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should not be exceeded. High-current optoisolators are available if
your application demands it. Second,
the transistor has a maximum rated
collector-emitter voltage, VCEO.
Typical values for VCEO are tens of
volts. VCEO is highest in this circuit
when the transistor is turned off.
When the transistor is on, its behavior
approaches that of a short circuit
and exhibits a saturated VCE of
approximately 0.3 V. VCE climbs as
high as 24 V in this circuit when the
transistor is off and no current is
flowing through RC and the load.
(DPDT) to many-pole-double-throw. + 5 V
They are simple
strengths are electrical isolation from
driver to switch and
a passive metal
The magnetically actuated
switch is comparable to a mechanical light switch. You
can connect the switch to any AC or
DC load, subject to the switch
element's maximum current rating.
Relay manufacturers specify how
much current the switch can handle
at certain voltages. Current ratings
are often in two categories: current at
a mid-range DC level (e.g., 24 VDC)
and at an AC-power level (e.g., 125
Relay coils are rated with a
voltage and current or a voltage and
resistance. The driving circuit must
steadily apply power to maintain the
relay in a switched state; however,
there are latching relays where this is
not the case. Relay coils are made
with different power levels, which are
related to the size of the switch element being actuated. A relay coil
+ 5 V
+ 24 V
Figure 3. Isolated input with optoisolator.
Optically isolating an input uses
the same basic circuit in reverse. The
circuit in Figure 3 drives the LED with
an output that swings from 0 to 24 V.
RL is chosen as before, with the
continued assumption of VF = 1.5 V at
5 mA: RL = 22. 5 V / 5 mA ~ 4. 3 KΩ.
On the control circuit side, a pull-up
resistor (perhaps 4. 7 to 10 KΩ) keeps
the input at logic-1 until the transistor
turns on and pulls the input
to ground. This circuit topology
inverts the control signal because the
isolated signal drives the LED when it
transitions from 0 to 24 V.
Figure 4. SPST and DPDT relay
may require 100 mW — or much
Driving a Relay Coil
Relay coils often require more
current than a digital IC output can
provide. A transistor circuit can be
used to amplify the relay control
output. Figure 5 shows a coil driver
Optoisolators are limited in their
power applications because they are
semiconductors. Very high voltages
and currents are difficult to deal with
in semiconductors. A basic transistor
circuit is also not suited to AC-powered loads. Relays, however, can
handle AC power and a wide range of
voltages and currents.
On one side of the relay is an
electromagnet; its coils are energized
by the driver.
At the other end is a magnetically
actuated switch. The basic schematic
symbol for a relay, shown in Figure 4,
reflects this structure. Relays are
made in many switch configurations,
ranging from single-pole-single-throw
(SPST) to double-pole-double-throw
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