Switches, Relay and Power
Kilian, C. T. (2001), Modern Control Technology: Components and Systems
Used extensively in control circuits as
both switches and power amplifiers.
A small current is used control a much
Current gain is expressed as:
Power is dissipated anytime there is
current through it and voltage across it.
Class A Operation
Biased in the middle of the operational
Input signal increases or decreases the
Current at the load is inverted.
Class B Operation
Transistor is biased just below 0.7V
(operational bias point).
Only half a signal will be amplified.
Class C Operation
Transistor is either fully saturated or
Acts as a switch.
Transistors are rated on the amount of
power they can dissipate.
The greater the current flow through the
transistor, the greater power, thus heat,
it must dissipate.
Excessive heat will cause damage to
Heat sinks are used to draw heat from
the device. Typically metal with fins for
Heat Sink compound is used to
increase thermal transfer.
Insulating the Case
On many power transistors, the case
itself is connected to the collector and
may need to be insulated.
A thin film of mica is typically used.
Integrated dual transistors that provide
a very large beta (>500).
Small current on base the 1
controls a large current biasing the 2
Field Effect Transistors
Performs job similar to a BJT.
Junctions are Gate, Drain, Source.
Source Voltage (V
) controls the
load current (I
JFET (Junction FET) is one version,
does not have large current control.
Channel use a negative gate voltage,
Channel use a positive.
MOSFETs capacitive couple the gate
preventing any direct path for current
Can control high currents.
Gain is called transconductance and
measured in siemens or mhos. It is the
inverse of resistance.
Change in current
Change in voltage
control device that is a
Thyristors are 4
that act as a switch.
Terminals: Anode, Cathode(K), Gate
Gate current is used to switch SCR on.
The gate cannot turn the SCR off.
An SCR will be off unless the V
at which time it will be in the
forward conduction region.
The SCR will remain in conduction until
the forward current drops below the
With a gate voltage, the breakover
voltage is dramatically reduced.
When gate voltage (thus current) is high
enough, the SCR starts conducting
almost immediately (V
SCR will remain in conduction even if
the gate voltage is removed.
Minimum current required to stay in
conduction is called Holding Current
Means of turning off the SCR (current < I
Open the load current.
Use Forced Commutation to short out the
AC Circuit automatically turns it off on the
SCRs are often used in AC circuits to
control power to the load, such as DC
Motors for speed control.
By controlling the resistance of the gate,
IGT is reached based on the
instantaneous AC voltage.
The SCR may be set to fire anywhere
within the 1
90 degrees of a cycle.
The resulting IAK wave form has delay
and conduction regions.
Addition of a capacitor creates a phase
shift allowing control over a wider range
of the alternation.
Through use of a full
power may be controlled during both
alternations for DC applications.
Other conditions that will cause an SCR
If the V
rises too quickly: dv/dt. A
snubber may be added to prevent this
are not held low long enough.
Excessive heat can lower the breakover
Activated SCR) is one which
used light entering a small window fires the
0.5A to 1000A
: Peak forward blocking voltage
: Peak reverse blocking voltage
: Peak surge current
: Gate trigger current
: Gate trigger voltage
DV/DT: Critical voltage rise
Similar to an SCR but can conduct in
Leads are labeled MT1, MT2 and Gate
Delay Time & Conduction Time
Delay time (or holdoff) is the time the thyristor
is shutoff preventing current flow.
Conduction time is length of time the thyristor
is in conduction.
The time for a half
0.5 x 1/60Hz = 8.33mS
Delay Time + Conduction Time = 8.33mS
If an SCR had a delay time of 3mS, it’s
conduction time would be 5.33mS
Conduction angle is the number of
degrees into a waveform the thyristor
With an alternation being 180 degrees,
if an SCR fires at 30 degrees, it will be
in conduction for 150 degrees.
Converting between conduction time
and conduction angle:
8.33mS/180 = 46.3uS/deg
Due to the low tolerances in both the
firing current of SCRs and Triacs,
triggering devices are often used to
more dependably fire the device.
The UJT fires when 2/3 of the voltage
between Base1 and Base2 is on the
emitter, the UJT will fire.
When Vc reaches the trigger voltage
(Vp), the UJT fires providing gate
current to the SCR.
The PUT behaves like the UJT, but the
firing voltage is programmable using
When the Anode voltage reaches the
gate voltage, the PUT fires gating the
which fires when the breakover voltage
is met in either direction.
Allows for symmetrical firing of TRIACS.
Solid State Relays (SSR)
Solid State relays are used for an
isolated control of high AC or DC loads.
A small voltage energizes an LED which
optically fires a triac control circuit.
Does not control based on AC firing
angle, but controls the length of time a
device is energized.