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028

AC Controller using MOSFETs

Particularly with low voltages, triacs are usually used as

control elements for ac voltages. The disadvantage, as so

often is the case, is in the power dissipated in the control

element, which is quite evident for currents greater than

1 ampère. In such cases, it is essential to use a heatsink for

the triac. If you want to control the brightness of a halogen

lamp using such an arrangement, for example, the voltage

drop across the triac also results in a significant reduction

in the maximum brightness of the lamp.

This disadvantage can be avoided by using two MOSFETs

for the control element, in place of a triac. The trick here is

to connect the two MOSFETs in series with opposite polar-

ity, with the gates connected in parallel to the control cir-

cuit. The junction of the two gate leads represents the virtual

ground of the circuit, which forms the reference for all other

potentials. Modern MOSFETs, such as the 20N03 from On

Semiconductor ( www.on-semi.com ) with an R DS(ON) of

0.035

D1

La1

T1, T2 = MTD20N03

1N4001

T1

D

G

µC

R1

100 Ω

S

T2

S

D3

C1

G

100µ

10V

D

5V1

500mW

D2

1N4001

014086 - 11

MOSFET with a low gate voltage (preferably a ‘logic FET’)

can be used.

Another benefit of this circuit is its ‘zero-power’ gate drive,

in contrast to triacs, which require drive currents of 10 mA

or more. This means that any microcontroller, TTL gate or

555 timer IC can be used as the controller.

, can be used in this circuit for controlling a 50-W

halogen lamp without any supplementary heatsink. The

loss in brightness is negligible, since the voltage drop is

only (0.035

4.2 A) = 0.147 V. Of course, you do not nec-

essarily have to use the 20N03; in principle, any n-channel

Ω ×

(014086-1)

42

Elektor Electronics

7-8/2001

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