Handy Tips - Relays
Why use another chip to control outputs? Well the answer to that is
that the PIC can only supply up to 25mA. This is fine for
logic levels, and even small devices like LED's, but you can't drive
a
relay, motor, piezo siren etc with a PIC..
The ULN2003 is a very cost effective chip that acts like a
switch.
It simply switches an earth to/from an external circuit, and can
withstand a continual 500mA current drain and a maximum 50V. All
this for $0.47 AUD.
Internally the ULN2003looks like this;
The
relays +Ve voltage must be
connected to Pin 9 to protect Back EMF, and the common ground is
connected to Pin 8. This is the ground supplied to the external
circuits when switched on. As you can see there are 7 channels per
chip, to control a
relay, hook up a circuit like this, and be sure
to connect Pin 8 of the
ULN2003 to
ground);
You don't need the ULN2003 to
control a
relay/load that the PIC can't drive on its own. You can
use a transistor like the following example (note that there are now
3 external components instead of 1);
Note the PIC's power supply/oscillator are not shown
Click here too see this circuit in action!
Now whenever
the output of the PIC goes high, an
earth is instantly supplied to the
relay, the coil will energize.
Note the diode, this is to negate back EMF (Built-in on the
ULN2003), and is vital in inductive
circuits like these.
The benefit of the
diode is that it provides a low voltage-drop path for the current
induced by the collapsing magnetic field to follow, thereby
preventing the build-up of a destructively high voltage on the coil
winding and/or the switch contacts. With the absence of the diode,
the voltage will rapidly build up on the coil until something brakes
down, allowing the energy to dissipate. That breakdown could be by
damaging arcing across the switch contacts or by punching through
the insulation on the coil wires, leading to eventual failure.
Where you can get the components;
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