Solenoids
A solenoid
is an electromagnet that activates a mechanical function, such as a plunger.
Solenoids are used to latch safety covers closed so they can’t be opened while
a machine is in operation, or to unlock the doors in your car when you push the
keyless entry button on the remote. Solenoids can open and close valves in industrial
processes or push the record head against the tape in a tape player.
Solenoids
come in many shapes and sizes, and are capable of exerting a force from less
than an ounce to several pounds. There are two basic varieties: continuous duty
and pulse duty. Continuous-duty solenoids are designed to be energized all the
time. An application such as holding a safety cover closed would use a continuous-duty
solenoid.
A pulse-duty
solenoid might be used for the doors in your car. Pulse-duty solenoids will
overheat if left energized all the time—they are designed for intermittent
operation. A pulse-duty solenoid allows a high-force solenoid to be smaller and
cheaper because continuous operation is not a concern.
Relays
A relay is a
solenoid that operates electrical contacts. When the relay is energized, the
contacts are shorted or opened, just like a mechanical switch. Interfacing to
Solenoids and Relays For the sake of simplicity, this section will address
relays, but the same considerations apply to solenoids. Figure 6.1A shows a
relay as it might be connected to a microprocessor. A single bit is used to
turn the relay on and off.
The figure
shows an NPN transistor connected to a port bit on the processor; you could
also use a MOSFET. Some microprocessors have outputs that are capable of
sinking sufficient current to activate a relay, as long as the relay is
operating from the same voltage as the processor.
Because the
relay or solenoid is activated by a coil, there is a flyback voltage that
occurs when the drive transistor is turned off and the magnetic field collapses
in the coil. This voltage can reach high enough levels to damage the drive transistor.
Figure 6.1B shows how a diode can be used to clamp the voltage across the coil
to safe levels.
When the
transistor turns on, activating the relay, the diode is reverse biased. When
the transistor turns off, the top end of the coil is tied to the drive voltage,
so a voltage spike appears at the lower end (transistor collector). As soon as
this voltage reaches the supply voltage plus one diode drop (about 0.6 V for a
silicon diode), the diode conducts.
1 comment:
Relays perform the best function in the transmission lines.. They help in overcoming the faults of the transmission lines.. Thanks for the enlightening post regarding the relays..
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