IN-CIRCUIT TRANSISTOR/SCR/DIODE GO-NOGO TESTER
The design shown will test PNP and NPN transistors, diodes and SCRs
both "in-situ" (equipment of course de-energised) and also by direct
connection to a stand-alone component.
It is a simple GO/NOGO test
which can identify diode and transistor action and will indicate diode
polarity and transistor type PNP/NPN, if this is unknown.
A 555 square-wave
oscillator gives a 2Hz square wave O/P and the second 555 inverts the 2Hz signal.
two complementary square wave voltages are used as the test supply
voltage to the Device Under Test (D.U.T.).
Transistor base bias is via
a 1000-Ohm resistor.
Two red LEDs are paralleled in ‘contra’ fashion
and connected across the output.
Current flow through either led is
limited by the 470 Ohm resistor and either LED’s P.I.V. is clamped to
about 1.7v, which is the ON-state voltage of the other LED (being in
the conducting state).
With no D.U.T. connected to the tester, and TEST
push button pressed, both LEDs will flash alternately.
If a healthy transistor is now connected: during the half-cycle
during which it will be biased ON and therefore heavily conducting,
LED will be effectively short-circuited and remain dark.
alternate half-cycle, the polarity of the applied signal will not cause
the D.U.T. To switch ON and current can then flow through one led
causing it to illuminate.
This appears as a 2Hz flashing of the LED.
It should therefore be evident that if the D.U.T. is:
- Shorted; both LEDs will be dark and
- Open Cct; both LEDs will flash.
The purpose of the two strings of series connected silicon diodes,
connected in series with the D.U.T. may require some explanation:
Their function is to allow only a current pathway through the D.U.T.
provided that it is in fact fully saturated (turned hard-ON) and that
insufficient current could flow through parallel circuit resistances
such that one or both of the LEDs would be dark due to this.
Remember this design is called an "in-circuit" tester (no messy de-soldering, to isolate a suspect faulty semiconductor!).
In order to test SCRs and diodes, S1 is thrown to the appropriate
in which one of each of the two series connected pair of
diodes is switched out.
This is necessary because: the forward ON-state
voltage of a healthy diode or SCR being about 0.7 volt,
conducting series junctions would present about 2.1 Volt across the
LED, which should normally be dark.
Therefore the LED would not extinguish as it should and therefore
give an erroneous test.
The Gate voltage for the SCR is via a 100-Ohm
Suffice to say that I have constructed this design and it has
provided good results with both small signal and power type
transistors, SCRs and diodes of various ratings.
From the picture shown
and the schematic, a simple front panel graphic is provided this
indicates what all the flashing and unlit LEDs actually mean!
used E-Z type hooks to connect the device to be tested.
Supply is a 9v
The original circuit and magazine scan.