Power and Grounding Considerations Diodes and resistors accomplish suppression in different ways. Diodes clamp the voltage across the coil to approximately -0.7 V when the switch opens. The current circulates in the loop until it eventually diminishes. Suppression resistors will allow the transient voltage to increase to a value determined by the forward current flow through the coil and the the value of resistance of the suppression resistor. However, resistor suppression causes the current in the loop to diminish much faster than would a diode. Diode selection should be based on the normal voltage and current seen by the coil.
For example, if the normal voltage applied to the coil is 24 V and the coil has a resistance of 50 ohms, then the current passing through the coil is 480 mA (I=V/R). The diode then would need to be able to withstand a reverse voltage of 24 V and a forward current of 480 mA when the current to the coil is interrupted by the opening of the switch since the collapse of the coil's magnetic field will attempt to maintain that current. The 189-3158 suppressor would function well in this application since it can withstand a reverse voltage of 600 V and a forward current of 2 A. Suppression resistors are low-cost alternatives and can be less stressful on relay coils.
Resistor selection should be determined by the voltage applied across the coil, the resistance of the coil, maximum power dissipation allowed, and the level of transient voltage to be tolerated. For example, if the coil is 50 ohms and the voltage applied is 24 V, an 82 ohm suppression resistor would allow the transient voltage to reach -39.6 V (V=IR= -.48 A X 82 ohms, the voltage is shown as negative since the polarity reverses when the switch opens). Using a 330 ohm resistor would allow a -158 V transient but the current would diminish in the loop faster. For power dissipation concerns, the resistor‘s power rating should be considered.
For example, if 24 V is applied across a 330 ohm resistor, the power dissipated by the resistor would be 1.75 W (P=V2/R). Therefore, the 330 ohm resistor should be selected that could dissipate at least 2 W. The heat generated by the resistor should be considered when selecting a resistor. NOTE: If the resistance of the suppression resistor is too low, the driver circuitry in the ECU may be loaded to a point where the relay or solenoid does not function properly. If the resistance is too high, the transient voltage may reach undesirable levels. There are other techniques that can be used for transient suppression.
Snubbers, Zener diodes, and varistors are all methods that have characteristics that make them better suited for some applications. But, for the simple applications of relays or solenoids, diodes or resistors should suffice. Inductive devices such as relays or solenoids should be located as far as possible from the components of the electronic control system. Wiring harnesses that are installed by the customer should be routed as far as possible from the wiring harness of the electronic control system in order to avoid problems that are associated with electrical noise.
4.6 Battery Disconnect Switch The machine OEM should incorporate a battery disconnect switch on the negative battery side of the battery circuit. The purpose of this switch is to disconnect the battery during long-term storage and to prevent electrical shock during machine service. This switch should not be used as an emergency shutdown switch or as an E-stop. Refer to Power Requirements (section 4.1) for information on ECU battery life. NOTE: The battery disconnect switch is not an emergency shutdown switch. NOTE: This is a change from previous generations of industrial engines. Previously, Caterpillar recommended that the battery isolation switch be on the battery +.
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