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The easy way to identify faulty electros ESR Meter Mk.2 Pt.1: By BOB PARKER Forget about capacitance meters – an ESR meter is the way to go when it comes to identifying faulty electrolytics. This well-proven design is autoranging, low in cost and simple to build. ESR Meter: Main Features • • • • • • • • • In-circuit testing, made possible by using <100mV test voltage which won’t forward bias diodes or transistors. Auto-ranging to cover 0.01-99Ω. Non-polarized test leads due to no DC component in the test signal. Single pushbutton to easily control all functions. Test lead resistance zeroing. Automatic switch-off after three minutes when the meter is idle. Low battery voltage warning – “b” blinks on the display. 13mm LED displays for easy viewing from a distance. Chart of typical electrolytic capacitor ESR figures on the front panel. 58  Silicon Chip I T’S HARD TO BELIEVE that it’s already eight years since my first ESR (equivalent series resistance) meter was described – in the January 1996 edition of “Electronics Australia”. It was designed on a 386 PC! The ESR meter allowed service technicians to quickly and easily identify defective electrolytic capacitors while they were still in circuit. It measures a characteristic of electrolytic capacitors which is very important: the “equivalent series resistance” or ESR. Back then, no-one (including myself) expected that a meter designed to measure a capacitor characteristic hardly anyone had ever heard of would become popular in Australia, let alone overseas. However, we didn’t consider the explosive growth of the Internet. Thanks to people discussing it on various newsgroups and by email, about 12,000 ESR meter kits have now been sold and sales (mainly outside Australia) continue to be strong. www.siliconchip.com.au Over those eight years, both Dick Smith Electronics (which sells the kit) and the author have received many suggestions from constructors on improving the ESR meter kit – particularly on making the construction easier. This upgraded version is the result and incorporates many of those ideas. As before, it will be available as a complete kit from DSE. What’s ESR? Before taking a look at what’s changed in this “Mk2” version of the ESR meter, let’s take a look at what an ESR meter does. First, we need to get into a bit of boring theory to understand how electrolytic capacitors (which I’ll refer to simply as “electrolytics” from here on) are constructed and work. This is necessary to understand why they cause so many electronic faults. Fig.1 is a simplified cross-section drawing which shows the basics. As with many other kinds of capacitors, the plates of an electrolytic consist of two long aluminium foil strips wound into a cylinder. The big difference is that the dielectric isn’t a strip of plastic or other insulating material separating these plates, but an extremely thin layer of aluminium oxide which is formed directly onto the anode foil itself during the manufacturing process. As part of an electrolytic’s electrochemical operation and to achieve the closest possible electrical contact with the cathode side of the oxide layer, a separating strip of porous material (generally paper) is sandwiched between the plates. This separator is soaked with a highly conductive liquid called the “electrolyte”, which effectively connects the negative plate to the oxide layer and gives the capacitor its name. In very old electrolytics, the electrolyte was water-based but they now use water-free formulas. Because electrolytics make use of a conductive liquid to complete the electrical circuit between the cathode plate and one side of the dielectric, the electrolyte’s electrical resistance is critical. It is the major component of the capacitor’s “equivalent series resistance” or “ESR”. Other components of ESR are the inductance of the wound capacitor element, the resistances of the internal connections and the impedance of the capacitance itself. In operation, electrolytic capacitors can function perfectly for decades. www.siliconchip.com.au Fig.1: simplified cross-section of an electrolytic capacitor. The dielectric consists of a thin layer of aluminium oxide on the anode plate and this is connected to the cathode plate via an electrolyte-soaked separator. Fig.2: as shown in this diagram, the electrical resistance of the electrolyte is in series with the capacitance of the oxide dielectric. It is the major component of the “equivalent series resistance” or “ESR” of an electrolytic capacitor. Fig.3: this block diagram shows the basic scheme for the ESR meter. S1 is an electronic switch and it allows the test capacitor to be alternately charged for 8µs from a constant current source and then discharged for 492µs. The resulting voltage waveform is then amplified and fed to a comparator, where it is compared with a reference voltage ramp. However, there are some conditions which will cause the electrolyte’s resistance (ESR) to increase. This can eventually reach a point where it causes problems for the circuit. Normally, a flexible rubber seal keeps the electrolyte contained inside the aluminium case of the capacitor. If the seal fails (as it regularly does in surface-mount electrolytics), the electrolyte will leak and/or dry out. The two other big killers are: (1) high temperatures where the electrolytic is located; and (2) high levels of ripple current through the capacitor, which cause elevated temperatures inside it. These conditions cause chemical changes to the electrolyte, increasing its resistance. This is why time after time, repair technicians find electrolytics failing in switchmode power supplies, the deflection stages of CRT TVs and monitors, and other power circuitry March 2004  59 or other problems but real world capacitors have ESR. The ripple voltage across this “equivalent series resistance” causes circuit losses as well as heating within the capacitor, if it becomes excessive. For example, in switchmode power supplies, high ESR can cause starting failure, loss of regulation and excessive high-frequency noise on the outputs. Similarly, deflection circuits can suffer from distorted and reduced scanning waveforms. In fact, high electrolytic capacitor ESR often causes strange problems which are hard to make sense of. It’s worth noting that ESR increases rapidly as the temperature drops. As a result, defective electrolytics are often indicated by faults which are worst in winter and when the equipment is first switched on, with the symptoms gradually diminishing as the temperature rises. Capacitance vs ESR meters Fig.4: this simplified flow chart shows how the microcontroller takes an ESR measurement. It simply counts the measurement pulses until the comparator output no longer goes high during one of them. such as electronically-commutated motors where both of those conditions are common. Why high ESR causes trouble The function of an electrolytic capacitor is to block DC while acting as a low impedance to any AC voltage across it. As a power supply filter, an electrolytic smooths rectified voltage and so has to pass the AC ripple voltage on it. This causes “ripple” current through the capacitor. In a perfect capacitor, such ripple current causes no internal heating 60  Silicon Chip In the past, technicians didn’t have much choice but to check suspect electrolytics using a capacitance meter. Unfortunately, capacitance meters are generally useless for weeding out electrolytics which are causing trouble. They’re generally designed to ignore the ESR and show only the actual capacitance which usually stays close to its correct value, even when the ESR has gone through the roof! In addition, the capacitor must be disconnected from the circuit before making capacitance measurements. Now you can see why ESR meters have become so popular with technicians. They’re designed to directly measure the very characteristic which is causing the fault. What’s more, this measurement can be made with the capacitor still in circuit (while the equipment is safely disconnected from power). This avoids the inconvenience of having to unsolder it, which incidentally also heats it up and makes the ESR drop, thereby masking the problem. Microcontroller-based meter Unlike most other ESR meters, this design is based on a microcontroller IC. This custom-programmed chip makes possible the extensive range of features offered (see panel). It also greatly contributes to the small size, low cost and simplicity of the ESR meter. The microcontroller drives two 7-segment LED displays to give a direct readout of ESR measurement. How it works An ESR meter’s job is to measure the resistance of an electrolytic capacitor’s electrolyte while (as far as possible) ignoring the capacitive reactance. Fig.3 shows a simplified diagram of how this is done in the ESR meter described here. As shown, switch “S1” (in reality, an electronic switch driven by the microcontroller) alternately connects and disconnects the capacitor being tested to a constant current source of either 0.5mA, 5mA or 50mA (depending on the range). In practice, the capacitor is alternately charged for 8µs (S1 in the “Charge” position) and discharged for 492µs (S1 in the Discharge” position). Because the test current pulses are so short, the voltage pulses developed across the capacitor are essentially proportional to its ESR. That’s because capacitors with values above about 1µF don’t have time to charge enough to significantly affect the reading. The voltage pulses across the capacitor are fed to a non-inverting wideband amplifier with a gain of 20. The resulting signal is then applied to the noninverting input of an op amp comparator (inside the microcontroller) and compared against a reference voltage which increases linearly with time. Analog-to-digital conversion In operation, the test current pulses are applied to the capacitor at a constant rate of one every 500µs (ie, 8µs charge, 492µs discharge). At the same time, capacitor C10 is charged via another constant current source, so that its voltage increases linearly at a rate of 10mV/500µs. The resulting linearly increasing voltage on C10 is applied to the inverting input of the comparator. As a result, the comparator’s output will go high during each ESR measurement pulse, until C10’s voltage exceeds the pulse amplitude. When that happens, the comparator’s output stays low and the missing output pulses are detected by the firmware in the microcontroller. Fairly obviously, the number of pulses that occur up until this point is directly proportional to the capacitor’s ESR. It’s simply a matter of using the www.siliconchip.com.au www.siliconchip.com.au March 2004  61 Fig.5: a Zilog Z86E0412 programmed microcontroller (IC2) forms the heart of the circuit. This IC automatically switches transistors Q3-Q5 to set the pulse current level, while Q7 & Q8 amplify the resultant voltage pulses across the test capacitor for comparison with a reference voltage ramp (across C10). the number of measurement pulses until the comparator output no longer goes high during one of them. General operation With the basics out of the way, let’s now take a look at the complete circuit. Fig.5 shows the details. As can be seen, it’s based on a Z86E0412 microcontroller (IC2). Starting with the power supply, Q1 is the main power switching transistor. In the meter’s “off” state, Q1 has no forward bias and so no significant current flows from the battery. Conversely, when switch S1 is pushed, base current flows from Q1 and through resistor R2 and diode D1 to ground. Q1 thus switches on and effectively connects the battery’s positive terminal to the input of 5V regulator IC1. This in turn provides a +5V rail to power microconHere’s a preview of the assembled PC board. troller IC2 and the rest of the The construction details are in Pt.2. circuit. microcontroller to count these pulses As soon as power is applied, IC2’s to obtain a reading on the display crystal oscillator (based on 3.58MHz (and microcontrollers are very good crystal X1) starts and IC2 begins at counting). executing the instructions in its Fig.4 shows the simplified flow firmware. The first “external” thing chart of how the microcontroller takes it does is drive pin 2 to +5V and this an ESR measurement. It simply counts turns on transistor Q2 via resistor R3 What’s Changed In The Mk.2 Version • Front panel chart figures updated to reflect current-generation electrolytic capacitors. • PC board now has silk-screened component overlay, solder masking and holes under the trimpots for adjustment after final assembly. • • • Improved appearance, with countersunk screws, etc. • Smaller more reliable pushbutton switch which is harder to accidentally bump in a toolbox. • Automatic self-testing of the meter’s circuitry added to the microcontroller firmware, to simplify fault-finding if a newly-built meter doesn’t work properly. Automatic switch-off time increased from two minutes to three minutes. Holder for 6 AAA cells instead of a 9V alkaline battery for longer times between battery replacements (and to finally end constructor confusion about how to keep the battery in place). 62  Silicon Chip (15kΩ). As a result, Q2 takes over from pushbutton switch S1 in maintaining Q1’s base current through R2, thus ensuring that the power remains on when S1 is released. Pulsed current sources Transistors Q3, Q4 and Q5 are driven by pins 15-17 of IC2 (via 2.2kΩ resistors) and function as switches. Depending on the range chosen, the Z86 pulses one of these transistors on for 8µs every 500µs, to apply short current pulses via C5 & C6 to the capacitor being tested. Resistors R6, R8 & R10 set the pulse current to either 0.5mA, 5.0mA or 50mA, while capacitors C5 and C6 block any DC component from reaching the test leads. Note that bipolar electrolytic capacitor C6 is in series with the current source resistors, so its own ESR is effectively “swamped” by the relatively high resistor values. C5 is included to preserve the highfrequency response of the pulse waveform and to further reduce the effect of C6’s ESR. Between the 8µs pulses, IC2 drives its pin 1 port to +5V. This turns Q6 on and discharges the series combination of C5/C6 and the capacitor under test. Pulse amplifier The current pulses developed across the test capacitor are fed via C7 and R12 to a fast non-inverting pulse amplifier based on transistors Q7 and Q8. These two transistors are wired as common-emitter stages, with feedback applied via R17 to give an overall gain of about 20, depending on the setting of VR2. The amplified signal output from this stage is then fed to the noninverting input of one of IC2’s comparators via pin 8, so that it can be compared with the reference voltage. Reference voltage generator Transistors Q9 and Q10 form a current mirror circuit which works with capacitor C10 to provide the reference voltage (see Fig.3). It works like this: when Q9 is on (ie, when pin 4 of IC2 is low), approximately 9.4µA flows through this transistor and R22. This current is “mirrored” by Q10, so the same amount of current also charges C10 (470nF) at a linear rate towards the +5V supply for as long as pin 4 of IC2 is held low. www.siliconchip.com.au The ramp voltage developed across C10 is applied to pin 10 of IC2. This pin is the common inverting input of the two voltage comparators inside the Z86. Q11 discharges C10 when IC2 switches its pin 4 port to +5V at the end of each measurement cycle. Range changing While ever the power is switched on, the Z86 goes through a regular measurement routine in which it starts C10’s voltage ramping up and then drives either Q3, Q4 or Q5 with 8µs pulses that are 500µs apart. This produces measurement ranges of 0.000.99Ω, 1.0-9.9Ω and 10-99Ω. If a reading is offscale, the unit automatically drops to the next lowest test pulse current and checks again. However, if it’s already on the 10-99Ω range and the reading is offscale, it will display “-” to indicate a reading above 99Ω. Conversely, if it gets a very low reading, it will keep going to the next highest test current, until it’s found the highest on-scale reading. The reading is then shown on the 7-segment LED displays. Driving the displays To display the reading, the Z86 What Are Typical ESR Readings? So what are typical ESR readings for various electrolytic capacitors? Unlike other electrical characteristics, there’s no such thing as a “normal” ESR value for an electrolytic of a given capacitance and operating voltage. The ESR to a large extent depends on the physical size of the capacitor and whether it’s a low-ESR or high temperature-rated type. It also varies between manufacturers. In addition, ESR increases rapidly as the temperature drops and vice versa. The chart on the front of the meter contains sample ESR values for a range of common electrolytic capacitor values and voltage ratings. These have been derived both from physical measurements on a range of capacitors and from manufacturer’s data sheets. It’s only intended as a rough guide, to give an idea of what to expect until you become familiar with using the ESR meter. microcontroller sends out eight bits of data (in sequence) every 5ms to IC3, a 4094 serial-to-parallel shift register. These data bits correspond to the LED display segments and to the decimal points which are formed using LEDs 1 & 2. In operation, the LED displays (DISP1, DISP2 and LEDs 1 & 2) are switched at a 100Hz rate by transistors Q12 and Q13. Q12 is driven (via R28) from the P23 output (pin 18) of IC2, while Q13 is biased on via R27, which connects directly to the +5V rail. Q13 toggles off when Q12 turns on and turns back on again when Q12 turns off. Due to the slow response of the human eye, the displays all appear to be constantly illuminated. This technique is called “multiplexing” and it allows the two displays to share a common drive circuit. Test lead resistance zeroing The resistance of the test leads can be compensated for by again pressing switch S1 (ie, after the unit has been Car Projects, Volume 2 THE PROJECTS: High-Energy Universal Ignition System; High-Energy Multispark CDI System;Programmable Ignition Timing Module; Digital Speed Alarm & Speedometer; Digital Tachometer With LED Display; Digital Voltmeter (12V or 24V); Blocked Filter Alarm; Simple Mixture Display For Fuel-Injected Cars; Motorbike Alarm; Headlight Reminder; Engine Immobiliser Mk.2; Engine Rev Limiter; 4-Channel UHF Remote Control; LED Lighting For Cars; The Booze Buster Breath Tester; Little Dynamite Subwoofer; Neon Tube Modulator. Available from SILICON CHIP Mail order prices: Aust: $14.95 (incl. GST & P&P) NZ/Asia Pacific: $18.00 via airmail Rest of World: $21.50 via airmail Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. www.siliconchip.com.au March 2004  63 bled if necessary (see the “Optional Modifications” panel in Pt.2). Parts List 1 PC board, code ZA1044, 95 x 57mm 1 3.58MHz crystal, HC-49/U case (X1) 1 16-pin IC socket 1 18-pin IC socket 1 28-pin IC socket 6 AAA alkaline cells 1 6 x AAA cell holder 1 plastic utility box, type UB3 1 miniature momentary-contact pushbutton switch 2 4mm banana sockets 1 pre-punched silk-screened front panel 1 red perspex display filter 4 15mm spacers 6 PC pins 8 washers 1 set of test leads 1 10kΩ PC-mount trimpot (VR1) 1 200Ω PC-mount trimpot (VR2) 1 Z86E0412 programmed microcontroller (IC2) 1 4094 CMOS shift register (IC3) 2 LSD5114 or LTS5503AE 7-segment LED displays (DISP1, DISP2) 2 3mm red LEDs (LED1,LED2) Semiconductors Resistors (0.25W, 1%) 4 1N4148 or 1N914 signal diodes (D1,D2,D5,D6) 2 1N4004 power diodes (D3,D4) 4 BC328 PNP transistors (Q1, Q3,Q4,Q5) 5 BC338 NPN transistors (Q2, Q6,Q11,Q12,Q13) 1 BC548 NPN transistor (Q7) 3 BC558 PNP transistors (Q8, Q9,Q10) 1 78L05 3-terminal regulator (IC1) powered up) while the test lead probes are held tightly together (to minimise contact resistance). When this is done, pin 3 of IC2 is pulled low via D2 and S1 and the microcontroller goes into its test lead zeroing routine. If the reading is less than 1Ω (as all test leads are), it saves this value for as long as the meter is switched on. It then subtracts it from all subsequent readings, so that only the ESR of the capacitor being tested is displayed (ie, so that the reading is unaffected by the test lead resistance). Switching off Pressing S1 while the test leads are separated (or connected to a resistance of 1Ω or higher) initiates the “switchoff” routine (assuming, of course, that the unit is already on). 64  Silicon Chip Capacitors 2 220µF 16V RB electrolytic (C3,C9) 1 100µF 16V RB electrolytic (C1) 1 47µF 50V bipolar RB electrolytic (C6) 1 22µF 16V RB electrolytic (C8) 1 470nF 63V MKT (C10) 4 100nF 50V disc or monolithic (C2,C4,C5,C13) 1 33nF 63V MKT (C7) 2 27pF 50V NPO disc ceramic (C11,C12) 1 470kΩ 1 220kΩ 1 100kΩ 2 47kΩ 2 15kΩ 7 10kΩ 1 6.8kΩ 3 4.7kΩ 1 2.7kΩ 4 2.2kΩ 2 1kΩ 1 680Ω 1 220Ω 1 180Ω 1 100Ω 1 68Ω (for calibration) 1 5.6Ω (for calibration) Miscellaneous Hookup wire & solder What happens is that the Z86 stops making measurements and switches its pin 2 port to 0V, in turn switching off transistor Q2. Then, when you release S1, Q1 switches off and the meter shuts down. In addition, the ESR Meter includes an automatic power-off function. This shuts the meter down if it has been idle for more than three minutes. It works like this: as long as the meter is actively taking readings, it keeps resetting a 3-minute timer function in the Z86 microcontroller. However, if the unit is left idle (even with the test leads touching), the Z86 automatically switches its pin 2 port low after three minutes, thus turning off the power. This automatic switch-off function may be a nuisance in some situations, however. Hence, it can be easily disa- Battery voltage warning A simple voltage divider consisting of trimpot VR1 and series resistors R25 & R26 makes up the battery warning circuit. This divider is connected across the switched battery voltage and VR1 is adjusted so that it applies 2V to pin 9 of IC2 when the battery voltage is at 7V (ie, the minimum at which the regulator will continue to regulate). Pin 9 is the non-inverting input of IC2’s second internal comparator. In operation, IC2 switches its pin 4 port to 0V for a period of 100ms several times per second, to allow C10 to charge up to a predictable 2V. The second comparator inside IC2 then compares this 2V reference against the voltage on VR1’s wiper. If the battery voltage is down to 7V, IC2 reduces the time each LED display is switched on by 50%. This reduces the load, which allows the battery voltage to slightly rise again and provide a bit more operating time. It also flashes a “b” on the righthand digit at a 1Hz rate until the power is turned off. Protection circuitry Last but not least, the meter needs to be protected against being connected to charged capacitors. This protection is partially provided by back-to-back diodes D3 and D4. If an external DC voltage (ie, a charged capacitor) is connected, one of these diodes conducts and forces non-polarised capacitors C5 and C6 to charge up to that voltage. Additional protection is provided by C7, R12, D5 & D6 which stop excessive input voltages from damaging transistors Q7 and Q8 in the pulse amplifier circuit. In particular, diodes D5 & D6 acts as voltage clamps – D5 ensures that the voltage on Q7’s base cannot go above 5.6V, while D6 ensures that this voltage cannot go below -0.6V. Finally, extra “heavy-duty” protection can be added by connecting a pair of back-to-back high-power diodes (not shown on the circuit) between the test terminals. The “Optional Modifications” panel in Pt.2 next month has the details. Next month, we’ll show you how to build the ESR Meter and describe how it is used. There’s also a full troubleshooting and diagnostics procedure in SC case you encounter difficulties. www.siliconchip.com.au