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 flows 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 flow 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 flow 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 field 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 field 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 flows through the coil creating an electromagnetic field indicated by the dotted lines and the two arrows pointing outward to show the electromagnetic field’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, flow 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 field 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 field which quickly builds up as electrons flow through the coil. Maximum intensity is reached shortly after coil electron current begins to flow.

The electromagnetic field is sustained and the polarity of the voltage drop remains constant as long as electron current flows 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 flows 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 field are shown pointing inward to illustrate the electromagnetic field immediately collapses as soon as the electron current through the coil stops.

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

A voltage spike also briefly 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 briefly displayed the voltage spike which was as high as 135 volts.

Notice that during the time the electromagnetic field 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 flow from the negative (-) to the positive (+).

Surge electrons leave the top of the coil which is a negative (-) voltage while the field 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 field 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 field 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.

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