e00a060.pdf

BASIC CIRCUITS

One-Minute Light

Elementary Circuit

by B. Kainka

Loss-free switching, using a Field Effect Transistor (FET) and a capacitor‘s

discharge function, is the base theme of this elementary circuit.

La1

The one-minute light circuit shown

in Figure 1 consists of six compo-

nents, whose functions are easy to

point out. The power source is a 6-

volt battery, paralleled to the branch

point with a lamp and transistor, act-

ing an on/off switch.

Likewise, parallel to the battery

is a node linking a pushbutton

switch coupled to a very high ohmic

valued resistor. The BUZ10 is a Field

Effect Transistor (FET), which differ-

entiates itself from a normal transis-

tor, in so much that almost no base

current is needed for it to switch on

— instead, a voltage at the Gate pin

is required. Thus switching without

performance loss is possible, which

is extremely practical for battery

switching circuits.

The voltage potential between

the pushbutton switch and resistor

determines whether current passes

to the lamp through the switching

transistor or not.

teristics of this transistor in Figure 2

shows, when the Gate/Source-Volt-

age V GS is +6 V, a maximum current

of 12 A flows through the Drain-

Source junction. Strictly speaking,

the diagram is only valid for a Drain-

Source-Voltage of 25 V. Actually, such

a high current does not ﬂow, due to

on the one hand the battery having

an relatively high internal resistance

and on the other hand the sudden

resistance change of the lamp fila-

ment from a low ohmic value in its

cold state to about 60 Ω in a few mil-

liseconds as the ﬁlament glows hot.

The current flowing through this

branch is easy determined using

Ohm’s law.

100mA

BT1

BUZ10

100µ

004107 - 11

Figure 1. One –minute light circuit diagram —

there are just of six components.

Typical Transfer Characteristics

I = U / R = 6 V / 60 Ω

= 100 mA

I D =f ( V GS ); V GS = 25V

s pulse test, T j = 25 °

By the way, the potential loss across

the conducting FET is marginal

thanks to the low drain-source resis-

tance of less than 0.1

I D

[A]

Standby mode

Discharging

In standby mode the pushbutton is

open. The capacitor is discharged

through the resistor. The transistor

then blocks, allowing no trickle cur-

rent to ﬂow to the lamp.

Pushing on the button, something

else also happens, the capacitor gets

a kick-start charge of +6 V. When

releasing the pushbutton, this poten-

tial remains at the Gate of the

BUZ10, due to the fact that the

capacitor can only discharge itself

very slowly through the high ohmic

value resistor. The voltage across the

capacitor u e decreases not linearly,

but rather in an exponential fashion:

Switch mode

V GS

[V]

One push on the button instanta-

neously brings a potential of +6 V to

the node. This voltage is also seen at

the Gate of the Field Effect Transis-

tor BUZ10. As the Transfer charac-

004107 - 12

Figure 2. The BUZ10 Field Effect Transistor

Transfer characteristics.

u c = U

e -t/RC

Elektor Electronics

10/2000

BASIC CIRCUITS

Here, U is the initial capacitor poten-

tial and e the base of natural loga-

rithms, or 2.718. The corresponding

curve (Voltage against Time) is

shown in Figure 3 .

Due to the fact that the capacitor

is slowly discharged, and the BUZ10

conducting sufficiently until the Gate

voltage drops to about 3.75 V, the

lamp glows a short spell after the

pushbutton is released.

The afterglow effect can even be

determined exactly, that is, mathe-

matically. This involves solving the

above formula in the time domain

and using an instantaneous voltage

of u c = 3.75 V:

1.0

0.8

0.625

U C

0.6

0.4

0.2

ln ( u c / U )

= –(4,7

10 6

Ω ⋅

100

10 -6 F)

ln (3.75

t [s]

V/6 V)

= –470 s

004107 - 12

ln 0.625

= –470 s ⋅ –0.47

= 221 s

Figure 3. The discharge curve of an ideal capacitor.

not be attained due to the fact that

capacitors (especially electrolytic

ones) also discharge themselves.

This theoretical value will however

(004107-1)

10/2000

Elektor Electronics

t= RC

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