Specifications

CIRCUIT IDEAS
current converter. The design procedure
is as follows:
Calculate R
I
according to the applica-
tion from one of the following equations:
(a) DC voltmeter: R
IA
= full-scale E
DC
/I
FS
(b) RMS AC voltmeter (sine wave only):
R
IB
= 0.9 full-scale E
RMS
/ I
FS
(c) Peak reading voltmeter (sine wave
only): R
IC
= 0.636 full-scale E
PK
/I
FS
(d) Peak-to-peak AC voltme-
ter (sine wave only): R
ID
= 0.318
full-scale E
PK-TO-PK
/ I
FS
The term I
FS
in the above
equations refers to meters full-
scale deflection current rating in
amperes.
It must be noted that neither
meter resistance nor diode volt-
age drops affects meter current.
Note: The results obtained
during practical testing of the cir-
cuit in EFY lab are tabulated in
Tables I through IV.
A high-input-resistance op-
amp, a bridge rectifier, a
microammeter, and a few other
discrete components are all that
are required to realise this versatile cir-
cuit. This circuit can be used for mea-
surement of DC, AC RMS, AC peak, or
AC peak-to-peak voltage by simply chang-
ing the value of the resistor connected
between the inverting input terminal of
the op-amp and ground. The voltage to
be measured is connected to non-invert-
TABLE II
Position 2 of Function Switch
E
rms
input Meter Current
5V 46 µA
4V 36 µA
3V 26 µA
2V 18 µA
1V 10 µA
H
ere is a very simple circuit which
can be used for testing of SCRs
as well as triacs. The circuit
could even be used for checking of pnp
and npn transistors.
The circuit works on 3V DC, derived
using a zener diode in conjunction with a
step-down transformer and rectifier ar-
rangement, as shown in the figure. Alter-
natively, one may power the circuit using
two pencil cells.
For testing an SCR, insert it in the
socket with terminals inserted in proper
slots. Slide switch S3 to on position (to-
wards ‘a’) and press switch S1 momen-
tarily. The LED would glow and keep
glowing until switch S2 is pressed or
mains supply to step-down transformer
is interrupted for a short duration using
switch S4. This would indicate that the
SCR under test is serviceable.
With switch S3 in off position (to-
wards b), you may connect a
RUPANJANA
milliammeter or a multimeter to monitor
the current flowing through the SCR. If
the SCR is no good, the LED would never
glow. If the SCR is faulty (leaky), the LED
would glow by itself. In other words, if
the LED glows only on pressing switch
S1 momentarily and goes off on pressing
switch S2, the SCR is good.
For testing a triac, initially connect
its MT1 terminal to point A (positive),
MT2 to point K (negative), and its gate to
point G. Now, on pressing switch S1 mo-
mentarily, the LED would glow. On press-
ing switch S2 momentarily, the LED
would go off. Next, on pressing switch S5,
the LED will not glow.
Now reverse connections of MT1 and
TRIAC/TRANSISTOR CHECKER
PRAVEEN SHANKER
Fig. 1
Fig. 2
TABLE III
Position 3 of Function Switch
E
Pk
input Meter Current
5V peak 46 µA
4V peak 36 µA
3V peak 26 µA
2V peak 16 µA
1V peak 6 µA
TABLE IV
Position 4 of Function Switch
E
Pk-To-Pk
Meter Current
5V peak to peak 46 µA
4V peak to peak 36 µA
3V peak to peak 26 µA
2V peak to peak 16 µA
1V peak to peak 7 µA
ing input of the op-amp.
38