Thyristors, DIAC, SCR, TRIAC

1. THRYSTORS
Electrónica
Analógica
Eduardo Emanuel Tovias
Garza
Engineer Aurelio García
2A2M
Técnico
Mecatrónica Área
Automatización
10th August 2020 - Matamoros,
Tamaulipas

2. Material Structure,Symbol and
Terminals of SCR
Sillicon Controlled Rectifier(SCR)
A silicon controlled
rectifier or semiconductor
controlled rectifier is a four-
layer solid-state current-
controlling device. The
principle of four-layer p–n–p–
n switching was developed by
Moll, Tanenbaum, Goldey and
Holonyak of Bell
Laboratories in 1956.

3. Equivalent Circuit of SCR Using
BJT’S
A thyristor is a solid-
state semiconductor
device with four layers of
alternating P- and N-
type materials. It acts
exclusively as
a bistable switch, conducting
when the gate receives a
current trigger, and
continuing to conduct until
the voltage across the device
is reversed biased, or until the
voltage is removed (by some
other means).

4. Characteristic curve of SCR
Thyristors are mainly used
where high currents and
voltages are involved, and are
often used to control alternating
currents, where the change of
polarity of the current causes
the device to switch off
automatically, referred to as
"zero cross" operation.

5. Description of Electrical
Parameters of SCR
Holding current (IH)
Holding current This is the current level circulating through anode and
cathode (or A2 and A1 for a TRIAC) under which the device turns off,
without gate current.
Latching current (IL)
Latching current This is the current level circulating through anode and
cathode (or A2 and A1 for a TRIAC) to keep the device conducting after
removal of the gate current.
Peak Repetitive Off State Forward Voltage (vDRM) This is the maximum peak voltage allowed across the device (50-60Hz).
Peak Forward Blocking Current (IDRM)
It specifies the peak forward transient voltage that a SCR
can block repeatedly or periodically in forward blocking mode.
Peak Repetitive Off State Reverse Voltage (vRRM)
This is the maximum peak voltage allowed across the device. This
parameter is specified up to the maximum junction temperature and the
leakage currents.
Peak Reverse Blocking Current (IRRM)
With no gate signal applied, peak reverse blocking current is the
maximum reverse current that flows with the SCR in the blocked (Off)
state. Peak reverse blocking current is equal to peak forward blocking
current but in the opposite direction.

6. Description of Electrical
Parameters of SCR
Peak Forward On State Voltage (VTM)
This is the voltage across the device while it is on-state. It is
specified at the peak current corresponding to the IT(RMS) current
of the device.
Peak On State RMS Current (ITRMS)
This is the maximum rms current allowed in the device for a
specified case temperature (Tc), or ambient temperature (Ta) or lead
temperature (Tl ), depending on the type of package.
Peak On State Average Current (ITAVE):
This is the maximum average current allowed in the SCR at a
specified case temperature (Tc), or ambient temperature (Tamb) or
lead temperature (Tl ), depending on the type of package.
Peak Non Repetitive Surge Current (ITSM)
This is the maximum peak current allowed in the device under pulse
conditions. For TRIACs, it is defined for a single full cycle sine wave
of 20 ms corresponding to the 50 Hz mains, and 16.6 ms for the 60
Hz mains. If the absolute rating is exceeded, the component may be
damaged.
Gate Trigger Voltage (VGT)
•This is the voltage to apply across gate and cathode (or gate and
electrode A1 for TRIAC) to reach the IGT current and then to trigger
the device.
Gate Trigger Current (IGT)
This is the current to apply between gate and cathode (or gate and
electrode A1 for TRIAC) to turn-on the device. This parameter
defines the sensitivity of the component

7. How to test a SCR using a
ohmeter
Connect the negative lead of your ohmmeter to the
anode of the SCR and the positive lead to the
cathode of the SCR.

8. Datasheet SCR C106B

9. Example – DC Motor Speed
Control Using an SCR
Consider the figure in which SCRs
are used to control the speed of
the DC motor. As we that DC
motor consists of a field and
armature windings. By controlling
the voltage applied to the
armature, the speed of the DC
motor is controlled.
The AC mains supply is connected
to transformer primary and to the
secondary winding , two SCRs are
connected in parallel as shown in
figure. The output from these
SCRs drives the DC motor. The
field winding is connected
through the diodes which gives
uncontrollable DC power to the
field winding.

10. DIAC (Diode for Alternating
Current)

11. Material Structure, Symbol and
Terminals of DIAC

12. Characteristic Curve of DIAC

13. Datasheet DIAC D83

14. Example Using a DIAC
This circuit starting with the Diode
(D1) transforms the actual current in
a one ripple shape with a positive
Pick, the resistor transforms the
majority high current, the capacitors
in decoupling smooth the current
and the DIAC triggers it to the LED

15. Triode for Alternating Current
(TRIAC)

16. Material Structure, Symbol and
Terminals of TRIAC

17. Equivalent Circuit of TRIAC Using
SCR’S

18. Characteristic Curve of
TRIAC

19. Description of Electrical
Parameters of TRIAC
Holding current (IH)
Holding current This is the current level circulating through anode
and cathode (or A2 and A1 for a TRIAC) under which the device
turns off, without gate current.
Latching current (IL)
Latching current This is the current level circulating through anode
and cathode (or A2 and A1 for a TRIAC) to keep the device
conducting after removal of the gate current.
Peak Repetitive Off State Forward Voltage (vDRM)
This is the maximum peak voltage allowed across the device (50-
60Hz).
Peak Forward Blocking Current (IDRM)
It specifies the peak forward transient voltage that a SCR
can block repeatedly or periodically in forward blocking mode.
Peak Repetitive Off State Reverse Voltage (vRRM)
This is the maximum peak voltage allowed across the device. This
parameter is specified up to the maximum junction temperature and
the leakage currents.
Peak Reverse Blocking Current (IRRM)
With no gate signal applied, peak reverse blocking current is the
maximum reverse current that flows with the SCR in the blocked
(Off) state. Peak reverse blocking current is equal to peak forward
blocking current but in the opposite direction.
Peak Forward On State Voltage (VTM)
This is the voltage across the device while it is on-state. It is
specified at the peak current corresponding to the IT(RMS) current
of the device.

20. A multimeter can be used to test
the health of a triac. First put the
multimeter selector switch in a
high resistance mode (say 100K),
then connect the positive lead of
multimeter to the MT1 terminal of
triac and negative lead to the MT2
terminal of triac (there is no
problem if you reverse the
connection).
The multimeter will show a high
resistance reading (open
circuit).Now put the selector switch
to a low resistance mode, connect
the MT1 and gate to positive lead
and MT2 to negative lead. The
multimeter will now show a low
resistance reading (indicating the
switch ON).If the above tests are
positive then we can assume that
the triac is healthy. Anyway this
test is not applicable triacs that
require high gate voltage and
current for triggering.
How to Test a TRIAC
100KΩ

21. Datasheet TRIAC BT138-600E

22. Example – AC Motor Speed
Control Using A TRIAC
How dimmer / AC motor speed controller works?
Resistor R3 and capacitor C3 are used to filter the high voltage
transients that may appear on the circuit. The potentiometer P
and capacitor C2 are elements necessary to trigger the triac.
(using phase control)
The triac controls the flow of alternating current to the load,
switching between the conduction state and the cut-off state,
during the positive and negative half cycles of power supply
(110/220 VAC that comes from the electric outlet of our homes).
It should be clarified that the capacitor in an AC circuit, has an
out of phase voltage with respect to the original signal. The
following image shows the moments the TRIAC is triggered
(small red triangles) and the voltage waveform that reaches the
load.
Moving the potentiometer knob, we can control the amount of
current flowing to the load and thus the electrical power
dissipated in it. To correct the hysteresis of the
TRIAC, resistors R1 and R2 an capacitor C1 are used.
Shape of the ripple