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Blocking Diodes, Isolating Door Triggers and Sensors, Diodes Across the Coil of Relays
Here's another installer friendly component you should
always have handy. Blocking diodes (1N4001/L) are one way valves
used in electrical
circuits. These are very simple devices
that are often real time savers. Other than the amperage
rating of the diode, there are only three basic things to
remember:
1. Cathode (side with the stripe)
2. Anode (side without the stripe)
3. Anytime the cathode is more positive than the anode,
no current will flow.
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Isolating Door Triggers:
Some vehicles have two separate (-) door triggers that are isolated
from each other, most commonly found on newer GM vehicles. One is for
the driver's door and the second is for the rest of the doors. Below is an
example of connecting them to one alarm trigger. If you were just to
connect to one of these and not both, one or more doors of the vehicle
would not be protected by the alarm. When installing an alarm in a vehicle
with this type of door trigger (dome lamp) circuit, you must connect to both
door triggers for all doors to trigger the alarm. If you were to tie each of these
together without the blocking diodes, some features of the vehicle would no
longer function properly. Some of the things that could happen are: the door
chime / buzzer sounding when any door is opened, rather than just the
driver's door, or indicators in the instrument cluster showing false information
as to which door is actually opened, and so on.
This diagram would also apply to connecting
the (-) outputs of two sensors,
such as a glass mic and an impact sensor, to one input of an alarm.
If you have two or more positive triggers to isolate, simply connect the anode
side of each diode to each trigger and the cathode sides to the positive
input of the alarm.
Unless specified, all diodes seen in these diagrams
are rated at 1 ampere (1N4001/L).
Diode across the coil of a relay:
The diode provides a path for current when the current path to the relay is interrupted
(i.e. switched off). This allows the coil field to collapse without the voltage spike that
would otherwise be generated. The diode protects switch or relay contacts and other circuits
that may be sensitive to voltage spikes.
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