Devices and Applications
Ctec 201.
Silicon Controlled Rectifiers (SCRs)
Supplement
Prepared by Mike Crompton. (Rev. 2 July 2003)
Silicon Controlled Rectifiers (SCRs)
Silicon Controlled Rectifiers, almost exclusively referred to as SCRs, are one of a small
group of devices known as Thyristors. They (SCRs) are three terminal devices that
perform in a manner similar to a silicon diode with an ‘On’ switch but no ‘Off’ switch.
That is, they will not conduct even if forward biased until the ‘On’ switch is momentarily
activated. Once activated the SCR conducts, often at it’s saturation rate, and can only be
turned off by ‘open circuiting’ the anode/cathode circuit, reverse biasing the
anode/cathode circuit or reducing the current flow through the SCR to a value below the
‘hold on’ current rating. De-activating the ‘On switch’ has no effect.
Constructed of four pieces of doped silicon in a NPNP configuration, it’s three terminals
are labeled as Anode, Cathode and Gate. The symbol is similar to that of a normal silicon
diode, but with an extra connection for the Gate.
In order for it to conduct at all (when gated on),
the anode must be more positive than the cathode,
which is normal forward bias. If reverse biased the
SCR can not conduct unless the reverse bias is so
high that it breaks down the SCR and destroys it.
The amount of forward bias required to allow
conduction varies from SCR type to type, but is
generally in the range of 1 to 2 Volts. If correctly
forward biased, no conduction will occur until the gate is activated with a positive
voltage of approximately 0.7V. Once current (electrons) starts to flow from cathode to
anode, removing the gate voltage will have no effect. The SCR will continue to conduct
until one of the three ‘turn off’ conditions is met. (Open circuit/reverse bias the
anode/cathode or reduce the current below it’s minimum hold on value).
The other SCR specifications that are of most concern are as follows:
a) Peak Forward Blocking Voltage – The maximum amount of forward bias voltage
the SCR can withstand without breaking down and conducting (Gate off). This
can vary from 30 to 1200 Volts depending on type.
b) Maximum Forward Current – The maximum amount of current, DC or RMS,
that the SCR can handle. Varies from 0.8 Amps to 100 Amps dependant on type.
These values can be increased with the use of special heat sinking devices.
c) Peak Inverse Voltage (PIV) – The maximum reverse bias that can be applied
without causing breakdown. Often this voltage is the same as the Peak Forward
Blocking Voltage.
d) Holding Current – The minimum cathode to anode (forward) current that will
sustain conduction. Once again dependent on type, this can vary from less than
1mA to 20mA.
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Other ratings include gate/cathode currents and voltages, maximum peak surge current,
maximum temperatures and fusing requirements. Anyone involved in SCR circuit design
would of course have to be concerned with all specifications.
The SCR is most often used in electrical control circuits and is particularly useful in large
voltage or current applications. Control is maintained with the 0.7V gate voltage,
eliminating the danger of direct switching much larger values. Another application is the
Crowbar circuit used to protect electronic devices from surges in voltage. This circuit,
with a brief explanation, appears below.
If the power supply voltage is say
12V, and the device to be protected
can only stand a 10% over-voltage,
something must shut the circuit
down if the voltage goes above
13.2V. This is where the Zener
diode comes in. Presume the Zener
voltage is 13V. As long as the
power supply voltage is less than
13V the Zener remains off. If the
voltage goes above 13V the Zener conducts, putting a positive voltage on the gate of the
SCR. The SCR fires and it’s conducting/saturated resistance is so small, almost a short
circuit, that so much current is drawn from the supply the fuse blows, thus protecting the
device from the over-voltage.
A series of very simple circuits that demonstrate the control aspects of the SCR follow.
The circuit at left illustrates the use of an SCR to control a DC
voltage being applied to a lamp. R1 and R2 form a voltage
divider that provides a positive voltage at the push button
switch. When momentarily pressed, the push button places that
positive voltage on the gate of the SCR which fires and turns
on the lamp. For the sake of clarity the switch to turn the lamp
off is omitted from the circuit.
Slightly more complicated than
the simple DC control circuit
above, the circuit at right uses a
DC as both the control voltage
to fire the SCR, and the ‘hold on’ voltage that maintains
the SCR conducting during the negative half cycle of the
AC being applied to the lamp. Remember that the SCR
will not conduct at all if reverse biased. R3 supplies a
positive voltage to the anode during the negative half
cycle of the AC source, maintaining conduction. Once
again the ‘off’ switch is omitted.
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This final circuit can be used as a simple half wave
dimmer circuit. During positive half cycles of the
AC source, VR1 and C1 form a CR charging
circuit. When the capacitor charges to a sufficiently
positive voltage (the time taken will be determined
by the setting of VR1) D2 conducts allowing the
+ve voltage to reach the gate and fire the SCR. The
SCR will conduct for whatever portion of the
positive half cycle remains. VR1 can be adjusted to
fire the SCR very close to the start of the positive
half cycle allowing almost all of it to be fed to the
lamp which will glow brightly, or adjusted so that
almost none of the half cycle is fed to the lamp,
causing it to glow dimly. D1 discharges C1 during
negative half cycles, it’s cathode being negative from the AC source and it’s anode being
positive until the voltage across C1 is discharged.
Points to remember.
1) An SCR, like a diode, will not conduct at all if reverse biased.
2) When forward biased , an SCR will only conduct once it’s gate has been momentarily
activated with a positive (0.7 or more) voltage.
3) When an SCR is conducting, deactivating the gate has no effect.
4) To turn an SCR off it must be reverse biased, it’s anode or cathode circuit must be
open circuited or it’s current flow must be reduced below the ‘hold on’ value.
5) SCRs are used primarily to control other circuits or devices.
6) The most important specifications of SCRs are: Peak Forward Blocking Voltage,
Maximum Forward Current, Peak Inverse Voltage (PIV) and Holding Current.
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