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SMALL CIRCUITS COLLECTION

001

Electrically Isolated

Zero-Crossing Detector

C. Volt

All power control circuits that use phase

control require an adjustable delay element

synchronised to the mains voltage. The

adjustable delay determines the phase

angle at which the thyristor, triac or tran-

sistor connects the mains voltage to the

load. Simple passive dimmer circuits use

RC phase shifters having a ﬁxed relation-

ship to the mains frequency. Nowadays,

microprocessors are often used for preci-

sion controllers and complex control tasks.

A zero-crossing detector is thus required

for synchronisation with the mains fre-

quency. If the microprocessor does not

have to be electrically isolated from the

mains, the detector can be constructed

very simply: a voltage divider and two pro-

tection diodes at the interrupt input of the

microcontroller are sufficient.

However, if the microcontroller must be

electrically isolated from the mains net-

work, a more elaborate solution is neces-

sary. Due to reactive, non-uniform loads,

zero crossing points can be exactly deter-

mined on the secondary side of the trans-

former only under certain conditions. The

non-linear transfer characteristic often

causes the secondary voltage to be

deformed and offset in phase, so it cannot

be assumed to be a clean, phase-aligned

sine-wave signal.

Using a separate, dedicated secondary

winding just for zero-crossing detection (or

even a separate transformer with the

appropriate protection class) is not partic-

ularly elegant in terms of circuit design, nor is it especially eco-

nomical in terms of power consumption, due to unavoidable

internal losses.

The circuit proposed here derives zero crossings directly

from the primary-side mains voltage, using an optocoupler for

electrical isolation. It uses only inexpensive standard compo-

nents. Furthermore, it features low power consumption and

small thermal dissipation. It generates precisely timed, clean

and uniform pulses for all zero crossings (thus 100 times per

second).

U+

R6

R7

7V3

D1

R1a

330k

K2

R4

R5

INT

0ms3

1N4148

R1

T3

10ms

K1

IC1

GND

1

4

BC

547B

B1

230V

S250

R2

2

3

T2

PC817

T1

R2a

100k

2x

BC547B

R3

C1

220n

50V

024026 - 11

1

6

1

6

2

5

2

5

3

4

PC817

CNY17

age (smoothed to around 7.3 V by R1/C1), as are the zero-

crossing pulses (via voltage divider R2/R3). The amount of

charge provided on each half-cycle is dimensioned to be just

enough to provide power to the subsequent stage for the next

zero-crossing pulse. Diode D1 prevents current from ﬂowing

from C1 back into the base of T1 when a zero crossing occurs,

since otherwise T1 could not detect zero crossings.

The circuit…

A full-wave rectiﬁer (B1) generates a pulsating DC voltage. The

supply voltage for the optocoupler is derived from this volt-

20

Elektor Electronics

12/2002

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SMALL CIRCUITS COLLECTION

R1

R1A

024026-1

K2

D1

H2

+

IC1

C1

K1

T2

B1

H4

T1

T3

024026-1

COMPONENTS LIST

The circuit generates zero-crossing pulses as follows: tran-

sistor T2 receives its base current directly from the rectiﬁed

mains voltage via high-resistance resistor R4. Due to the rapid

rise of the mains voltage, T1 is almost always switched fully

on, being cut off for only approximately 50 µs before and after

each zero crossing. Inverter T2 causes a current limited to

approximately 15 mA by R5 to ﬂow through the LED of the

optocoupler for the exact duration of the 100-µs zero-crossing

pulse. Since the LED of the optocoupler is driven only by very

short pulses, the exact magnitude of the current is not critical.

The phototransistor is wired conventionally. Here it is con-

nected to an inverting transistor, since a positive pulse was

needed for the interrupt input of a microcontroller.

Semiconductors:

B1 = B250C500 or 4 off

1N41004

D1 = 1N4148

T1,T2,T3 = BC547B

IC1 = PC817 (Sharp)

Resistors:

R1,R1A = 330k

R2 = 120k Ω

R2A,R4 = 100k

R3 = 10k Ω

R5 = 270

Ω

R6,R7 = 10k Ω

Miscellaneous:

K1 = 2-way PCB terminal

block, lead pitch 7.5mm

K2 = 3-way PCB terminal

block, lead pitch 5mm

Capacitor:

C1 = 220nF, 50V

into account in the layout, particularly the region of the opto-

coupler. By contrast, a separation of 3 mm must be main-

tained between components bearing mains voltage. In oper-

ation, only R2 becomes slightly more than lukewarm (but

please don’t try to verify this yourself!). The four-pin Sharp

optocoupler is ﬁtted at an angle, as can be seen in the com-

ponent layout, while the alternative CNY17-2 is ﬁtted straight.

The PCB shown here is unfortunately not available ready-

made.

…and the PCB

Nearly the entire mains voltage appears across R1 and R2. For

this reason, each of these resistors is split into a pair of resis-

tors on the printed circuit board (R1 + R1A and R3 + R2A).

All other components and the associated board layout can be

designed for voltages less than 30 V. Naturally, adequate iso-

lation distance (> 6 mm) for electrically isolated circuit ele-

ments bearing a protective low voltage must always be taken

(024026-1)

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