Controlling Output
Transducers
1.1
1.1.1
Types of output transducers
Produce “information”
These devices require low power. Low power is typically
.
• meter
• oscilloscope
1.1.2
Produce “action”
These devices require high power. High power is typically
.
• lights
• solenoids
An operational amplifier is a voltage device; it can’t produce more than
of power.
ii
Controlling Output Transducers
1.2
Types of control
• on/off
like a switch
• proportional
like a rheostat or potentiometer
There are trade-offs involved in choosing a type of control.
• efficiency
on/off is 100% efficient
proportional
• noise
on/off produces
noise
proportional produces
1.3
noise
Devices for control
Any device used for control is basically used in a voltage divider using the
and
supply voltage, the control device, and the load. The order of the
the
in the voltage divider has some effects on the circuit operation.
1.3.1
Relay
Since a relay is inductive, all of the precautions for an inductive device must
be taken.
The relay can be inserted on either side of the load in a voltage divder.
1.3 Devices for control
iii
Vsupply
Vout
Vin
Figure 1.1: Relay; floating load configuration
Vsupply
Vin
Vout
Figure 1.2: Relay; grounded load configuration
iv
Controlling Output Transducers
1.3.2
MOSFET
A MOSFET (or Metal Oxide Semiconductor Field Effect Transistor) is a
three terminal device. The terminals are
• drain
• source
• gate
The current from the drain to the source is controlled by the
the gate and the source.
There are two kinds of MOSFET
between
• enhancement mode (E type)
As Vgs increases,
• depletion mode (D type)
Actually, all MOSFETs operate in
operate in that mode.
mode. It’s just that some only
Vsupply
Vout
Vin
Figure 1.3: MOSFET; grounded load configuration
The MOSFET, like the relay, can be placed in either position of a voltage
divider. The effects are the same as for the relay.
1.3 Devices for control
v
Vsupply
Vout
Vin
Figure 1.4: MOSFET; floating load configuration
1.3.3
BJT
A BJT (or bipolar junction transistor) is a three terminal current device.
The terminals are
• collector
• emitter
• base
into
The current from the collector to the emitter is controlled by the
the base.
Like the previous devices, either position in a voltage divider can be used.
vi
Controlling Output Transducers
Vsupply
Vin
Vout
Figure 1.5: BJT; grounded load configuration
Vsupply
Vout
Vin
Figure 1.6: BJT; floating load configuration
1.3.4
TRIAC
A TRIAC is a three terminal ac voltage device. The terminals are
1.3 Devices for control
vii
• main terminal 1
• main terminal 2
• gate
A triac will begin to conduct when it receives a voltage pulse on the
is
.
It will continue to conduct until the
It can operate in two
.
• burst
• delayed trigger
MT1
gate
MT2
Figure 1.7: TRIAC
.
viii
Controlling Output Transducers
Figure 1.8: Optoisolator block diagram
Vsupply
Vout
Figure 1.9: Optoisolator; non-inverting configuration
1.3.5
Optoisolators
For an optoisolator, both the input and output sides are voltage dividers.
Similar considerations apply as with the control devices.
An optoisolator can be connected either to have the output voltage increase when the input increases, or to have the output voltage decrease when
the input increases. You should be comfortable with both.
1.3 Devices for control
ix
Vsupply
Vout
Figure 1.10: Optoisolator; inverting configuration