Learn The Easy How
You Can Build Your Own SCR/TRIAC
Although you don’t
find SCR and Triac in all electronic equipment, this does not mean
that both are not important components. You need to learn how to
test it so that when you come across one in the future you will
know if it is good or bad during troubleshooting. You can test
these components using meters but with the SCR/Triac tester you
will find that it is easier to test as compare to using Multimeter.
Before I start describing how to build this tester, I guess that it
is important to know some of the basic operation of SCR and
SCR stands for
silicon-controlled rectifier (or semiconductor-controlled
rectifier). It is a four-layer solid state device having an input
control terminal (gate-G), an output terminal (anode-A) and a
terminal common to both input and output (cathode-C or Kathode-A).
It generally operates as an AC switch for lighting and heating
control. In the normal "off" state, the SCR restricts current to
the leakage current. When the gate-to-cathode voltage exceeds a
certain threshold, the device will turns "on" and conducts current.
The device will remain in the "on" state even after gate current is
removed so long as current through the device remains above the
holding current. Once the current falls below the holding current
for an appropriate period of time, the device will switch "off".
The SCR can be found in switch mode power supplies (SMPS). For your
information not all SMPS use SCR.
The TRIAC is a
three-terminal device similar in construction and operation to the
SCR. The TRIAC controls and conducts current flow during both
alternations of an ac cycle, instead of only one. Both the SCR and
the TRIAC have a gate lead. However, in the TRIAC the lead on the
same side as the gate is "main terminal 1-MT1, T1 or even A1," and
the lead opposite the gate is "main terminal 2-MT2, T2 or even
It can be triggered
by either a positive or a negative voltage being applied to its
gate electrode (with respect to T1, otherwise known as MT1 or A1).
Once triggered, the device continues to conduct until the current
through it drops below a certain threshold value, the holding
current, such as at the end of a half-cycle of alternating current
(AC) mains power.
A TRIAC is generally
used for motor speed control and in light dimmer. If you repair
Laser printer you will find a Triac in the power supply area to
control the heating element.
Diagram Of The SCR/Triac Tester
Components needed to
build the SCR/Triac Tester
1- 1x circuit
2- 1x small
4- 3x 30cm
5- 9 volt
6- 9 volt battery
7- 1x 270 ohm ¼ watt
8- 12 volt 3 watt
9- 12 volt light
10- 3x test clips
(preferably three different colors)
11- 1x on/off
12- 1x normally open
switch (push to on switch)
13- 4x circuit board
You may also
1-A power drill to make holes in
the circuit board and the small box.
2-Double sided tape to secure the
3-Sand paper to clean up the
dirty layer on the circuit track
4-Different color of
5-A pen knife to cut off unwanted
circuit track and
6-Printed words or alphabet plus
glue for labelling
7-Different part number of SCR
and Triac for testing purposes.
The procedure of
fixing this tester can be seen from the photos
Please click on the
youtube video to see how I test the MCR100-6 SCR.
pinout of a Triac is usually T1 (MT1 or A1), T2 (MT2 or A2) and
Gate (G). Just connect the test probes as follows: The 'K' probe to
'T1', the 'A' probe to 'T2' and the 'G' probe to 'GATE'. Turn on
the switch and then press the Gate button on the tester and you
should expect the lamp to light.
Conclusion-By having this tester you will not have doubt if
the SCR or the Triac will breakdown when under load as compare to
using multimeter to test it. By looking at the light bulb you can
judge if the component is working or not. If there is a constant
light you can be sure it is working. If the light have intermittent
flashes and do not light at all then you can suspect that the
component under test may have problem. I suggest that you compare
with a known good working component for best result. If using 9
volt does not provide a good result to you then try use 12 vdc to
retest and see the result. By the way , you have the choice on how
you place the circuit board or components, this entirely depends on
your preference and not necessary have to follow the above design.
Take care and may God always bless you and your