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Diodes
in Vehicle Electronics
Spike
Suppression and Polarity Diodes
This email is Spike Suppression Diodes which weren't
necessary until computer circuits began to appear in vehicles. In BC
time (before computers), spike suppression wasn't necessary. Here's
why.
Fig. 1
In Figure 1 above,
a solenoid (or coil) is operated by a mechanical switch wired to
operate the solenoid from the ground side.
Fig. 2
In Figure 2 above,
the switch CLOSES to operate or "power up" the solenoid.
Electron current flows from the negative terminal of the power source
(vehicle battery or generator) and travels up through the solenoid.
Electron flow enters the solenoid at the bottom and exits at the top
of as it flows back to the positive terminal of the voltage source or
B+. The polarity of the voltage drop across the solenoid is negative
at the bottom and positive at the top. This field voltage drop can be
measured with a DMM and is indicated by the arrows pointing outward.
As electrons flow
through the solenoid winding an electromagnetic field is created
around the solenoid winding with the same polarity voltage drop. That
is, positive at the top and negative at the bottom of the solenoid
winding. The arrows pointing outward indicate the expanding movement
of the electromagnetic field.
As long as the
switch remains CLOSED the electron current continues to flow and
sustains the energy of the electromagnetic at a specific level which
moves and holds the solenoid's plunger (not shown) creating the
movement to accomplish a task.
Fig. 3
In Figure 3 above,
the switch OPENS to de-energize or "power down" the
solenoid. Electron current from the power source stops flowing
through the coil. The electromagnetic field suddenly collapses as
indicated by the arrows pointing inward. The action of the collapsing
electromagnetic field is kindly referred to as an "energy
dump" as stored electrical energy is released back into the
circuit.
During an energy
dump the voltage drop is reversed across the coil being driven by the
lines of force moving inward this time. Electrons pour out of the top
terminal of the solenoid, now negative, and travel around the circuit
through the power source to supply electrons to the highly positive
voltage at the bottom of the solenoid.
All the stored
energy in the electromagnetic field immediately dumps back into the
circuit.with significant electrical force that causes arcing across
the switch contacts that have just begun to OPEN.
In one of our
electronic classes we demonstrated with a lab scope how a 30 ohm
solenoid operated at 13.0V creates an energy dump with a voltage
spike that rises to about 140 volts. The time duration of the energy
dump only lasts for a few milliseconds but the voltage spike
amplitude produced is sufficient to create an arc across the opening
switch contacts. Notice in Figure 3 a small arc effect is shown as
the contacts OPEN. The only negative impact of the energy dump is a
little arcing at the switch contacts. That is not problem for a
switch with heavy duty contacts.
If the switch
fails due to contact damage from the arcing it is possible to prevent
the arcing by controlling the energy dump A diode is connected across
the solenoid terminals as shown below in Figure 4. REMEMBER, electron
flow is against the arrow in the diode symbol.
Fig. 4
The collapsing
electromagnetic field causes the voltage drop across the solenoid to
reverse its polarity during the energy dump. That is the same
polarity voltage that allows electrons to pass through a diode. The
spike suppression diode turns ON during the energy dump and electrons
pour out of the top of the solenoid. They see a low resistance path
to the opposite side of the solenoid (now positive) through the
diode.
Energy dump
electrons flow through the diode to satisfy the need for electrons at
the bottom of the solenoid. There is no energy dump electron flow the
long way around through the power source and there is no arcing
across the switch contacts when they OPEN. The energy dump is safely
contained at the solenoid terminals and the switch contacts do not
experience arcing. In BC days (before computers) the arcing at the
switch contacts was dealt with by using big switch contacts to
survive arcing for a long time.
However, with
computer control of solenoids the energy dump must be suppressed with
spike suppression diodes. Mechanical switches to control solenoids
are replaced with solid-state switches called Transistor Drivers
mounted inside on-board control units. Transistor drivers cannot
withstand the powerful energy dumps that would pass through the
transistor. The only option is using spike suppression diodes to
protect the transistor driver from the energy dump..
Below in Figure 5
there are 2 spike suppression diodes to suppress the energy dump that
occurs when a solenoid is turned OFF.
Fig
5
Spike Suppression
Diode #1 is placed across the starter solenoid. This diode was
added to surpass the energy dump when the starter solenoid powers
down after the engine begins to run. When this energy dump is not
suppressed there could be problems with the PCM loosing adaptive
memory from the energy dump running the long way around the circuit.
The result was cold engine driveability problems that did not clear
up until the PCM had enough drive time to relearn adaptive
strategies. The problem reappeared with the next crank when another
energy dump occurred.
Spike Suppression
Diode #2 is inside the relay and wired across the relay coil. It
protects the transistor driver from a very destructive energy dump
when the driver stops electron current trough the relay coil. One or
two energy dumps is all it takes to kill the transistor if the spike
suppression diode inside the relay becomes OPEN. Some relays use a
small value resistor instead of a diode to control the energy dump.
Diode #3 is a
special purpose diode. It has nothing to do with spike voltage suppression
because it is not connected across a solenoid or relay winding. The
diode is inside the PCM and connected in series with the B+ voltage
applied to PCM circuits. This diode is a polarity sensing diode and
only allows electron flow through PCM circuits when the correct
voltage polarity is applied to the vehicle. That is when the positive
battery cable is positive and the negative cable is negative or
normal polarity. The polarity diode allows electron current through
PCM circuits. Anytime reverse voltage is applied to a vehicle such as
when jumper cables are crossed in an attempt to jump start a vehicle,
the polarity diode prevents electron current through PCM circuits and
saves the PCM from damage.
You are probably
thinking ..... "How do I check a spike suppression diode or a
polarity diode?" That becomes a major topic of discussion
covered in a few lessons of the 60 Lesson Home Study Vehicle Electronics Course.
It is too big a topic to handle in these short emails.
Till next
time,
Vince
Fischelli
Director of
Training
Veejer Enterprises
Web site: www.veejer.com
Email: vince@veejer.com
Phone: 972.276.9642
Fax: 972.276.8122
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