Installation Manual

Electrical & Electronic Application And Installation Manual
Production Release Version 1.0
Page 44
4.6 Suppression of Voltage Transients
4.6.1 Suppression Methods & Best Practice
Note: The installation of transient suppression at the source of the
transient is required.
The use of inductive devices such as relays and solenoids can result in the
generation of voltage transients in electrical circuits. Voltage transients that
are not suppressed can exceed SAE specifications and lead to a degradation
in the performance of the electronic control system.
The customer should specify relays and solenoids with built-in voltage
transient suppression. Refer to figure 4.7 for ways to minimise voltage
transients from relays and solenoids without built-in voltage transient
suppression. Techniques include the installation of a diode or resistor of the
proper size in parallel with the solenoid or the relay coil.
Diodes and resistors accomplish suppression in different ways. Diodes clamp
the voltage across the coil to approximately –0.7V 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 value of the resistance of the
suppression resistor. However, resistor suppression causes the current in the
loop to diminish much faster than would a diode.
Figure 4.9 Voltage Suppression Diagram
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 24V and the
coil has a resistance of 50 Ohms, then the current passing through the coil is
480mA (I = V/R). The diode then would need to be able to withstand a
reverse voltage of 24V and a forward current of 480mA 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.