STATIC CHARACTERISTICS OF SILICON CONTROLLED RECTIFIER
INTRODUCTION
An SCR is so called because silicon is used for its construction and its operation as a
rectifier (very low resistance in the forward conduction and very high resistance in the reverse
direction) can be controlled. Like the diode, a SCR is a unidirectional device that blocks the
current flow from cathode to anode. Unlike the diode, a SCR also blocks the current flow from
anode to cathode until it is triggered into conduction by a proper gate signal between gate and
cathode terminals.
For engineering applications of SCRs, their terminal characteristics must be known. In
this experiment we are studying the V-I characteristics of SCR.
SPECIFICATIONS
Sl.No. SPECIFICATIONS RATINGS
1 Standard voltage rating of SCR used : 400 V
2 IRMS (RMS value of current) :4A
3 Fuse :1A
PROTECTIONS
1. The device is mounted on proper heat sink.
2. Fuse is provided for the over current protection.
FRONT PANEL DETAILS
1. The terminals of the SCR are connected to front panel and marked the terminals as A for
anode, C for cathode and G for gate.
2. A replaceable fuse is provided on front panel in series with the anode terminal.
3. Two variable power supplies are provided
i. 0 – 15V/250mA
ii. 0 – 35V/500mA.
4. Variable resistance (2.5 Kohms/25W) is provided for loading.
5. Three no’s digital indicators are provided, two digital ammeter for measuring IG and IA
another digital voltmeter for measuring anode current VAK.
6. All the terminals are brought out to front panel through BTI-15 terminals for
connections purposes. Students should externally interconnect components and meters
to get characteristics.
HOW TO CONDUCT EXPERIMENT
AIM: To study the static characteristics of a SCR.
APPARATUS: SCR Characteristics study unit, connecting wires.
THEORY: The silicon controlled rectifier (SCR) is a semiconductor device which acts as an
electronic switch. SCR is the most popular member of the thyristor family. It is a four layer
three junction PNPN device having three terminals and is used as a controlled switch. It
converts AC to DC and also controls the amount of power fed to the load. SCR has two states,
i.e. either it does not conduct or it conducts heavily. There is no state in between. SCR behaves
like a switch. There are two ways to turn on the SCR, the first method is to keep the gate open
and make the supply voltage equal to the break over voltage. The second method is to operate
SCR with supply voltage less than break over voltage than turn it on by means of a small current
applied to the gate.
SCR Forward characteristics
SCR symbol is shown in fig. 1.1 and SCR structure is shown in fig. 1.2. Fig. 2.1 shows
two transistor model of SCR & fig. 2.2 shows equivalent circuit of SCR. When SCR anode is
connected to positive through an external resistance, Junction J1 (P1 N1) and junction J3 (P2 N2)
are forward biased. But junction J2 is reverse biased. Hence the total voltage applied to the
device appears across reverse biased junction J2. The block over capability of this junction is the
maximum forward blocking voltage with gate open. When a small gate current is applied
between gate and cathode (i.e. junction J3 (P2 N2), the junction tries to conduct resulting in
reducing the depletion layer on P2 side. Hence the forward break over voltage decreases. If the
gate current is increased, the forward break over voltage further decreases. When SCR is fired
gate loses its control, the current in the circuit is maintained by the external load resistance. The
anode current must be more than a value known as latching current IL, in order to maintain the
required amount of carrier flow across the junction, otherwise the device will revert to the
blocking condition as the anode-to-cathode voltage is reduced. The minimum current which
keeps the SCR in ON state immediately after a thyristor has been turned on and gate signal has
been removed is called latching current IL.
Fig 1.1 SCR symbol Fig 1.2 SCR structure
Fig. 2.1 Two transistor module of SCR Fig. 2.2 Equivelent circuit of SCR
The static characteristics of a SCR are shown in fig. 3. When a thyristor conducts, it
behaves like a conducting diode and there is no control over the device. The device will
continue to conduct because there is no depletion layer on the junction J2 due to movement of
carriers. However, if the forward anode current is reduced below a level known as the holding
current IH, a depletion region will develop around junction J2 due to reduced number of carriers
and the thyristor will be in the blocking state. The holding current is in the order of milliamps
and is less than the latching current IL. Holding current IH is the minimum anode current to
maintain the thyristor in the on-state. The holding current is less than the latching current.
When the cathode voltage is positive with respect to the anode, the junction J2 is forward
biased, but junctions J1 and J3 are reverse biased. This is like two series-connected diodes with
reverse voltage across them. The thyristor will be in the reverse blocking state and the reverse
current is of the order of few micro amps due to minority carriers. However thyristor can be
made on in reverse direction by applying large reverse voltage of the order of few hundred volts.
PROCEDURE
FORWARD CHARACTERISTICS
1. The connection are made as shown in the circuit diagram.
2. Switch on the characteristics unit. Apply constant VAK voltage say 35V by varying VAA.
3. Gradually increase the gate current till the SCR becomes on. Note down IG, VAK and IA.
4. Now VAK is increased/decreased gradually by VAA and IA is noted for two to three
readings, load resistance may also be varied to vary VAK/ IA.
5. Steps 3 to 4 are repeated for another values of VAK say 25V.
6. Tabulate the readings in the table.
7. Plot a graph of VAK verses IA for different (two) values of IG.
8. To determine latching current IL
1. Keep proper VAK to trigger SCR by gate current. Trigger SCR by applying gate
current. Keep sufficient load current by varying load resistance in fully clockwise
direction.
2. Now reduce load current slowly & check SCR is till under on by removing gate
current.
3. If the SCR is under on insert gate circuit further & reduce the load current again.
4. Step (3) is repeated till the SCR jumps to blocking state by removing & inserting
gates.
5. The minimum current for which SCR suspend under ON condition is noted which
is latching current IL.
9. To determine holding current IH
1. Keep proper VAK to trigger SCR by gate current. Trigger SCR by applying gate
current. Keep sufficient load current by varying load resistance in fully clockwise
direction.
2. Open the gate circuit; now reduce load current till SCR jump to blocking state.
3. The minimum current for which SCR suspend under ON condition is noted which
is holding current IH.
TABULAR COLUMN:
Forward Characteristics:
Sl. IG1 = mA IG2 = mA
No.
VAK = V IA = mA VAK = V IA = mA
RESULT: Static characteristics of SCR are drawn.
