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TEST&MEASUREMENT
Capacitor ESR Tester
the good, the bad and the leaky...
Design by Flemming Jensen
How about an in-circuit capacitor tester to take the strain out of tracking down faulty capacitors? No need to solder out any capacitor, simply check it in-circuit, from thousands of microFarads down to a hundred nanoFarads. In most cases, parallel coils or low value resistors are no problem. Even shorted caps may be revealed in-circuit and polarity is irrelevant for the tester. High ESR? Replace!
The most significant property of a capacitor is its capacitance but besides that there's another important factor, namely the so called ESR, or Equivalent Series Resistance. An ideal capacitor is a purely reactive component with a 90-degree phase angle between current and voltage. In the real world, however, a capacitor needs to be modelled as an ideal capacitor in series with a resistor representing the losses introduced by the component. The equivalent circuit is shown in Figure 1. Sure, capacitance can be measured by means of a capacitance meter, which is pretty common nowadays, but unfortunately this test won't tell anything about the capacitor's quality -- we need to know the ESR as well. Over time, electrolytics tend to dry out, which will raise their ESR and inevitably the voltage drop inside the capacitor. Evidently, the pure reactance Xc can not produce heat, due to the 90-degree phase shift between voltage and current, but the ESR can, and in switching circuits the resultant heat will cause a further degradation of the capacitor's quality i.e., a further rise in ESR. It's fairly common to find electrolytics that on the face of it have only lost just a few
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How does ESR influence circuit behaviour?
In (fast) switching circuits, a low ESR may be crucial for proper circuit behaviour. For example, in a TV set, high capacitor-ESR may lead to inability to quit stand-by mode, incorrect picture height or width, synchronisation problems, interference or hum bars. In Switch Mode Power (SMPSUs) supplies, high ESR caps may lead to blown semiconductors, blown fuses or no start up. In power circuits, a rising ESR will make the capacitor warm up, leading to even higher ESR and eventually circuit breakdown. The usual method to troubleshoot these problems involves soldering out the capacitors, measure the capacitance and solder the good ones back in. A tedious task, but what's even worse, ailing capacitors often don't show a low capacitance, get soldered back in again and then the troubleshooting gets really time consuming.
ideal capacitor ESR
reactive part X C =
1 2 f . C
012022 - 11
Figure 1. The most important property of a capacitor is its capacitance but beside that there's another important factor, namely the socalled ESR, or Equivalent Series Resistance.
percent of their rated capacity although their ESR is in the hundred Ohms range. Obviously, such a component acts as a load just running hot and wasting a lot of energy.
The measuring principle
The capacitor under test, C.U.T., is fed with a 100-kHz constant-current square wave signal. The ESR value is determined by measuring the AC voltage drop across the C.U.T. If the capacitance is high enough compared to the frequency, the voltage drop over the internal reactance is negligible and the drop is caused by the ESR only. This voltage is converted to DC and fed to the voltmeter section. AC to DC conversion of a 100-kHz signal in the millivolts range pre-
sents a real design challenge. Furthermore, the conversion needs to be as linear as possible because we want to use an ordinary 200-mV DVM readout. It goes without saying that an ordinary diode rectifier will not suffice, and an active diode rectifier with opamps will have a hard time working at 100-kHz and a few millivolts. The solution we came up with is a synchronous rectifier -- essentially a polarity changer controlled by the same generator that supplies the 100-kHz test signal. This circuit works surprisingly well and is cheap, too! A simplified version of the circuit is shown in Figure 2. Here, the C.U.T. is assumed to be a 100-µF with an ESR of 10 . As shown, the reactance is negligible and the ESR, which is purely resistive, is dominating.
Although the above principle works well, further reduction of the reactive influence is called for. Figure 3 shows an example where the C.U.T. is 0.1-µF cap whose ESR is zero Ohms. As mentioned above we use a relatively high frequency to make the reactance negligible while enabling even the smallest electrolytics like 0.1-µF to be tested. For this it is necessary to reduce the influence of the beginning integration of the waveform even further. The ESR is zero and the reactance is 15 . As can be seen, the integrated waveforms presented to the differential amplifier inputs result in a sawtooth centred around zero volts at the output. After integration, in the RC network that follows, a DC level of zero volts is fed to the voltmeter circuit. If the C.U.T. also represents an ESR of, say, 10 , the sawtooth at the output will still have the same waveform. However, it will be DC-shifted in the positive direction by an amount representing
+5V 0
2k2 2k2
1.255V 1.244V 0 1.255V 0
1M
S C D R
XC = 0 Q ESR 1M ESR = 10
11mV 0
Q 2k2
1M
+5V 0
1.244V 0
2k2
C.u.T. 100µF
1.255V 1.244V 0
1M
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Figure 2. Illustrating the principle of operation. Assuming that the C.U.T. is a 100 µF capacitor with an ESR of 10 Ohms, the reactance is negligible and the ESR, which is pure Ohmic, is dominating.
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TEST&MEASUREMENT
50mV +5V 0
2k2 2k2
0 50mV 0
1M
0 50mVpp 0V DC
S C D R Q ESR
XC = 15 1M ESR = 0
Q 2k2
1M
+5V 0
0
2k2
50mV
C.u.T. 100nF
0
1M
50mV
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Figure 3. Second hypothetical test: C.U.T. is a 0.1-µF capacitor with an ESR of 0 .
the ESR value. After integrating the sawtooth away, the output will give the proper reading of 10 , excluding the 15 reactance.
Low-ESR cap or shorted cap?
You may question if you're testing a low-ESR cap or simply a shorted one. A simple DC Ohms test is usually enough to decide this. No need to get out the multimeter -- with a push of a button the ESR tester becomes a DC Ohmmeter and your display should change to a higher Ohm reading. If it doesn't, the odds are you have a shorted cap on your hands.
Some practical ESR values, please?
So how high will the ESR be then? Well, that depends on where the capacitor is used, the type, the make, the voltage rating, etc. A 2,200-µF reservoir capacitor with an ESR of 10 may be fine in a linear power supply, while a 2,200-µF one having 1 ESR may be grossly inadequate in a switch-mode PSU. In general, if a large capacitor, as in this example, reads more than one Ohm, you should be suspicious and run a comparison on a similar component. But don't worry! It won't take long before you are able to distinguish bad caps from good ones. If you regularly are repairing SMPSUs, TV sets, monitors etc. you will soon appreciate the ESR tester.
IC1. This signal is divided in IC2.A which in fact constitutes our bipolar 100-KHz test signal generator. Series resistors R6 and R3-P3 on the Q and Q outputs of IC2.A give the generator a high output resistance compared to the low ESR, and, essentially, make the generator act as a 100-kHz, balanced, constant-current generator. The voltage drop across the C.U.T. is taken to IC3, four bilateral switches coupled as a controlled polarity changer, changing polarity in sympathy with the outputs of IC2.A. This enables IC3 to act as a (rudimentary) ADC. IC4.A, a differential amplifier, converts the differential signal into a single-ended signal, i.e., one which is referenced to ground. IC4.B amplifies the signal such that it can be applied to a 200mV voltmeter. IC9 is the voltmeter IC, here the ICL7106 is used with an LCD, all in a standard configuration. The LM358 in position IC8 is a comparator that tells you when it's time to change the battery. IC7, finally, generates the negative supply rail for the circuit. As shown in the circuit diagram, the test probes carry two screened wires each. Each probe carries a signal wire (e.g., `A') and a measuring wire (e.g., `B'). More about the probes under `Construction'.
Circuit diagram
Let's have a look at the circuit diagram of the Capacitor ESR Tester -- see Figure 4. A 200kHz square wave generator is built around
Construction
A compact printed circuit board was designed for the Capacitor ESR Tester by the Elektor labs. The
resulting double-sided throughplated board design is shown in Figure 5. As appropriate for a test instrument, the board is designed such that all adjustment points are easily accessible, in this case, from the sides of the board (multiturn presets P1, P2, P4, P5) and from the top (preset P3). Although the construction of the board follows standard practice (of which the main maxim is: work carefully), a few things should not be left unmentioned. Firstly, the circuit board has a screening ground plane at the component side, so care should be taken to avoid short-circuits by solder blobs or solder hairs between component terminals and the ground plane. Secondly, ascertain, check and double-check the polarity of any polarized component, in particular the tantalum capacitors in positions C3, C4 and C5. Tantalum capacitors when reverse polarised have a nasty habit of exploding and emitting hideous fumes. Finally, we recommend using sockets for all ICs (except IC9) and the LCD. The latter is easily made by cutting a 40-pin IC socket in two (lengthwise) and using the two 20-way socket strips. Small holes should be drilled in the two long sides of the ABS case to allow P1, P2, P4 and P5 to be adjusted from the outside. Regarding the probes, their basic construction is illustrated in Figure 6. These two wires are soldered together as close to the probe tip as possible. In this way the voltage
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+5V
R2 4k7 P4 5k MT R23 100k C1 180p R1 10k
+5V +9V U BATT IC5 LM2931-5,0 +5V
14 C12 100n 2 BT1 1 C2 100n C6 100µ 16V 13 R 11 C 10 S 9 10µ 16V C4 2 C5 7 10µ 16V 4
+5V
FREQ. ADJUST
IC2
7
8
1 BOOST
- 5V
IC1.C
10 8
IC1.B
9 4 5
+9V
C+
IC1.A
3
IC7
OSC VOUT 5
& +5V
&
6
&
S1
ICL7660
C LV 3 6
9V
0
14 C13 100n
C3 10µ 16V
IC1
7
TEST CAP. SHORT
12
IC2.B
D
8
IC3.D
R3 TPC R9 56 R10 56 11 12 10 R22 1M TPD R26 1k C15 1n 3 5 13 R8 R20 2k2 R16 1M R15 1M 2
+5V
1k8
C
2k2 P3 1k R7
D
IC3.B
4
1 R 3 C
4 S 5 D1 D2
2
IC2.A
D
6
SYMMETRY ADJUST
Cx
D4
D3
IC4.A
3
1
+5V
R18 10k P2 100k MT R17 10k
2x 1N4002 IC1 = 4093 IC2 = 74ACT74 IC3 = 74VHC4066 IC4 = LF412 IC6 = 4070 +5V
R6 2k2
2x 1N4002
R11 56
IC3.A
A B
R12 56 8 9 6 47
1M
1
2
ZERO ADJUST
R21
IC3.C +9V U BATT +5V +5V
- 5V
14 C11 100n 3 R5 33k R31 10k 5 6
IC3
7
IC8
8
LO BATT
1 2 1
IC6.B LCD1
=1
4 16 P1
LM358
2 4
8 P3
12 P2
+5V
C10 100n
R4 22k
R13 10k
=1 IC6.A
3 38 ARR PL BP 1 40 28
8
39 2
IC4
4 C9
+5V
3 BC4 G3 E3 D3 C3 B3
VARITRONIX VI-302 DPRC
G2 G1 E2 E1 A3 D2 C2 B2 A2 D1 C1 B1 F3 F2 F1
BP A1
100n 14
+5V
32 31 9 20 19 AB POL
10 11 29 30
27 26 13 14 15 24 25 25 13 14 9 G2 E2 D2 F2 10 11 12 C2 B2 A2
23 22 17 18 19 20 21 7 G1 6 F1 8 E1 2 D1 3 C1 4 B1 5 A1 21 1 36 P5 R28 22k 1k
- 5V
R19 R25 10k 100k R29 1M 5 7 R14 10k TPA
IC6
7
22 17 18 15 24 16 23 G3 E3 D3 C3 B3 F3 A3
37 TEST 31 IN HI C8 10n 30 IN LO
BP
IC9 ICL7106
OSC 1 OSC 2 OSC 3 C REF C REF
V REF HI
6
IC4.B
REF LO BUFF A/Z INT V
35 26
22k
C7 220n
TPB R24 P1 5k MT
COMM 32
40
39 R27 100k
38 C14
34 C16 100n
33
29 C17
28 R30 100k
27 C18
- 5V
100n 012022 - 14
100p
220n
Figure 4. Circuit diagram of the Capacitor ESR Tester. Cx is the capacitor under test.
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TEST&MEASUREMENT
C13 TPC R28
drop along the signal wire will not add to the measurement. The screening ensures that the test leads do not pick up noise, and that you maintain a stable zero adjustment.
P4 IC1 C1 T R1 A B C6 R12
H3
P5
C12 IC2 IC6
H1
012022-1
R6 R23
LCD1
P3
T
R2 R11 R8 S1 D3 D4 IC5 C2 IC7 C3
C18
R5 D2 D1 R7 R9 R10 C15 R22 R16 R26 R15 R20 R21 IC8 R4 R31 R13 R3
The ESR Tester as an add-on
The most costly parts in the circuit are the display and the 7106 A-D converter. Money can be saved if you decide to use the ESR Tester as an add-on for an existing digital multimeter (DMM). Switch the multimeter's range selector to the 200.0 mV/DC position and connect the inputs to GND and the wiper of P1. You should not be tempted to supply the ESR Tester from the multimeter's battery. Remember, the ESR Tester has its output referenced to ground, so if you run it off the multimeter's battery the Tester will have its battery minus connected to the input common terminal, which is far from advisable. Use a separate battery for the ESR Tester to avoid any problems. Or if you really want to use just one battery, give the ESR Tester an add-on 9-volt battery, connecting the ESR Tester's regulated plus 5 volt to the plus terminal of the multimeter's battery connector and the ESR Tester's minus 5 volts to the multimeter's minus terminal.
C17
R30
1-220210
ROTKELE )C(
C D
IC9
C9 C10
C8
+9V BT1 0 C4 C5
C16
R29
R18 R17
IC3
IC4
TPB
TPA
C7 TPD P2 R14 R25 R24 R19
C11
R27
C14
P1
H2
(C) ELEKTOR
012022-1
A few words of warning
Though the ESR Tester has diodeprotected inputs, it is still a good idea to discharge any largish capacitors you want to test. Some reservoir capacitors in power circuits contain so much energy that the protection circuit may burn out. If this should happen, the defective components are usually to be found in the protection circuit alone. The remedy should therefore be pretty straightforward and inexpensive.
Figure 5. Copper track layout and component mounting plan of the PCB designed for the instrument. Doublesided, through-plated board, available ready-made (see Readers Services).
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COMPONENTS LIST
Resistors: R1,R13,R14,R17,R18,R19, R31 = 10k R2 = 4k7 R3 = 1k8 R4,R24,R28 = 22k R5 = 33k R6,R7,R8 = 2k2 R9-R12 = 56 R15,R16,R20,R22,R29 = 1M R21 = 47 R23,R25,R27,R30 = 100k R26 = 1k P1,P4 = 5k multiturn preset, vertical mounting, side adjust (Bourns 3266X, Farnell #347-747) P2 = 100k multiturn preset, vertical mounting, side adjust (Bourns 3266X, Farnell #347-784) P5 = 1k multiturn preset, vertical mounting, side adjust (Bourns 3266X, Farnell #347-723) P3 = 1k preset, horizontal mounting Capacitors: C1 = 180pF C2,C9-C13,C16,C18 = 100nF C3,C4,C5 = 10µF 10V radial C6 = 100µF 16V radial C7 = 220nF C8 = 10nF C14=100pF C15 = 1nF C17 = 220nF Semiconductors: D1-D4 = 1N4002 IC1 = 4093 IC2 = 74ACT74 PC IC3 = 74VHC4066 IC4 = LF412-CN IC5 = LM2931-5,0 IC6 = 4070 IC7 = ICL7660 IC8 = LM358-N IC9 = ICL7106-CP Miscellaneous: LCD1 = 3.5 Digit LCD with LOBATT indicator, e.g., Varitronix VI302 DPRC (Farnell #478-660) S1 = pushbutton, 1 make contact Battery holder On/off switch 2 miniature probes, e.g., Hirschmann PRUF1 (Farnell #523-483) Length of 2-core screened cable ABS enclosure with LCD window and battery compartment, e.g. Multicomp type BC4, (Farnell # 645-758) 40-pin IC socket cut in half (see text) PCB, order code 012022-1, see Readers service page or www.elektor-electronics.co.uk
the voltage source. Connect TPA to TPB, short the test leads together, and adjust P2 for a `000.0' reading. Remove the connection. Reconnect P1. 2. Connect a frequency counter or an oscilloscope between TPC and GND. Adjust P4 for 200 kHz counter reading or 5 µS period time on the oscilloscope. Connect the test leads to a 10-Ohm resistor. Connect an oscilloscope (in AC mode) between point TPD and GND. Turn P3 (symmetry adjust) so that the two half cycles line up and produce a straight line. Adjust P1 for a `10.0' DVM reading. If you do not have a counter or a `scope available, turn P3 and P1 to the centre of their travel. To ensure that the ESR Tester works properly you can connect different (known, good) capacitors in series with different resistors and have these simulate capacitor ESR.
Component considerations
The LF412 in position IC4 is a good choice for the differential amplifier. Since we are dealing with high frequency signals in the millivolts range, low drift, low offset and high bandwidth are crucial. Many different opamps have been tested but most resulted in DC drift problems. The LF412 emerged as a good, low cost choice causing minimal drift. IC5, then, is a 5-volt regulator that works just fine at a voltage drop less than 600 mV and so ensures long battery life. This regulator enables the circuit to keep working down to a battery voltage of less than 6 V. IC2, a 74ACT74, is capable of delivering enough current at 100 kHz to produce a nice clean square wave. IC3 is a high speed (VHC) version of the well known 4066. Compared to the common 4066, the effect of unwanted reactance is halved. For best performance the specified components should be used, but all in all, quite acceptable performance is achieved still if you use an ordinary 4066 for IC3, and a 74HCT74 for IC2.
(012022-1)
ESR Tester adjustment
Before adjusting the instrument, be sure that you have a regulated plus 5 V from IC5 and minus 5 V from IC7. If you don't, you'll have to troubleshoot your circuit board.
1. Start with the voltmeter circuit. P1 should be disconnected at this point. Connect a known, accurate voltage source of less than 200 mV to point TPA (test point A) and adjust P5 until the LCD shows the right value. Remove
max. 0.5 m
A
Probe 1
B
C
Probe 2
D
012022- 15
Related websites:
www.awiz.com/cwinfo.htm www.flippers.com/esrkttxt.html
Figure 6. Here's how to make the 4-wire test leads between the probes and the instrument proper.
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