Transcript Relays and Resistors

Relays and Resistors
Mike Battaglia
Will Mills
Relays
• A relay is a special kind of
switch that is electrically
actuated.
• A relay can be thought of as
another use for an inductor.
Rather than using the inductor
coil to resist changes in
current, the magnetic field
produced is used to pull a
magnetically charged, spring
loaded plate from one contact
to another, opening or closing
the switch.
• Relays were invented by
Joseph Henry in 1835. This is
the same Henry whose name
has become the unit for
inductance.
An “ice cube” style
AC coil DPDT relay
Switching Arrangements
•
Relays have the same
switching arrangements that
switches do (SPST, SPDT,
DPDT, etc).
•
The diagram to the left shows
some common relay
arrangements, along with their
respective circuit symbols.
•
Note that the relay symbol is
actually two parts; the first part
is the symbol for an inductor
with two parallel lines next to it,
and the second part is a
separate circuit representing
the switch itself. The two
circuits will not be connected.
Relays – Terms
• The voltage needed to activate a given relay is
referred to as the excitation voltage.
• The voltage used to activate a given relay may be
either AC or DC. A relay with an AC coil will need an
AC voltage, a DC coil will need DC voltage.
• The coil resistance is the resistance limiting the
current flow through the coil. For a DC coil, this will
the only factor in determining the current. For an AC
current, the inductance of the coil itself, combined with
the coil resistance, will be used to determine the
current.
How do relays work?
• There are three basic kinds of relays that each work in different
ways: Mechanical relays, reed relays, and solid-state relays.
• Mechanical relays, which are older, are designed for high
currents (2 A to 15 A), but take a relatively long time to operate
(10-100 ms).
• Reed relays are designed for moderate currents (500 mA to 1 A)
and moderately fast switching (0.2-2 ms). Reed relays are DCactuated only, and are generally limited to SPST switching.
• Solid-state relays are newer, and use semiconductors to operate.
They come with a wide range of current ratings (from the
microamp range up to 1500 A), and have switching speeds of 1 –
100 ns. Solid state relays are also generally limited to SPST
switching.
• Some relays are designed to be Normally Open (NO), and others
Normally Closed (NC).
Mechanical Relays
• A current sent through the coil
magnet acts to pull a flexible,
spring-loaded, conductive
plate from one switch contact
to another.
• Many mechanical relays come
equipped with a “latching”
feature that keeps the switch in
its last set position until
another pulse is applied.
• Mechanical relays come in two
main varieties: Miniature
and subminiature relays.
Two kinds of Mechanical Relays
• Subminiature relays are designed to switch
large currents. They can be either DC or AC
actuated.
• DC-actuated relays typically come with
excitation-voltage ratings of 6, 12, 24 VDC, and
coil resistances of 40, 160, and 650 Ω
respectively.
• AC-actuated relays have excitation voltages of
110 and 240 VAC, and coil resistances of 3400
and 13600 Ω respectively.
• Switching speeds are moderately slow and
range from about 10-100ms. Current ratings
range from about 2-15 A.
Two kinds of Mechanical Relays
• Miniature relays are designed to be more
sensitive than subminiatures and work with
lower-level currents. They are almost always
DC-actuated, but they may be designed to
switch AC currents.
• They generally come with excitation voltages of
5, 6, 12, and 24 VDC.
• Their coil resistances range from 50 to 3000 Ω.
Reed Relays
• In a reed relay, two metal strips (reeds) act
as the contacts themselves.
• They are placed in a glass capsule that is
surrounded by a coil magnet. When current
is sent through the coil, the resulting
magnetic field forces the reeds together,
closing the switch.
• The switching time is faster than mechanical
relays at around 0.2 – 2 ms.
• The reeds are sometimes wet with mercury
to eliminate contact bounce. The low
resistance of mercury means that the relay
can work with lower voltage signals.
• Reed relays are DC-actuated only, and can
only be used in an SPST arrangement. They
are often made for PCB mounting.
• They are designed to switch low-level
currents, and come with excitation voltages
of 5, 6, 12, and 24 VDC. Their coil
resistances range from 250-2000 Ω.
• Reed Relays can be easily damaged by
power surges.
Solid-State Relays
• A solid-state relay switches states by
applying the excitation voltage to a
semiconductive junction. There is no coil.
• They are the newest kind of relay
invented. They have extremely fast
switching speeds in the nanosecond
range, and can work with currents as low
as a few microamps or as high as 1.2 kA.
• There are no moving parts, so contact
wear isn’t an issue.
• These devices often have high “on”
resistances, are often more finicky to
work with, and can blow out much more
easily than electromechanical relays.
• Examples of Solid-State relays include
transistors and thyristors.
Common Issues with Relays
•
•
•
The voltage applied to the magnetic coil
should be within 25% of the excitation
voltage rating. Too small of a voltage will
not be enough to trip the relay, and too
large of a voltage will damage or destroy
entirely the magnetic coil.
The coil of a relay is an inductor. Inductors
resist changes in current, so if the current
through the coil is suddenly interrupted (if a
switch is thrown), the inductor will respond
with a huge voltage spike (V=L*dI/dt).
These spikes can be damaging to
neighboring devices, the relay itself, or
even the person touching the switch that
interrupted the current.
Transient Suppressors are used to block
these voltage spikes. They can be easily
made out of different circuit components,
often ones that we have already worked
with.
Don’t let this happen to you!
Transient Suppressors
• The symbol with a arrow and a line is a
Diode. Placing the diode in “reverse bias”
(so that the negative end is lined up with the
positive end of the voltage source) will
effectively block the spike for a DC-actuated
relay.
–
+
V
+
–
• An AC-actuated relay is more simple – a
resistor and a Capacitor in series will block
the voltage spike. The capacitor will absorb
the DC spike, and the resistor will control the
capacitor discharge.
Examples of Relay-Based Circuits
• When the switch in the DC-actuated
circuit is thrown, the relay pull the
contact downward and close the lower
circuit. This means that the lower bulb
will light. When the switch is opened,
the relay will relax, and the upper bulb
will light again. The diode blocks the
voltage spike.
• The AC-actuated circuit behaves the
same way that the preceding circuit
does. The only difference is in the
transient suppressor. When the switch
is thrown or opened, the voltage spike
will be intercepted by the capacitor,
which will slowly discharge through the
resistor.
Resistors
• A resistor is a simple electrical
device that serves to reduce
current flow and lowers voltage
levels in a circuit.
• They are commonly used to set
signal levels, act as damping
agents in oscillators, and
provide feedback networks for
amplifiers.
• Resistors can be of either fixed
or variable resistance. Some
resistors are dependant on an
external light or heat source.
A 30-ohm Carbon
Film Resistor
How do resistors work?
• An electric current flows
through the resistor. Some
of the electrons collide with
atoms inside, transferring
their kinetic energy into
atomic vibrations.
• These vibrations are
converted into heat energy
and dissipate into
surrounding air molecules
(or a heat sink).
• This basically serves to
limit the current through the
resistor.
Resistor Operation
• Placing a resistor in series with a load
will limit the current going through the
load, and drop the voltage across the
load.
• The higher the resistance of R, the less
current flows through Rload. The current
can be computer using V=IR.
• The voltage through Rload can be
computed with the voltage divider
equation:
VR lo ad
R load

 Vbat
R  R load
Types of Resistors
• A carbon film resistor is one of the
most popular resistors used today.
• A carbon film is deposited onto a small
ceramic cylinder. A small spiral groove
cut into the film controls the amount of
carbon between the leads, which
controls the resistance.
• Carbon film resistors are known for
excellent reliability, solderability, and a
high tolerance to noise, moisture, and
heat.
• Typical power ratings range from ¼ to
2 W, and resistances range from 10 Ω
to 1 MΩ, with tolerances around 5%.
A 2.7k Ω, ½ W, 5%
tolerance, carbon film
resistor
Types of Resistors
• Carbon composition resistors are
made from a mixture of carbon powder
and a glue-like binder. To increase the
resistance, less carbon is added.
• These resistors are known for being
predictable, with low inductance and
low capacitance.
• Power ratings range from ⅛ to 2 W.
Resistances range from 1 Ω to
100 MΩ, with tolerances of 5%.
A 4.3kΩ, ¼ W, 5%
tolerance, carbon
composition resistor
Types of Resistors
• Metal-Oxide resistors use a
ceramic core coated with a metal
oxide film.
• These resistors are known to be
very stable during hightemperature operation.
• They contain a special paint on
their outer surfaces making them
resistant to flames, solvents, heat,
and moisture.
• Resistances range from 1 Ω –
200 kΩ, with tolerances of 5%.
Various metal oxide resistors
Types of Resistors
• Precision metal film resistors are
known for being very accurate and
ultra-low noise.
• It is made from a ceramic
substrate coated with a metal film,
encased in an epoxy shell.
• These are often used in highprecision devices, such as test
instruments, D-A and A-D
converters, and A/V devices.
• Resistances range from about
10 Ω to 2 MΩ, with power ratings
from ⅛ to ½ W, and tolerances of
1%.
Various metal film resistors
Types of Resistors
• These wire wound resistors are
used for high-power applications.
• Types of wire wound resistors
include vitreous enamel coated,
cement, and aluminum housed
wire-wound resistors.
• The resistance comes from a
long resistive wire, coiled around
a cylinder.
• These are the most durable
resistor. Resistances range from
0.1 Ω to 150 kΩ, with power
ratings from 2 W to 500 W or
more.
Types of Resistors
• These two resistors are special
types that vary the resistance
based on an external heat or
light source.
• Photoresistors are often made
from semiconductive materials.
Increasing the light level
decreases the resistance.
• Thermistors are temperature
sensitive resistors. Increasing
the temperature decreases the
resistance, in most cases.
Power Ratings
• Two resistors may have the
same resistance values, but
different power ratings (wattage
ratings).
• Resistors with higher power
ratings can dissipate heat
generated by a current more
effectively.
• To determine what power rating
your resistor should have, use
2
the equation P  I R.
• This is the minimum power
rating necessary for the resistor
to survive without melting
Needed higher power rating!!
Variable Resistors
• A special case of resistor is a variable resistor.
• These resistors can be adjusted to set the resistance to a user-defined
value.
• Potentiometers, rheostats, and trimmers are all different types of variable
resistors.
• Potentiometers and rheostats are both essentially the same thing. They are
adjusted by a knob and can be used, for example, as a volume control.
• Potentiometers are generally used for low-level DC electricity, and rheostats
are used for high-power AC electricity. They are designed for frequent
adjustment.
• Trimmers are designed to be adjusted infrequently, and usually come with
PCB-mounting pins. They are used for fine-tuning circuits, and are usually
hidden.
Variable Resistors
• Variable resistors can be linear-tapered and nonlinear-tapered.
• The “taper” is the way in which the resistance changes as the knob is turned.
• A nonlinear taper, or a logarithmic taper, is used in resistors that work with
the human perception.
• For example, a light switch dimmer, or an amplifier gain knob should be
nonlinear.
RESISTORS
ARE