inverted version of this signal is used to drive SZ and S4 so that the two pairs of CMOS switches operate 180° out of phase. Shunt input When the motor is running (or power is applied to the load), the voltage developed across shunt resistor RSHUNT is proportional to the current. This voltage is fed to a low pass RC filter (100kQ and 0.lµF) and limited to ±600mVby clipping diodes Dl and DZ before being fed to pin 10 of op amp stage IClc. Normally, however, the voltage across the shunt is less than zoom V. VRl, its series 330kQ resistor and the lOOkQ input resistor form a voltage divider which allows adjustment of the voltage applied to pin 10 of IClc from the shunt resistor. IClc has a gain of about 34, while the 0. lµF capacitor across its feedback resistor rolls off the response above 4Hz. The amplified and filtered signal from IClc is now fed to voltage controlled oscillator VCO1 (ICZd, ICZc & IC3a). ICZd operates as an integrator by virtue of the 0.lµF capacitor connected between its output (pin 14) and the inverting input at pin 13 . When the output ofIClc goes positive with respect to ground, pin 13 ofICZd will be more positive than pin 12 due to the voltage divider (2 x ZZ0kQ) at the non-inverting input and so pin 14 of ICZd will swing towards -5V. The 0. lµF capacitor on pin 14 now charges towards the negative supply rail via the series ZZ0kQ resistors on v+ = 5V S2 8 I 3 .,. I c1 - I I I 28 SILICON CHIP S4 I 4 CLOCK SIGNAL I I I 531 Miscellaneous Hookup wire, resistor or enamelled copper wire for motor shunt (see text). pin 13 and the resulting signal fed to the inverting input (pin 9) of Schmitt trigger stage ICZc. When ICZd 's output voltage reaches the negative threshold of ICZc, pin 8 of ICZc switches high and closes CMOS switch IC3a. IC3a now connects a 220kQ resistor to ground and this in turn pulls the inverting input ofICZd below the noninverting input. As a result, ICZd's output now swings high and the 0. lµF capacitor charges towards the positive supply rail. When the output of ICZd reaches the positive threshold of Schmitt trigger ICZc, pin 8 of ICZc goes low again and IC3a opens . ICZd now begins charging the 0. lµF capacitor towards the negative supply rail and so the process is repeated indefinitely. Thus, we have an oscillator which increases in frequency as the control voltage at the output ofIClc increases. VCOZ operates in exactly the same manner as VCOl. It consists of ICZa, ICZb and CMOS analog switch IC3b. In this case, however, the control voltage is fixed at 1.2V (VREF1) by two forward biased diodes, D5 & D6. When link LK1 is in place, VCOZ is enabled and the circuit operates in "Pacer" mode. When LKZ is in position, VCOZ is disabled and the circuit op erates in "discharge" mode. Note that the outputs of Schmitt triggers ICZc and ICZb both swing between -5V and +5V. In each case, this is converted to a 0V to +5V swing by a voltage divider consisting of two l00kQ resistors connected in series to the +5V rail. The outputs of the voltage dividers in turn drive NAND gates IC4b and IC4c. These two NAND gates simply buffer and invert the Schmitt trigger outputs. Thus, when ICZc's output switches high , pin 4 of IC4b sw itches low and pin 3 of IC4a remains high. INVERTER c2 I: 5 o,-.....--ovour = -v+ = -5V Fig.3: how the LMC7660 negative voltage generator IC works. It use an internal oscillator to drive two pairs if switches 180° out of phase so that C2 charges to -5V. CHARGE USED MODE CHARGE REMAINING MODE Fig.4: before mounting the parts on the PC board, decide on the mode you wish to use & install thl),necessary solder brides on the copper side of the board as shown here. Make sure that you install the bridges exactly as shown & that you don't short out adjacent tracks. Once the bridgl)s are in place, you can attend to the linking options on the component side of the board (see text & Fig.5). Conversely, when IC2c's output goes low, pin 4 of IC4b switches high and pulls pin 2 of IC4a high. At the same time, pin 1 ofIC4a is pulled high via a .0lµF capacitor and so IC4a's output (pin 3) goes low. The .0lµF capacitor on pins 1 and 2 of IC4a now charges via its associated 82kQ resistor. When the voltage on pin 1 falls below the lower threshold of the NAND gate input, the output of IC4a goes high again. Diode D4 prevents a large negative voltage from appearing on pin 1 ofIC4a when IC4b's output subsequently switches low again. Thus, IC4a provides a brief (0.8ms) negative-going pulse each time pin 8 of IC2c goes low. Similarly, IC4d provides a brief negative-going pulse each time pin 7 of Schmitt trigger ICZb goes low. Counters NAND gates IC4a and IC4d drive the UP & DOWN clock inputs of binary counter IC5, either via links LK3 & LK4 or links LK5 & LK6. LK3 & LK4 are used for the "Pacer" and "Charge Remaining" modes, while LK5 & LK6 are used for "Charge Used" mode. IC5 is a presettable UP/DOWN binary counter. This means that it can initially be set to a particular count under the control of the load input at pin 11. The preload count inputs are at pins 15, 1, 10 & 9 (A, B, C & D) and these are linked either to +5V or ground to obtain the necessary preload count. IC6 and IC7 are also binary presettable UP/DOWN counters and are connected in cascaded mode to IC5. Note that the CARRY output of IC5 is connected to the UP input of IC6, and the CARRY output of IC6 is connected to the UP input of IC7. Similarly the BORROW outputs of IC5 and IC6 connect to the DOWN inputs of the following stages. This configuration allows the three counters to operate together as a 12bit UP/DOWN counter. However, only the most significant eight bits from the counter are connected to Digital to Analog Converter IC8 (ie, IC5 operates only as a divide-by-16 stage). The LOAD inputs of IC5, IC6 & IC7 are all tied together so that the counters are simultaneously preloaded with their required counts. Initially, when power is first applied, the LOAD inputs are all pulled low via the lµF capacitor across SZ, and the counts at the preload inputs are loaded into the counters. The 1µF capacitor then charges via an 82kQ resistor, at which point preloading ceases and the counters are ready to begin counting. Alternatively, RESET switch SZ can initiate preloading at any time (ie, reset the counters) by simply discharging the 1µF capacitor. Digital-to analog converter IC8 is an 8-bit digital-to-analog converter (DAC) which has differential current outputs (!OUT & !OUT-bar) at pins 2 & 4. It is a relatively easy DAC to connect up. The inputs are at PACER MODE B1-B8 (pins 5-12) and these control the output of the DAC. In addition, the DAC requires two current reference inputs, one at pin 14 (VREF+) and the other at pin 15 (VREF-). There are many ways to configure the reference inputs and this circuit uses a positive current reference derived via a 4. 7kQ resistor from VREF1 (1.2V), while the VREF- input is connected to ground via a second 4. 7kQ resistor. Diodes D8 and D9 at the VLC terminal (pin 1) set the IC for CMOS input levels (connecting this pin directly to ground sets the IC for TTL input levels). Compensation for the DAC is provided by the .0lµF capacitor from pin 16 to the -5V supply. The differential current outputs . from IC8 at pins 2 & 4 are converted to a voltage output using differential amplifier IClb. The output of this op amp varies from -1.ZV if all zeros (lows) are applied to the B1 -B8 inputs to+ 1.2V if all ones (highs) are applied to the B1 -B8 inputs. When the inputs are all zeros except for a one at the most significant input (Bl), IClb's output is at ground (0V). For example, let's say that we want the unit to operate in "Pacer" mode; ie, with the meter starting off at centre zero. In this case, we simply preload 0000 into counters IC5 & IC6 and 1000 into counter IC7. (ie, a count of 800 hex is loaded). Similarly, if we want the unit to operate in "Charge Used" mode, 0000 is loaded into all counters (ie, 000 hex) to get -1.ZV at the output ofIClb. Finally, for the "Charge Remaining" mode, a count of 1111 r'nust be preloaded into each counter (ie, FFF hex). IClb drives the positive terminal of ]ULY 1991 29 +5-30V GND VRSHUNT Fig.5: before mounting any of the parts, install the mode select links as described in the text, depending on which mode you wish to use. The remaining parts can then be installed but don't mount RSHUNT until after the calibration procedure. For low-current applications, RSHUNT can be a standard resistor while for heavy current applications, it should be made up from a length of tinned copper wire (see table) & mounted off the board adjacent to the load. the 1mA meter via series 100Q and 2.2kQ resistors. The negative terminal of the meter is held at -1.2V by VREF2 (IC1a) so that we get a centrezero reading when IC1b's output is at ov. IC1a is simply a buffer ampl,ifier which has an output equal to the voltage on its non-inverting input at pin 3. VR2 and the 36kQ and lO0kQ resistors set this voltage to -1 .2V, while the 27kQ resistor equalises the source resistance on the inverting input with that on the non-inverting input to minimise output voltage drift. Construction Most of the parts for the Battery Discharge Pacer are mounted on a PC board coded SCl 1108911 and measuring 123 x 135mm. Before starting construction, check your PC board carefully against the published pattern. It will be much easier to locate and repair any board defects at this stage. 30 SILICON CHIP At this stage, you also have to decide on the mode of operation you require; ie, "Charge Used", "Charge Remaining", or "Pacer". Each option requires different linking arrangements on the PC board, to connect the VCOs to counter IC5 and to set the preload values for the counters. Fig.4 shows the preload connections for each of the three options. They are implemented by installing solder bridges across the copper tracks in the positions indicated. Fig.5 shows the linking options for the VCOs. Here's what to do for each mode: Charge used mode: this mode uses only VCO1. Install solder bridges to preload 0000 into all counters as shown in Fig.4. Install link LK2 (to disable VCO2) and links LK5 & LK6, as shown in Fig.5. Charge remaining mode: similar to "charge used" mode. Install solder bridges to preload 1111 into all counters (Fig.4) and install links LK2, LK3 and LK4. Pacer mode: install solder bridges as shown on Fig.4, then install links LK1, LK3 & LK4 as shown on Fig.5. Once the programming links and preload bridges have been installed, the remaining parts can be mounted on the PC board. Do not place anything in the RSHUNT position at this stage, since this value must be calculated to suit your particular application. · The order of parts assembly on the PC board is unimportant but make sure that all polarised parts are correctly oriented. These parts include the ICs, the diodes and the electrolytic capacitors. Install PC stakes at all external wiring points. Final assembly The completed PC board assembly can now be installed inside the case, along _with the front panel switches and the meter. Begin by drilling the mounting holes for the two switches, then mark out the mounting holes for the meter using the drilling template supplied. Position the meter so that it's right in the centre of the panel. This done, drill numerous small holes around the inside circumference of the large meter clearance hole and knock out the centre piece. Clean up the hole with a file, then drill the four screw-mounting holes using the correct sized drill. The front panel label can now be stuck to the front panel and the holes cut out using a sharp utility knife and reamer. It's up to you as to whether or not you replace the meter scale with a new artwork. To remove the old scale, first unclip the plastic cover, then carefully undo the two meter scale screws. The new meter scale can then be installed and the cover clipped back into place. Note that the scale published here is suitable for both the "charge remaining" and "charge used" modes. For the "Pacer" mode, only a centrezero mark is required on the meter scale. The meter and the switches can now be installed on the front panel and the rear panel drilled to take a single rubber grommet. This done, secure the PC board to the integral standoffs using self-tapping screws and complete the wiring as shown in Fig.5 . Testing The first step in testing the unit is to apply power (6-30V DC) and check the voltages at the supply pins of the ICs. ICl and ICZ should have +5V on pin 4 and -5V on pin 11; IC3 should have +5V on pin 14 and -5V on pin 7; IC4 should have +5V on pin 14 and OV on pin 7; ICs 5, 6 and 7 should have +5V on pin 16 and OV on pin 8; ICB should have +5V on pin 13 and . ;- -- 7 ! - -- - - : " 7 ' ~ . µ , - - - - - - W ] - - - ,I; ur- _r , ,. < Take care with the ICs when you are installing them on the board since they don't all face in the same direction. In particular, note that ICl & IC2 face in the opposite direction to IC3 & IC4 (see Fig.5). The board is secured to integral standoffs on the bottom of the case using self-tapping screws. RESISTOR COLOUR CODES 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 No. Value 4-Band Code (5%) 5-Band Code (1 %) 1 1 10 8 3 2 390kQ 330kQ 220kQ 100kQ 82kQ 47kQ 36kQ 27kQ 22kQ 12kQ 4.?kQ 2.2kQ 100Q 10Q orange white yellow gold orange orange yellow gold red red yellow gold brown black yellow gold grey red orange gold yellow violet orange gold orange blue orange gold red violet orange gold red red orange gold brown red orange gold yellow violet red gold red red red gold brown black brown gold brown black black gold orange white black orange brown orange orange black orange brown red red black orange brown brown black black orange brown grey red black red brown yellow violet black red brown orange bll!e black red brown red violet black red brown red red black red brown brown red black red brown yellow violet black brown brown . red red black brown brown brown black black black brown brown black black gold brown 1 1 1 5 JULY 1991 31 Diameter (mm) 3.149 2.500 2.000 1.600 1.250 1.000 0.800 0.630 0.500 0.400 0.315 Current Rating (A) Resistance mQ/metre 15 7.5 4.5 2.9 2.3 1.5 1.1 0.7 0.3 0.2 0.1 2.212 3.512 5.488 8.575 14.05 21.95 34.30 55.31 87.81 137.2 221.2 -5V on pin 3; and IC9 should have +5V on pin 8, 0V on pin 3 and -5V on pin 5. If any of these voltages is incorrect, check the PC tracks for shorts or open circuits. If the -5V supply is not present, check the circuit around IC9. When power is first applied or when the Reset is pressed, counters IC5, 6 and 7 are preloaded as discussed previously. You can check this by measuring the voltage at pin 7 ofIClb. This voltage should be -1.ZV when the preload is 000 (charge used mode); 0V when the preload is 800 (pacer mode); and + 1.2V when the preload is FFF (charge remaining mode). Assuming everything checks out so far, the voltage on the meter negative terminal can be set to -1.ZV using VRZ. To do this, press the RESET switch and adjust VRZ so that the meter reads 0% when the preload is 000, 50% or mid-scale when the preload is 800 and 100% when the preload is FFF. creased and deflect to the right if the voltage is decreased. In the charge used mode, the meter should rise gradually when 52mV is applied to the VRSHUNT input, until eventually it reaches full scale on the meter and then falls to zero again. Similarly, in the "charge remaining" mode, the meter reading should gradually fall from full scale to zero reading and then jump to full scale again. Calibration The unit can now be checked for correct operation by applying a voltage of 52mV to the shunt input. This can easily be done using a 22kQ resistor and lkQ potentiometer. One end of the resistor is connected to the +5V supply and the other end to one side of the pot. The other side of the pot is connected to circuit ground, while the wiper is connected to the VRSHUNT input. It's now simply a matter of adjusting the trimpot to obtain 52mV at the shunt input (check this voltage with your multimeter). If you are set up in the "Pacer" mode, the meter should remain close to the centre reading but may have some drift to the left or right. Check that this reading can be reset with the RESET switch. Now check the effect on the meter when you increase or decrease the shunt voltage. It should gradually deflect to the left if the voltage is in- To calibrate the unit, set VRl to mid-scale, apply 52mV to the VRSHUNT input and observe the meter. Adjust this voltage until it takes exactly 1 hour for the meter to travel from 0 to 100% in the "charge used" mode or from 100% to 0% in the "charge remaining" mode. For the "Pacer" mode adjust the input voltage until the meter needle remains stationary. By the way, you don't have to spend an hour observing the meter before making each successive adjustment. For example, in the "Charge Used" or "Charge Remaining" modes, the meter needle should move by 20% over a 12-minute period. Use this shorter time interval for your initial adjustments, then use a longer interval to make sure that the shunt voltage is spot on. The shunt voltage should now be measured and recorded as the calibration voltage for your Battery Discharge Pacer. The shunt resistor The calibration voltage is now used to calculate the value of the shunt resistor (RSHUNT) required. For the "Charge Used" and "Charge Remaining" modes, RSHUNT is equal to the BATTERY:-, DISCHARGE PACER • • RESE~ 32 SILICON CHIP Fig.6: this full-size artwork can be used as a drilling template for the front panel. The meter is supplied with its own drilling template . 00 I I!.! ~ -0-0 ~ 0 Fig. 7: check your PC board against this full-size pattern & repair any defects before mounting the parts. calibration voltage divided by the A.h capacity of the battery. This means that if the battery is discharged at the same rate as its A.h capacity, the meter would travel over its full scale in one hour. The wattage rating is equal to the maximum discharge current squared divided by the resistance of the shunt. For the "Pacer" mode, RSHUNT is calculated by selecting an optimum discharge rate for the battery. This is the discharge which will provide the racing vehicle with just sufficient battery capacity to finish by the end of the race. Any more and the battery will go flat before the end of the race; any less means that the car could have been driven faster. The value ofRSHUNT for the "Pacer" mode can now be calculated by dividing the calibration voltage by the optimal discharge rate. The wattage rating of the shunt is equal to the maxi- mum discharge rate squared divided by the resistance of the shunt. Once the shunt value has been calculated, you can decide on what to use for the shunt. In low current applications, you can use a standard resistor (eg, 0.H2). This can be installed in the RSHUNT position on the f..~ .l CLASS-2.5 MU -45 Fig.8: if you are using an MU-45 meter, this full-size artwork can be used to replace the existing scale. PC board, as shown in Fig.5. For heavier current applications, a shunt made up using a short length of enamelled copper wire may be more practical. In this case, the shunt must be mounted outside the case (preferably adjacent to the load) so that the heavy current flows directly through the shunt to ground. The VRSHUNT input to the circuit is then simply a voltage sensing connection between ground and the top of the shunt. Table 1 gives details on standard gauge copper wires, their resistance in milliohms per metre and the nominal current rating. For higher current shunts , you will need to use copper bus bar (use manufacturers' data sheets for resistance and current rating details). Once you are set up with a suitable shunt, any final adjustments (if necessary) can be made using trimpot VRl. SC ]UL Y 1991 33