VEEJER ENTERPRISES, INC.

COMPUTER CONTROLLED
CRANKING CIRCUITS: PART 4
STARTER CONTROL CIRCUIT ACTIVATED WITH
RELAY CONTROL
There is a lot to be said about electromechanical relays which is covered
extensively in our 60 lesson electronics course on-line. Lessons 38, 39, 40, 41 cover
relays in depth for a total of 31 pages. For purposes of this brief training program,
we will limit our discussion to major points about relay operation to continue this
series of articles. Below in Figure 4-1 a Starter (mechanical) Relay is used to control
the Starter Solenoid.

Fig. 4-1 The Starter Relay Circuit

The relay used in the circuit, illustrated in Figure 4-1, is a standard 5 pin relay. Pins
86and Pin 85 connect to the relay coil. Across the relay coil a semiconductor diode
is mounted inside the relay to provide spike voltage suppression when the diode is
turned OFF (deactivated). Spike voltage suppression will be discussed a little later.

In this relay circuit Pin 86 connects to B+. Pin 85 connects to B- through the closed
PARK NEUTRAL (P/N) switch and the START switch which are drawn in the
conventional way switches are drawn in schematic diagrams. They are always
shown as OPEN (not CLOSED). In this way Pin 85 is not connected to ground or B-
until both switches are CLOSED at the same time. When reading a schematic
diagram, the technician mentally closes the two switches to activate the relay.

As soon as both switches are CLOSED, electron current passes through the relay
coil creating an electromagnet, as shown in Figure 4-2 below indicated by the
dotted lines around the relay coil. Trace the electron current through the relay coil
circuit starting at -BATT (B-) since the battery is the voltage source during engine
cranking and ending at +BATT.

Fig. 4-2 The Starter Relay is activated.

Mentally CLOSE the Ign Sw to enable the cranking circuit. To activate the relay the
P/N and START switches must both be closed. When both are CLOSED Pin 85 is
now grounded (connected to -BATT or B-) through the switch contacts. Electron
current ows up from ground through the relay coil, through fuse F2 and on to B+.
The electron current through the relay coil changes the relay coil into an
electromagnet which attracts the relay’s movable contact at Pin 30 and pulls it
inward to contact Pin 87. The B+ at Pin 30, through fuse F11 now appears at relay
Pin 87 through the closed relay contacts as shown.

Pin 30 and Pin 87 connect to the relay contacts which act as a mechanical switch to
operate a circuit, such as the Starter Solenoid. A relay at rest is said to be
deactivated (or de-energized). When a relay is at rest, Pin 87A is the normally
closed (N/C) contact while Pin 87 is the normally OPEN (N/O) contact. When the
relay is turned ON or activated, Pin 30 moves from Pin 87A and moves to Pin 87 as
shown in Figure 4-2.

The wire from Pin 87 connects to the B+ terminal on the starter solenoid. Since the
solenoid is permanently grounded by a wire from Pin G to the starter motor
housing, electrons ow up from ground through the grounded outer housing of the
starter motor to supply electrons through the starter solenoid winding which
becomes another electromagnet. The starter solenoid plunger is attracted into the
center of the starter solenoid winding closing the circuit between the B and M
terminals of the starter solenoid heavy-duty contacts. This applies battery voltage
(B+) directly to the starter motor.

Since the starter motor housing is grounded by mounting bolts, electrons ow

through the starter motor winding as long as the starter solenoid’s heavy-duty
contacts remain closed and the engine cranks.

The cranking action ceases when either the P/N or START switch are opened which
causes the Starter Relay to deactivate. As soon as electron current through the
relay coil stops, the electromagnetic eld around the coil quickly collapses and
dumps its energy back into the circuit. This action is often referred to as an “energy
dump.”

The diode placed across the relay coil is called a spike suppression diode. It allows
the energy dump to remain in the relay and not cause arcing across the contacts of
the P/N and START switches as they OPEN.

UNDERSTANDING SPIKE SUPPRESSION

DIODES
Spike suppression diodes serve a vital purpose protecting electronic circuits. What
follows is a brief explanation of how a spike suppression diode protects a circuit by
preventing "energy dumps" and the surging electron current that can damage a
solid-state component (such as a PCM, transistor and/or an integrated circuit.)
when a coil powers down and the electromagnetic eld collapses.

Fig. 4-3

The illustration above, Figure 4-3, shows the schematic of a coil connected to B+
and a control switch on the ground side of the coil. Think of this coil as the coil
inside a relay.

Think of the switch performing the function of the P/N and START switches. When
the switch closes, as shown, electron current ows through the coil creating an
electromagnetic eld indicated by the dotted lines and the two arrows pointing
outward to show the electromagnetic eld’s lines of force build up around the coil.

Notice the polarity of the voltage drop across the coil while electrons pass through
the coil. Electrons enter the bottom of the coil, ow through the coil and exit the
coil at the top to go to B+. This creates a measurable voltage drop across the coil
which is negative (-) at the bottom and positive (+) at the top of the coil. A DMM can
measure the voltage drop across the coil with the red test lead at the top of the coil
and the black test lead at the bottom of the coil. Reading should be close to B+.
The electromagnetic eld represents electrical energy stored (held) around the coil.
This energy is taken from the circuit during the time the relay is activated to create
an electromagnetic eld which quickly builds up as electrons ow through the coil.
Maximum intensity is reached shortly after coil electron current begins to ow.

The electromagnetic eld is sustained and the polarity of the voltage drop remains
constant as long as electron current ows through the coil. During this time, the
relay is said to be “ON” or energized and the relay contacts are closed. At this point
in the circuit’s operation there is no problem in the circuit which is performing
precisely as it should. The relay contacts remain in the closed position as long as
current ows through the coil. The problem occurs when the switch OPENs and the
electron current through the coil stops. The problem that arises is called an “energy
dump.”

In the illustration below, Figure 4-4, the switch is shown in the OPEN condition to
deactivate the relay or turn the relay ”OFF” which also serves to open the relay
contacts (not shown).

Fig. 4-4
The two arrows indicating the electromagnetic eld are shown pointing inward to
illustrate the electromagnetic eld immediately collapses as soon as the electron
current through the coil stops.

At the exact instantaneous split-second moment the switch is ipped OFF, the
electron current through the coil stops and at the same exact time the
electromagnetic eldIMMEDIATELY collapses. All the energy stored in the
electromagnetic eld is dumped back into the circuit at that moment creating a
signi cant energy dump that produces high electron current surge back into the
circuit.

A voltage spike also brie y appears which can be viewed with an oscilloscope.

In an electronics class I used to teach, I had a 30 ohm coil connected to 14 volt B+ source. When the coil
was turned OFF an oscilloscope brie y displayed the voltage spike which was as high as 135 volts.

Notice that during the time the electromagnetic eld is collapsing the voltage drop
across the coil reverses polarity because the lines of force are now moving inward,
the opposite direction. Remember electrons always ow from the negative (-) to
the positive (+).

Surge electrons leave the top of the coil which is a negative (-) voltage while the
eld is collapsing and are forced through the power source by the power of the
energy dump and travel through the ground circuit and up through the switch
OPEN contacts. This energy dump occurs so powerful (high voltage) that electrons
jump across the gap of the open switch contacts. Over time this causes erosion of
the switch contacts. Look closely at the illustration above and noticed the little arc
appearing across the open switch contacts as electrons seek to get to the high
positive (+) voltage at the bottom of the coil. This is the magnitude of the force of
the energy dump causing electrons to jump across the gap of the open switch
contacts. (The same principle of a collapsing electromagnetic eld is used to create
a spark across a spark plug gap.)

Once the electrons induced into the circuit by the energy dump travel through the
circuit and jump across the gap of the OPEN switch contacts, electrons are supplied
to the bottom of the coil. The circuit comes to rest again once the energy dump is
dissipated in the circuit.
 Fig. 4-5

In the schematic above, Figure 4-5, there is a diode connected the relay coil. The
negative (-) voltage at the top of the coil indicates the eld is collapsing. The
electron surge of the energy dump travels through the diode to arrive at the
positive side the coil without traveling through the external circuit. There is no
arcing across the switch contacts. The spike suppression diode allows the energy
dump to pass to the positive side of the coil. The total time period of the energy
dump is shorter than the blink of an eye. But it must be controlled so that it does
not pass through the external circuit

If the electron current induced by the energy dump is allowed to pass through
electronic circuits they could be permanently damaged. In our next article, Part 5,
we get into more electronics as we use an onboard computer to control the relay.
Stay tuned for more when we control the relay with a computer next time.

CONTINUE READING...

Intro Part 1 Part 2 Part 3 Part 4 Part 5 Part 6 Part 7 Part 8

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