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In-Circuit LC Meter

A Prototype Study

In-Circuit LC Meter
In-Circuit LC Meter
When troubleshooting a board, the ability to test passive components without desoldering them from the PCB is most useful but at the same time critical, since interference from the rest of the circuit during testing must be minimized. This prototype circuit can measure in-circuit capacitance from 1 pF to 80 mF or inductance from 1 mH to 40 H. It uses a low voltage method that is robust against parasitic parallel or series resistance. The signal processing is mainly analog; the computation to extract L, C, and “parasitic” resistance is performed by an Arduino UNO.  

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Materials

Component list

Component List

Resistors (All 1%, metal film, ≥1/8W)

R1, R71 = 4.7 kΩ

R2, R4, R34, R35, R36, R37 = 2.2 kΩ

R3 = 1.0 kΩ (1)

R5, R6, R7, R10, R13, R66, R72, R73 = 100.0 kΩ (2)

R8 = 100.0 Ω (1)

R9 = 100.0 MΩ (3)

R11 = 10.0 MΩ (3)

R12, R27 = 1.0 MΩ (3)

R14, R26, R46, R54, R62 = 100.0 kΩ (3)

R15, R28, R53, R55, R57 = 10.0 kΩ (3)

R16, R25, R52, R60 = 1.0 kΩ (3)

R17, R30, R39, R75 = 100.0 kΩ

R18, R19, R20, R21, R22, R31, R40, R48, R58, R59, R63, R64 = 10.0 kΩ

R23, R45, R65 = 1.0 kΩ

R24, R29, R32, R33, R38, R41, R42, R43 = 1.0 kΩ (2)

R44 = 1.0 MΩ

R47, R51 = 100.0 Ω (3)

R49 = 0.0 Ω (3)

R50, R56 10.0 Ω (3)

R61 = 1.0 Ω (3)

R67, R69 = 4.7 kΩ (1)

R68, R70 = 100.0 kΩ (1)

R74 = 5 kΩ trimmer

 

Capacitors

C1 = 22 nF, low temp. coefficient

C2…C14 = 1 µF, ceramic, multilayer

 

Semiconductors

D1,D2 = 1N4007, protection diodes

Q1, Q8 = J113, n-j FET switch

Q2, Q3, Q4…Q7 = BS170, MOSFET switch

Q2,Q3 = Si4136DY, MOSFET (4)

U1, U12 = TL084, quad opamp

U2, U3, U13 = MC34072PG, dual opamp

U2, U3, U13 = LM393B +1k collector R, dual comparator (5)

U16 = OPA2182, fast zero-offset opamp

U4, U11 = TL431D, 2.5V reference

U10 = DG413Y, quad MOSFET switch (current drive)

U5 = CD4016B, quad MOSFET switch (sampling)

U7 = CD4001B, quad 2-input NOR

U9 = CD4070B, quad 2-input XOR

U18 = CD4093B, quad 2-input NAND

 

Miscellaneous

SW1 = rotary switch, 6×2

SW2 = rotary switch, 4×3

SW3 = toggle switch, 2×2

 

Notes:

(1): Tolerances in these components can be compensated through the amplification and attenuation values inputs in the Arduino program.

(2): The equality of values among these resistors affects the accuracy.

(3): These resistors are critical for accuracy. Use precision (low-tolerance) resistors.

(4): More accurate alternative to BS170, with a lower Ron.

(5): Alternative to MC33172.

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