SC_Digital_LC_meter.pdf

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Here's a handy piece of test gear you can

build for yourself - a Digital LC Meter for

measuring inductance and capacitance over

a wide range. It's based on an ingenious

measurement technique, delivers surprising

accuracy and is easy to build.

ANY MODERN DMM's (digital

multimeters) have capacitance

measuring ranges, especially the up

market models. So it's not hard to

measure the value of capacitors, as

long as their value is more than about

50pF or so.

Below that level, DMMs are not very

useful for capacitance measurements.

Dedicated digital capacitance meters

are available, of course, and they gen

erally measure down to a few pF or

so. But if you want to measure things

like stray capacitance, they too are of

limited use.

SILICON CHIP

It's even worse when it comes to

measuring inductors. Very few DMMs

have the ability to measure induct

ance, so in many cases you have to use

either an old-type inductance bridge or

'Q'

meter. Both of these are basically

analog instruments and don't offer

either high resolution or particularly

high accuracy.

It's different for professionals who

for the last 20 years or so have been

able to use digital LCR meters. These

allow you to measure almost any pas

sive component quickly and automati

cally, often measuring not just their

primary parameter (like inductance or

capacitance) but one or more second

ary parameters as well. However, many

of these you-beaut instruments also

carried a hefty price tag, keeping them

well out of reach for many of us.

Fortunately, thanks to microcon

troller technology, that situation has

changed somewhat in the last few

years with much more affordable dig

ital instruments now becoming avail

able. These include both commercial

and DIY instruments, along with the

unit described here.

Main features

As shown in the photos, our new

Digital LC Meter is very compact. It's

easy to build, has an LCD readout and

fits snugly inside a UB3 utility box. It

won't break the bank either - we esti

mate that you should be able to build

it for less than $75.

Despite its modest cost, it offers

automatic direct digital measurement

over a wide range for both capacitance

siliconchip.com.au

. - - - - - - - - - - - - _ +5V

HOW IT WORKS: THE EQUATIONS

(A) In calibration mode

(1) WithiustLl andCI:

(2) With C2 added to Cl:

Fl- _.;"l==

F2 _

lOOk

ex/Lx

2.".

(IITT

2.".

..J

Ll.(Cl +C2)

;-~l;:;;:;::~i

>---+---..

47k

Foul

(3) From

(1)

and (2), we can Rnd C

F2'

Cl· (Fl' _ F2') .C2

(4) Also from

(l)

and (2), we can Rnd Ll:

Ll -

4.,,~

Fl'.Cl

I "

(8) In measurement mode

(5)

When Cx is connected: F3 _

2.". " Ll.(Cl +Cx)

Fl'

)

so Cx - Clo ( F3' -

(6)

Or when Lx is connected:

;:,-~=;;1~"=""

Fig.l: the circuit uses a wide-range test oscillator, the frequency of which

varies when an unknown inductor (Lx) or capacitor (Cx) is connected. This

oscillator is in turn monitored using a microcontroller which accurately

calibrates the unit and measures the change in oscillator frequency. The

microcontroller then calculates the unknown component's inductance or

capacitance and displays the result on an LCD.

F3 _

2.".

..J

(Ll +Lx).Cl

Fl'

)

Lx - Ll. ( F3' - 1

NOTE: F2

F3 should always

lower than Fl

resolution. In fact, it measures capaci

tance from just O.lpF up to 800nF and

inductance from 10nH to 70mH. Meas

urement accuracy is also surprisingly

good, at better than ±1

of reading.

also operates from 9-12V DC,

drawing an average current of less

than 20mA. This means that it can be

powered from either a 9V alkaline bat

tery inside the case or from an external

plugpack supply.

How it works

The meter's impressive performance

depends on an ingenious measure

ment technique which was developed

about 10 years ago by Neil Hecht, of

Washington state in the USA.

uses

a wide-range test oscillator whose

frequency is varied by connecting the

unknown inductor or capacitor you're

measuring. The resulting change in

frequency is measured by a micro

controller which then calculates the

component's value and displays it

directly on an LCD readout.

So there are basically only two key

parts in the meter: (1) the test oscilla

tor itself and (2) the microcontroller

which measures its frequency (with

and without the component being

measured) and calculates the compo

nent's value.

To achieve reliable oscillation over

a wide frequency range, the test oscil

lator is based on an analog comparator

siliconchip.com.8u

with positive feedback around it - see

Fig.l. This configuration has a natural

inclination to oscillate because of the

very high gain between the compara

tor's input and output.

When power (+5V) is first applied,

the comparator's non-inverting (+)

input is held at half the supply volt

age (+2.5V) by a bias divider formed

by two lOOW resistors. However, the

voltage at the inverting input is ini

tially zero because the IOIlF capacitor

at this input needs time to charge via

the 47kQ feedback resistor. So with

its non-inverting input much more

positive than its inverting input, the

comparator initially switches its out

put high (ie, to +5V).

Once it does so, the IOIlF capacitor

on the inverting input begins charging

via the 47kQ resistor and so the volt

age at this input rises exponentially.

As soon as it rises slightly above the

+2.5V level, the comparator's output

suddenly switches low.

This voltage low is fed back to the

comparator's non-inverting input via

a 100kQ feedback resistor.

is also

coupled through the IOIlF input ca

pacitor to a tuned circuit formed by

inductor L1 and capacitor Cl. This

makes the tuned circuit "ring" at its

resonant frequency.

As a result, the comparator and

the tuned circuit now function as an

oscillator at that resonant frequency.

In effect, the comparator effectively

functions as a "negative resistance"

across the tuned circuit, to cancel its

losses and maintain oscillation.

Once this oscillation is established,

a square wave of the same frequency

appears at the comparator's output

and it is this frequency (F

out)

that is

measured by the microcontroller. In

practice, before anything else is con

• Inductance Range:

from about 1OnH to over 70mH (4-digit resolution)

• Capacitance Range:

from about 0.1 pF to over 800nF (4-digit resolution)

• Range Selection:

automatic (capacitors must be non-polarised)

• Sampling Rate:

approximately five measurements per second

• Expected Accuracy:

better than ±1% of reading, ±0.1 pF or

1OnH

• Power Supply:

9-12V DC at less than 20mA (non-backlit LCD module).

Can be operated from an internal 9V battery or an external plugpack.

MAY 2008

1 PC board, code 04105081,

125 x 58mm

1 PC board, code 04105082, 36

x 16mm

1 PC board, code 04105083, 41

x21mm

1 US3 utility box, 130 x 68 x

44mm

1 16x2 LCO module (Jaycar OP

5515 or OP-5516 - see panel)

1 5V 10mA OIL reed relay

(Jaycar 8Y-4030)

1 100llH RF inductor (L1)

1 4.0MHz crystal, HC-49U

1 OPOT subminiature slider

switch (81)

1 8P8T momentary contact

pushbutton switch (82)

1 8POT mini toggle switch (83)

1 18-pin OIL IC socket

1 2.5mm PC-mount OC

connector

1 4x2 section of OIL header strip

1 7x2 section of OIL header strip

1 jumper shunt

1 binding posVbanana socket,

red

1 binding posVbanana socket,

black

2 PC terminal pins, 1mm

diameter

4 M3 x 15mm tapped spacers

4 M3 x 6mm csk head machine

screws

5 M3 x 6mm pan head machine

screws

1 M3 nut (metal)

2 M2 x 6mm machine screws

(for 81)

4 M3 x 12mm Nylon screws

8 M3 Nylon nuts

4 M3 Nylon nuts with integral

washers

1 9V battery snap lead

1 10kQ horizontal trimpot (VR1)

Semiconductors

1 PIC16F628A microcontroller

programmed with 0410508A.

hex (IC1)

1 7805 +5V regulator (REG 1)

llN4148 diode (01)

1 1N4004 diode (02)

value of the unknown component

using one of the equations shown in

the lower section of the equations box

- ie, the section labelled "In Measure

ment Mode".

From these equations, you can see

that the micro has some fairly solid

"number crunching" to do, both in the

calibration mode when it calculates

the values of Ll and Cl and then in

the measurement mode when it cal

culates the value of Cx or Lx. Each of

these values needs to be calculated to a

high degree of resolution and accuracy.

To achieve this, the micro's firmware

needs to make use of some 24-bit float

ing point m:aths routines.

Circuit details

How this ingenious yet simple meas

urement scheme is used to produce a

practical LC meter can be seen from the

full circuit diagram of Fig.2. It's even

simpler than you might have expected

because there's no separate comparator

to form the heart of the measurement

oscillator. Instead we're making use

of a comparator that's built into the

microcontroller (lCl) itself.

As shown, microcontroller ICl is a

PIC16F628A and it actually contains

two analog comparators which can be

configured in a variety of ways. Here

we are using comparator 1 (CMP1) as

the measurement oscillator. Compara

tor 2 (CMP2) is used only to provide

some additional "squaring up" of the

output from CMPl and its output then

drives the internal frequency counting

circuitry.

The oscillator circuitry is essentially

unchanged from that shown in Fig.l.

Note that the micro controls RLYl

(which switches calibrating capacitor

C2 in and out of circuit) via its I/O

port B's RB7 line (pin 13). Diode Dl

prevents the micro's internal circuitry

from being damaged by inductive

spikes when RLYl switches off.

In operation, ICl senses which po

sition switch Sl is in using RB6 (pin

12). This is pulled high internally

when Slb is in the "C" position and

low when Slb is in the "L" position.

Crystal Xl (4MHz) sets the clock fre

quency for IC1, while the associated

33pF capacitors provide the correct

loading to ensure reliable starting of

the clock oscillator.

The results of ICl 's calculations are

displayed on a standard 2x16 line LCD

module. This is driven directly from

the micro itself, via port pins RBO-RB5.

siliconchip.com.au

Capacitors

1 221lF 16V RS electrolytic

2 lOIlF 16V RS electrolytic

1 10llF 16V tantalum

1 100nF monolithic

2 1nF MKT or polystyrene (1

if possible)

2 33pF NPO ceramic

Resistors (O.25W,

3100kQ

168kQ

147kQ

24.7kQ

41kQ

nected into circuit, F

out

simply cor

responds to the resonant frequency of

Ll, Cl and any stray capacitance that

may be associated with them.

When power is first applied to the

meter, the microcontroller measures

this frequency (Fl) and stores it in

memory. It then energises reed relay

RLY1, which switches capacitor C2

in parallel with Cl and thus alters the

oscillator frequency (ie, it lowers it).

The microcontroller then measures

and stores this new frequency (F2).

Next, the microcontroller uses these

two frequencies plus the value of C2

to accurately calculate the values of

both Cl and Llo Uyou're interested, the

equations it uses to do this are shown

in the top (Calibration Mode) section

of the box titled "How It Works: The

Equations".

Following these calculations, the

microcontroller turns RLYl off again

SIUCON CHIP

to remove C2, allowing the oscillator

frequency to return to Fl. The unit is

now ready to measure the unknown

inductor or capacitor (Cx or Lx).

As shown in Fig.l, the unknown

component is connected across the test

terminals. It is then connected to the

oscillator's tuned circuit via switch Slo

When measuring an unknown capaci

tor, Sl is switched to the "C" position

so that the capacitor is connected in

parallel with Clo Alternatively, for an

unknown inductor, Sl is switched to

the "L" position so that the inductor

is connected in series with Llo

In both cases, the added Cx or Lx

again causes the oscillator frequency

to change, to a new frequency (F3).

As with F2, this will always be lower

than Flo So by measuring F3 as before

and monitoring the position of Sl

(which'is done via the C/L-bar line),

the microcontroller can calculate the

Trimpot VRI allows the LCD contrast

to be optimised.

Firmware

link functions

The firmware in ICI is designed to

automatically perform the calibration

function just after initial start-up.

However, this can also be performed

at any other time using switch S2.

Pressing this switch simply pulls the

micro's MCLR-bar pin (4) down, so

that the micro is forced to reset and

start up again, recalibrating the circuit

in the process.

Links LKI-LK4 are not installed for

normal use but are used for the initial

setting up, testing and calibration. As

shown, these links connect between

RB3-RBO and ground respectively.

For example, if you fit LKI and then

press S2 to force a reset, the micro will

activate RLYI (to switch capacitor C2

into circuit) and measure oscillator

frequency F2. This is then displayed

on the LCD.

Similarly,

you fit LK2 and press

S2, the micro simply measures the

initial oscillator frequency (Fl) and

displays this on the LCD. This allows

you to not only make sure that the os

cillator is operating but you can check

its frequency as well. We'll have more

to say about this later.

LK3

LK4 allow you to perform

manual calibration "tweaks" to the

meter. This is useful if you have access

to a capacitor whose value is very ac

curately known (because it has been

measured using a full-scale LCR meter,

for example).

With LK3 fitted, the capacitance

reading decreases by a small amount

each time it makes a new measure

ment (which is about five times per

second). Conversely, if LK4 is fitted

instead, the microcontroller increases

the capacitance reading by a small

increment each time it performs a new

measurement.

Each time a change is made, the ad

justment factor is stored in the micro's

EEPROM and this calibration value is

then applied to future measurements.

Note also that although the calibration

is made using a "standard" capacitor,

it also affects the inductance measure

ment function.

In short, the idea is to fit the jumper

to one link or the other (ie, to LK3 or

LK4) until the reading is correct. The

link is then removed.

As mentioned above, links LKI-LK4

are all left out for normal operation.

siliconchip.com.au

JAYCAR 16x2 LCD MOOUIfQP-6515/QP-5516

1 - - - . - - - - - - - - - - - - - - - - - - - - - -...,

'--------1

Fig.3: follow this layout

diagram to build the Digital

LC Meter but don't solder

in the switches or the test

terminals until after these

parts have been mounted

on the front panel. The 2·

way pin headers for

links

LKt·LK4

are

installed on

the copper side of the board

- see text.

The PC

board

usembJy

atbu:hed to the case lid using M3 x t5mm spacers

and M3 x

8JDm

csk·head

machine screws. Make sure that the assembly

secll1"8 before

IOIderJns

the

switch

lup and test terminals.

They're only used for troubleshooting

and calibration.

Power supply

Power for the circuit is derived from

an external 9-12V DC source. This can

come from either a plugpack supply

or from an internal 9V battery. The

switched DC input socket automati

cally disconnects the battery when the

plugpack supply is connected.

The incoming DC rail is fed via re

verse polarity protection diode D2 and

power switch 83 to regulator REG1 - a

standard 7805 device. The resulting

+5V rail at REG1's output is then used

to power ICl and the LCD module.

Const!-"uction

Because it uses so few parts, the unit

is very easy to build. All the parts, ex

cept for switches 81-83 and the Cx/Lx

input terminals, are mounted on a PC

board coded 04105081 and measur

ing 125 x 58mm. The LCD module

connects to a 7x2 DIL pin header at

one end of the board and is sup

ported at either end using M3

Nylon screws and nuts.

Fig.3 shows the parts layout

on the PC board. Here's the

suggested order of fitting the

components to the PC board:

(1). Fit DC power connec

tor CONl and the two Imm PC

board terminal pins for the internal

battery connections.

(2). Fit the six wire links, four of

which go under where the LCD mod

ule is later fitted. Don't forget the link

immediately below switch 81.

(3). Install the 4x2 DIL pin header

used for links LKI-LK4. Note that this

item must be mounted on the copper

side of the board (not on the top), so

that a jumper can later be fitted to any

of the links when the board assembly

is attached to the box lid).

To install this header, just push the

ends of the longer sides of the pins into

the board holes by 1-2mm, then solder

them carefully to the pads. That done,

push the plastic strip down the pins so

that it rests against the solder joints,

No.

Value

100kQ

68kQ

47kQ

4.7kQ

1kQ

4-Band Code (1%)

brown black yellow brown

blue grey orange brown

yellow violet orange brown

yellow violet red brown

brown black red brown

5·Band Code (1%)

brown black black orange brown

blue grey black red brown

yellow violet black red brown

yellow violet black brown brown

brown black black brown brown

SILICON CHIP

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