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We’ve covered the theory – now here’s how to build it! SPARK ENERGY METER Design by Dr Hugo Holden This meter closely estimates the energy delivered to actual sparks in the ignition system under test, either a CDI or MDI system. E arlier in this issue, we described the ideal way to measure the output of an ignition system: to load it with a bidirectional 1000V zener diode which approximates the actual voltage drop when a spark is established. Our meter actually uses a 1500V zener which gives similar results, for reasons explained below. The meter has two ranges which are selected automatically, zero to 100 millijoules or zero to 1000mJ and it can work with a spark repetition rate up to 700 sparks per second (corresponding to more than 10000 RPM in a V8 engine) or down to just 1Hz. It can measure uni-polar or bi-polar spark voltages. The meter is portable and battery-powered. It also has a low battery indicator. It can be connected to a working engine one spark plug at a time or alternatively, it can be used to bench test an ignition system. It works with single or double-ended ignition coils. Circuit description Fig.1 shows the complete circuit. The HT connection from the ignition system is applied to a spark plug which is a 5k resistor type, BR8HS. The plug’s earth part of the electrode is cut off and the plug is used as a feed through connector. The 5k resistor in the plug helps to limit and isolate very brief high current transients caused by the stray and siliconchip.com.au February 2015  57 +8.6V +5.4V 5k SPARK PLUG INPUT K A A ZD1 A A K 5k HV K ZD29 ZD2 K K ~ K ZD30 D5 D4 K K D2 D1 A K 150 5W 8.2k 2 3 9.1M 100k 3 8 1 IC1a K D6 A ~ + 1 2 100nF 630V + A CALIBRATION TERMINALS 270k A D3 – A CASE 240k A 47nF Ctc 4 14 –As Vdd Rtc RCtc IC2 4047B 100nF 9 MR Osc Q +T Q +As –T Vss Retrig 5 7 12 6 13 D8 10 11 K A D7 K A 91k 20k 47 10nF 100V 1nF 47 - 100V CUR +8.6V POWER REG1 78L05 IN S1 K 10F 16V A K D16 1N5819 A 20k 13 10F 12V 1W A 9 5 6 62k 220nF K 10 33k 68k 100nF 150k 100k 14 1M 16V D9 IC1d 100nF 16V 510k 12 ZD31 100F IC1: LMC6484 +5.4V GND BATTERY 9V ALK. SC OUT IC1b 7 8 A 11 POWER 20k 510k 4 IC1c  LED1 K 100k 1.5k SPARK ENERGY METER                       Fig.1: full circuit of the Spark Energy Meter. ZD1-ZD30 are the 1.5kV dummy load. The resulting voltage is rectified by bridge D1-D4 and passes through a 1505W shunt resistor. The output is is integrated by IC3b while a sample & hold buffer comprising IC5b-IC5d and IC3c provide a steady signal for the LCD meter. Q1 discharges the hold capacitor if the spark train ceases while IC3d and IC4a switch the unit to a higher range for more energetic sparks. IC1b-IC1d monitor the battery voltage and flash LED1 if it’s low. 2015 distributed capacitance of the ignition coil, distributor and the wiring. The high voltage signal from the plug is fed to a string of 30 100V 5W zener diodes, wired to create a high-voltage, high-power bidirectional 1500V zener diode. The reason an effective or equivalent spark sustaining voltage of 1500V was chosen rather than 1000V is so that signal processing of the “Dwell Arte- fact” is avoided when testing ignition coils directly. Also it accounts for the 500V spark voltage drop in the distributor in a conventional ignition system and in fact, the spark energy delivered at 1500V is similar to that at 1000V in any case. After passing through the bidirectional zener diode assembly, the signal is fed to a bridge rectifier (diodes D1 to D4) with a 100nF capacitor across it, to suppress short-term variations in voltage. Its output goes to a 150 5W current-sense resistor shunted by a 10nF capacitor to provide further filtering. Neither capacitor significantly affects the signal waveform or the signal’s integrated value. The voltage across the 150 5W resistor is proportional to the spark current. The top end of this resistor is Specifications Range: ..................................... 0-100mJ (low range), 0-1000mJ (high range, automatic switching) Input: ........................................ standard spark plug connection with separate earthing lead Measurement Linearity: ........... ~4% Power supply: ........................... 9V alkaline battery (internal), ~17mA drain Low voltage indication: ............. power LED flashes below ~7.2V Calibration: ............................... onboard display zeroing and scale adjustment.   (Scale is set accurately using a calibrator board, described below.) 58  Silicon Chip siliconchip.com.au +8.6V +8.6V +5.4V +5.4V 100nF 11 IC3: LMC6484 2 D 1 IC3a Q IC4b 9 3 S CLK Q Vss R 10 7 HIGH 6 12 IC4: 4013B 4 S D13 Q R CLK 3 Q Q2 2N7000 G D S 2 G K Q3 2N7000 S D14 5.1k A A 1nF D 1 D 5 A  K IC4a K 10k 14 Vdd RLY1 D15 A LED2 14 IC3d 12 13 100nF 4 13 8 CLK K 1k 33F +5.4V +8.6V DRV RLY1 IC5b 180k IC5d 5 3 4 IC5c 12 10 11 6 14 Vcc Vss 7 9 100nF 20k 5 K D11 D10 K 11 G A D12 5.1M K 1F S 8 METER ZERO +5.4V 10M Q1 2N7000 11 7 10M D 7 A IC3c VR1 1M IC5: 4066B IC3b 8 100k 6 A 10k 10 8 9 +5.4V 470k 6 5 10k 1F 1F 9 10 470k 1 V+ DP REL INHI LCD METER 1.8.8.8 INLO COM RFH ROH V– 2 200 DISPLAY ZERO ZD1–ZD30: 1N5378BG A K D1–D4: UF4007 D13: 1N4004 ZD31: ! 2V, 1W A K connected to circuit ground via a 47 resistor while the negative end goes to the inverting input of op amp IC3b via an RC low-pass filter (47 & 1nF) and a series-connected pair of resistors (180k + 20k). IC3b operates as the integrator at the heart of this circuit. To measure the energy of the spark, we need to calculate the product of the voltage across the dummy load (fixed at 1500V) with the integral of load current over time. Another way to think of this integral is as the area under a curve plotting current against time. Luckily a simple op amp integrator performs this calculation for us. IC3b uses a 100nF integrator capacitor which is reset to 0V before each spark and charges at a rate proportional to spark current. The voltage across the 150 resistor is Ispk x siliconchip.com.au D16: 1N5819 A K D5–D12, D14: BAT46 D15: 1N4148 A K 150. Ignoring the 180k series resistor (which is initially shorted out by reed relay RLY1), the combination of a 20k resistor and 100nF capacitor gives an output at pin 7 of Ispk x 150÷ (20k x 100nF) = 75000V/A.s or 75V/ mA.s. Given the constant 1500V load voltage, this is equivalent to 50V/J (75000V / 1500V, 1J = 1V.A.s). Thus, the maximum output we can expect from rail-to-rail op amp IC3b running from a 9V battery is around 5V, representing 100mJ. To take higher readings, RLY1 switches off (as explained later) and this increases the source resistance of IC3b from 20k to 200k, reducing its sensitivity to 5V/J and thus readings up to 1J are possible. Note that because the shunt voltage is applied to a bridge rectifier before being fed to IC3b, both positive and 2N7000 78L05 LEDS GND K A IN OUT D G S negative spark voltages contribute to the reading. Sample and hold Because the spark duration is quite short but we want a steady reading on the display, the circuit incorporates sample and hold. The energy of every second spark is measured and once the reading is complete, it is “latched” in the hold buffer as soon as the next spark is detected, resulting in a steady reading on the LCD panel meter (assuming the spark energy is relatively consistent). Op amp stage IC1a is used to detect the start of each spark. Its non-inverting input, pin 3, has a reference voltage of 1.35V applied, generated by the 270k/91k resistive divider across the 5.4V regulated supply rail. The inverting input, pin 2 normally sits at February 2015  59 Just a little smaller than life-size, this inside shot shows how the PCB fits inside the diecast case, with the display mounted on the lid At left, just in view, is the base of the spark plug used as a termination point, along with the earth connection and double lug. Construction details will be provided in the second part of this project, next month. around 1.6V due to the 240k/100k divider between the 5.4V rail and the bottom of the sense resistor, which is at ground potential between sparks. When a spark occurs, once the cur- rent rises above about 3mA, this causes a voltage of 0.45V across the sense resistor and thus the voltage at pin 2 of IC1a drops below 1.3V, causing the output of IC1a to swing high. The 9.1M feedback resistor provides a small amount of hysteresis to prevent output oscillation. IC1a then triggers monostable IC2 which produces a 1ms output pulse at Q (pin 10). These two signals, from IC1a and IC2, are “ORed” by diodes D7 and D8 in combination with the 20k pull-down resistor. The purpose of IC2 is to ensure that the minimum pulse length fed to IC3a is 1ms. If the spark duration is longer then the output of IC1a will still be high while the output of IC2 is low but if the spark is less than 1ms, IC2 keeps the trigger signal high for that minimum period. This trigger signal then goes to flipflop IC4b, inverting the state of its Q and Q-bar outputs (pins 13 and 12) at the start of each spark pulse. When the Q output goes high, this turns on CMOS switch IC5b which discharges Mounted underneath the main PCB is the input PCB, as shown here. This board contains the thirty 100V, 5W zener diodes, which are all connected in series but half are connected in reverse polarity to the rest. A spark plug provides the input feedthrough connection. 60  Silicon Chip siliconchip.com.au Spark Energy Meter: Parts List 1 double-sided PCB, code 05102151, 110.5 x 85mm 1 double-sided PCB, code 05102152, 110.5 x 85mm 1 front panel label 109 x 84mm 1 diecast box 119 x 94 x 57mm (Jaycar HB-5064 or equivalent) 1 LCD panel meter (Jaycar QP5570 or equivalent) 1 5V reed relay (Jaycar SY-4036 or equivalent) (RELAY1) 1 SPDT PCB mount toggle switch (Altronics S1421 or equivalent) (S1) 1 resistive spark plug 14mm thread and preferably 12.7mm reach or similar (BR8HS) 1 9V U clip battery holder (Jaycar PH-9237, Altronics S 5050) 1 9V battery snap and lead 1 9V alkaline battery 1 TOP-3 silicone washer 2 6-way polarised headers with 2.54mm spacings (Jaycar HM3406 or equivalent) 2 6-way header plugs with 2.54mm spacings (Jaycar HM-3416 or equivalent) 8 stick-on rubber feet 1 alligator clip (Jaycar HM-3025 or equivalent) 1 M4 x 10mm screw 1 M4 nut 1 4mm star washer 1 crimp eyelet (1mm diameter cable entry) 1 6.3mm chassis spade connector 1 6.3mm crimp female spade connector (1mm diameter cable entry) 1 M3 x 6mm countersunk screw 1 M3 nut 4 M3 x 12mm countersunk screws 8 M3 tapped Nylon spacers 4 M3 x 5mm machine screws 1 100mm length of 9-way rainbow cable the integrator capacitor, thus resetting it. When the next spark occurs, the Q output goes low, releasing this reset and at the same time, Q-bar goes high, switching on IC5c which allows the output of IC3b (the integrator) to charge the 1F capacitor at the input of buffer IC3c. However, note that CMOS switch IC5d also must be enabled for this siliconchip.com.au 1 200mm length of 7.5A mainsrated cable 1 1m length of 7.5A green or black mains rated cable 1 200mm length of 4mm diameter heatshrink tubing 1 M205 fuse clip 2 PC stakes 1 1MΩ horizontal trimpot (VR1) Semiconductors 2 LMC6484AIN quad CMOS op amps (IC1, IC3) 1 4047B monostable/astable multivibrator (IC2) 1 4013B dual D flipflop (IC4) 1 4066B quad CMOS switch (IC5) 1 78L05 low power 5V regulator 3 2N7000 N channel FETs (Q1-Q3) 30 1N5378BG 100V 5W zener diodes (ZD1-ZD30) 1 12V 1W zener diode (ZD31) 4 UF4007 1A 1000V fast diodes (D1-D4) 9 BAT46 Schottky diodes (D5-D12, D14) 1 1N4004 1A diode (D13) 1 1N4148 switching diode (D15) 1 1N5819 1A Schottky diode (D16) 2 3mm LEDs (LED1,LED2) Capacitors 1 100F 16V electrolytic 1 33F 16V electrolytic 2 10F 16V electrolytic 3 1F MKT 1 220nF MKT 6 100nF MKT 1 100nF 630V polyester (greencap) 1 47nF MKT 1 10nF 630V polyester (greencap) or 3kV ceramic 1 1nF 1kV ceramic (Altronics R2889) 1 1nF MKT capacitor to charge and that is driven by op amp stage IC3a, configured as an inverter to invert the pulses from IC2. Hence, the sample-and-hold buffer only samples the output of the integrator after the spark duration and thus the integration of the spark current has been completed. The 100k resistor from the output of buffer IC3c to pin 9 of IC5c prevents leakage current through IC5c from Resistors (0.25W, 1%) 2 10MΩ 1 68kΩ 1 9.1MΩ 1 62kΩ 1 5.1MΩ 1 33kΩ 1 1MΩ 4 20kΩ 2 510kΩ 3 10kΩ 2 470kΩ 1 8.2kΩ 1 270kΩ 1 5.1kΩ 1 240kΩ 1 1.5kΩ 1 180kΩ 1 1kΩ 1 150kΩ 1 200Ω 4 100kΩ 1 150Ω 5W 1 91kΩ 2 47Ω Parts List For Calibrator 1 PCB, code 05101153, 47 x 61mm 2 2-way screw terminals with 5.08mm spacings 1 25mm length of 0.7mm tinned copper wire 3 PC stakes 1 100Ω horizontal trimpot (VR1) 1 50kΩ horizontal trimpot (VR2) Semiconductors 1 7555 CMOS timer (IC1) 1 LM317T adjustable 3-terminal regulator (REG1) 1 IRF540 N-channel Mosfet (Q1) 1 BC337 NPN transistor (Q2) 1 BC327 PNP transistor (Q3) 2 1N4004 1A diodes (D2) Capacitors 1 100F 16V electrolytic 2 10F 16V electrolytic 1 100nF MKT 1 10nF MKT Resistors (0.25W, 1%) 1 220kΩ 1 100Ω 1 240Ω 1 10Ω Alternative PWM circuit 2 1N4148 diodes (D3,D4) 1 1kΩ resistor in place of 220kΩ 1 250kΩ horizontal trimpot (VR2) slowly discharging the 1F capacitor. The output of IC3c therefore is a steady voltage representing the last energy value computed by the integrator and this is fed to the LCD panel meter via a resistive divider network with VR1 providing a zeroing adjustment. The resistors chosen set the correct full scale reading for the meter, so that with 5V at the output of IC3c, it will read either 100.0 (at 100mJ full-scale February 2015  61 mode) or 1000 (at 1J full-scale mode). shouldn’t just hold the last reading forever. We want it to drop to zero so we realise that there are no more sparks being detected (and thus no energy being measured). This is achieved by Mosfet Q1 which discharges the 1F hold capacitor after a few seconds without any spark pulses. The Q-bar output of IC2 goes low for 1ms on every second spark detected, discharging the two 1F capacitors at Q1’s gate and thus keeping it off. However, if the sparks stop for long enough, these capacitors charge via the 5.1M resistor and thus Q1 switches on, zeroing the reading. Auto-ranging As we mentioned earlier, reed relay RLY1 is initially switched on to provide the more sensitive 100mJ full-scale reading. Op amp IC3d is wired to compare the output of sample-and-hold buffer IC3c’s output to the 5.4V rail. Thus once the reading goes above 108mJ, its output goes high, setting flipflop IC4a. IC4a is initially reset by the 33F capacitor and 5.1k resistor at its pin 4 input, with D13 discharging the capacitor at switch-off (this same signal also resets IC2 initially). With IC4a reset, its Q output at pin 1 is low and thus Q2 is off, so the highrange indicator LED (LED2) is also off. At the same time, the Q-bar output at pin 2 is high, so Q3 is switched on and this powers the coil of RLY1. When the output of IC3d goes high and the flip-flop is set, LED2 switches on and RLY1 switches off. The only way to return to the higher-sensitivity 100mJ scale mode is to switch the unit off and on again, resetting IC4a. Power supply The unit is powered from a single 9V alkaline battery. Reverse polarity protection is provided by Schottky diode D16 while power switch S1 turns the unit on and off. 78L05 regulator REG1 has a Schottky diode in its ground leg to “jack up” its output to 5.4V. This is to ensure that it’s always above the output of IC3c even with the meter at its maximum reading of 100mJ/1J, which corresponds to 5V. Op amp stages IC1b-IC1d provide a low battery warning which flashes power indicator LED1 if the battery voltage drops below 7.2V. IC1d is Display zeroing Ideally, when sparks are no longer being delivered to the unit, the display D1 1N4004 CON1 A 7–12V DC IN* REG1 LM317T K ADJ 100F + +5V 0V 100nF 7 *NOTE: FLOATING SUPPLY NEEDED FOR CALIBRATOR 5V ADJUST D2 1N4004 VR1 100 K 6 10F 2 10nF A A BC327, BC337 E SC 2015 A IRF540 B G C D S D Trig TP1 10nF OUT IN SPARK ENERGY METER CALIBRATOR E D 3 Out IC1 Thr 7555 5 CV Q1 IRF540 10 E GND 6 K ADJ Disch 7 LM317T OUT 4 Rst – Q2 BC337 B VR2 50k G Q3 BC327 S C 220k (R1)# 1N4148 K 8 Vcc C B 1 #R1 MAY NEED CHANGING TO A HIGHER (eg, 270k) OR LOWER (eg, 180k) VALUE SHOULD THERE BE INSUFFICIENT RANGE ADJUSTMENT WITH VR2 TO SET THE 250Hz 1N4004 The meter must be calibrated before use to ensure accuracy and this is done by by applying a test signal with a repetitive 2ms -5V pulse across the 150 5W resistor. The display is then calibrated to show 100mJ. This is done by adjusting the internal trimmer on the LCD. We’ve designed OUTPUT CON2 100 10F 16V 240 Calibrator circuit +5V OUT IN the low-battery comparator, with its inverting input (pin 13) connected to the 5.4V rail as a reference and pin 12 connected to a voltage divider across the battery. A 1M positive feedback resistor provides hysteresis. If the battery level is low, the output of IC1d goes low, reducing the voltage at input pin 10 of IC1c. This op amp acts as an OR-gate, so while the battery voltage is above the 7.2V threshold, its output is always high and thus power LED1 is lit constantly. But once the voltage at pin 10 drops, astable oscillator IC1b driving its pin 9 input and causes the output to pulse, flashing LED1. The 510k and 220nF component values at IC1b’s inverting input (pin 6) in combination with the resistors connected to its pin 5 non-inverting input. set the flash rate to around 2Hz with a duty cycle of around 75%. 2 8 Vcc Disch Thr Trig 4 Rst IC1 7555 Out CV TP1 3 5 GND K D4 A 1 TO BASES OF Q2, Q3 A VR2 250k 1k (R1) D3 K D3, D4: 1N4148 ALTERNATIVE PWM DRIVE CIRCUIT Fig.2: the calibrator circuit. REG1 is adjusted to give a 5V output while VR2 allows the output frequency of IC1 to be set to 250Hz. This gives the required 2ms -5V pulses at CON2. With some small changes shown in the yellow box, the circuit can be used as a 1A, 5V/12V PWM motor speed controller or lamp dimmer instead. 62  Silicon Chip siliconchip.com.au a PCB to perform this task and the circuit is shown in Fig.2. Once you’ve finished using it to calibrate the Spark Energy Meter, it can be reconfigured to operate as a pulse width modulated (PWM) DC speed controller. Since a 2ms pulse is required, the simple solution is to generate a 250Hz square wave with the required amplitude. If the duty cycle is close to 50%, the frequency and voltage can be adjusted to the correct values using measurements from a DMM. The circuit operates from a 7-12V supply with reverse polarity protection by diode D1. REG1 is an adjustable regulator that is adjusted to give exactly 5V. Typically, the voltage between the OUT and ADJ terminal is 1.25V but could range between 1.2 and 1.3V depending on the particular regulator. The 100 resistor between the output and adjust terminal sets a nominal 12.5mA flowing through the 240 resistor and 100 trimpot. That current will allow the adjust terminal to be set to sufficient voltage for 5V at the output. CMOS timer IC1 runs from this 5V supply. It has a rail-to-rail output at pin 3. That means the output will swing to a few millivolts off 5V when pin 3 is high and to a few millivolts shy of 0V when the output is low. The output drives resistances VR2 and the 220k resistor in series to charge the 10nF capacitor connected to pins 2 & 6 when pin 3 is high and discharge when pin 3 is low. When the pin 3 output is high, this capacitor charges to 2/3rds the supply voltage, whereupon pin 6 detects this and sets the output low, discharging the capacitor. When the capacitor reaches 1/3rds the supply voltage, pin 2 detects this and the pin 3 output goes high. The cycle continues, alternately charging and discharging the capacitor. Since the capacitor is charged and discharged symmetrically between 1/3rds and 2/3rds the supply voltage via the same value resistance, the pin 3 output is a square wave with a 50% duty cycle. The pin 3 output also drives emitterfollower buffer transistors Q2 and Q3 to drive the gate of Mosfet Q1 via a 10resistor. When pin 3 is high, Q2 is switched on to charge Q1’s gate, switching it on in turn. When pin 3 is low, Q3 switches on instead and siliconchip.com.au the Mosfet’s gate is discharged, turning it off. The 5V supply rail and drain of the Mosfet are connected to the 150resistor in the Spark Energy Meter via CON2 to provide the calibration signal. Note that the supply for the calibrator needs to be floating relative to that of the Spark Energy Meter. So long as the same 9V battery is not used to power both circuits, that will be the case. The two circuits should not be joined except via CON2. Alternative circuit The circuit diagram shows an alternative circuit that could be used after the Spark Energy Meter has been calibrated. You can then use this circuit as a pulse width modulated power control for small DC motors or for lamps up to about 1A. The motor needs to be rated for 5V. For a higher voltage motor, you can connect between the minus terminal of CON2 and the + terminal of CON1 to run at the input supply voltage (eg, 12V). In this configuration, the 220k resistor is replaced with a 1k resistor and VR2 is replaced by a 250ktype. Diodes D3 and D4 are added so there will be a different charge and discharge path. When pin 3 is high, the 10nF capacitor is charged via D3 and the portion of VR2 to its wiper. During discharge, the capacitor is discharged via diode D4 and the opposite portion of VR2 to the wiper. So if VR2 is set to its mid point, the waveform should be close to a square wave as the resistance on either side of the trimpot wiper are the same. The more VR2 is adjusted off centre the more the waveform becomes asymmetric. At the extremes of VR2, the output will be high for the ratio of 1/250 of each cycle when the wiper is wound anticlockwise and high for 249/250 when the wiper is fully clockwise. That way the Mosfet can be switched to be on almost all the time or off most of the time or anywhere in between. SIGNAL HOUND USB-based spectrum analyzers and RF recorders. SA44B: $1,320 inc GST • • • • • Up to 4.4GHz Preamp for improved sensitivity and reduced LO leakage. Thermometer for temperature correction and improved accuracy AM/FM/SSB/CW demod USB 2.0 interface SA12B: $2,948 inc GST • • • Up to 12.4GHz plus all the advanced features of the SA44B AM/FM/SSB/CW demod USB 2.0 interface The BB60C supercedes the BB60A, with new specifications: • • • • • The BB60C streams 140 MB/sec of digitized RF to your PC utilizing USB 3.0. An instantaneous bandwidth of 27 MHz. Sweep speeds of 24 GHz/sec. The BB60C also adds new functionality in the form of configurable I/Q. Streaming bandwidths which will be retroactively available on the BB60A. Vendor and Third-Party Software Available. Ideal tool for lab and test bench use, engineering students, ham radio enthusiasts and hobbyists. Tracking generators also available. Next month In the part 2 article next month, we’ll go through building the three PCBs, assembling the two main boards into the diecast case and the calibration and set-up procedure. We’ll also go over how to connect the spark energy meter to a working engine. SC Silvertone Electronics 1/8 Fitzhardinge St Wagga Wagga NSW 2650 Ph: (02) 6931 8252 contact<at>silvertone.com.au February 2015  63