Using Inductive Loads with VT System
Version 1.0
2011-06-15
Application Note AN-IND-1-015
Author(s)
Restrictions
Abstract
Jan Großmann
Public Document
This application note describes, what has to be considered when connecting inductive
loads to the VT System.
Table of Contents
1.0
2.0
3.0
4.0
5.0
6.0
Overview ..........................................................................................................................................................1
Behavior of inductive loads..............................................................................................................................1
Impact of inductive loads to the VT System.....................................................................................................1
Protection mechanism for inductive loads .......................................................................................................2
Conclusion .......................................................................................................................................................3
Contacts...........................................................................................................................................................3
1.0 Overview
One feature of the VT1004 is – besides using the internal electronic load – also to connect an external original
load. For connecting external loads, especially inductive loads, some issues have to be considered.
2.0 Behavior of inductive loads
In opposition to resistive loads, inductive and capacitive loads have no straight proportional behavior between
current and voltage. This behavior gives inductive and capacitive loads the ability to store energy.
In case of an inductive load the current flowing through causes a magnetic field. By switching off this load the
current flow will be interrupted and the magnetic field collapses immediately. The inductive load tries to hold the
switched off voltage and creates a self-induction voltage with a reversed polarity.
This resulting voltage peak can easily be ten to twenty times higher than the applied voltage.
Expressed in a formula the voltage over an inductive load behaves in the following way:
u (t )  L 
di(t )
dt
For this reason the big current change from maximum to zero within a small time during switching off an inductive
load leads to the high voltage. It can also be seen the straight proportional influence of the inductive load
dimension itself.
3.0 Impact of inductive loads to the VT System
When using external inductive loads with the VT1004 as seen in a simple example in Figure 1, these voltage peaks
can cause a malfunction of the VT1004 and in worst case the VT1004 can be damaged. The voltage peaks can
thereby occur by switching an inductive load using PWM and also by static switching of the internal relay for the
original load.
1
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Contact Information: www.vector.com or +49-711-80 670-0
Using Inductive Loads with VT System
When opening the original load relay connected to an inductive load, the voltage peak can generate an electric arc
between the contacts which might destroy the relay or reduce its life cycle.
So it is strongly recommended to switch off the applied voltage from the ECU first before opening the relay to
remove the original load.
The voltage transients by switching inductive loads are causing also interferences which might be coupled to wires
and tracks and will be therefore distributed within the whole system and will influence its behaviour.
ECU
DC
VT1004
Switch
Original
Load
Ch1
Ext.
Load
R
Ch2
L
Figure 1: Simplified VT1004 application with a pulse width modulated inductive load
4.0 Protection mechanism for inductive loads
To protect the VT1004, the high voltage peaks have to be suppressed by offering the inductive load the possibility
to keep up the current flow after switching off. Therefore additional parts are needed which are discharging the
peak voltage without influencing the original function of the application circuit.
It is therefore recommended to place at least a diode in parallel to the inductive load. This diode, called freewheeling diode, is placed in reverse direction to the applied voltage and will limit the voltage peak to its threshold
voltage.
Because of its small threshold voltage the diode has nearly no resistance where the energy can dissipated and so
it behaves nearly as short-circuit. Therefore it takes time until the energy of the inductive load is completely
discharged. To speed this up a resistor or a Z-diode has to be placed in series to the free-wheeling diode.
In the easiest case general purpose diodes or Z-diodes can be used. For special cases suppressor diodes are
recommended, which behave similar to the Z-diode but have a more rectangular characteristic especially at higher
currents.
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Application Note AN-IND-1-015
Using Inductive Loads with VT System
Diodes are for DC use only and might be destroyed if the wrong polarity is applied to. For polarity independent
applications bipolar suppressor diodes, voltage dependent resistors or RC-circuits have to be used.
A voltage dependent resistor has also a threshold voltage and is able to absorb high energies.
The dimensioning of a RC-circuit is depending on the applied voltage and current. Compared to diode and voltage
dependent resistor this circuit has no threshold voltage and will damp voltage peaks immediately. Therefore the
RC-circuit is very suitable for AC applications.
If using Z-diodes, suppressor diodes or a voltage dependent resistor, the threshold voltage depends on the
application but should not exceed the allowed input range of the VT1004 which is -32.7 V … 32.7 V. For example
in case of combining a diode with a Z-diode the voltage peak will be limited to
U lim.  U batt .  U D  U Z .
U batt .
Applied Voltage
UD
Forward voltage of diode
UZ
Reverse voltage of Z-diode
An overview over the various protection circuits is shown in Figure 2.
Figure 2: Protection of high voltage peaks
5.0 Conclusion
By switching inductive loads high voltage peaks can occur. To protect the VT1004 from these voltage peaks there
are various possibilities according to the case of application.
The basic protection is to use diodes to discharge the peak voltages. For DC use special care has to taken for the
right polarity of the used diodes.
Switching off the VT1004 internal relay for original load should be done without applied voltage when an inductive
load is connected to.
6.0 Contacts
Germany
and all countries not named below:
France, Belgium, Luxemburg:
Sweden, Denmark, Norway,
Finland, Iceland:
Vector Informatik GmbH
Ingersheimer Str. 24
Vector France SAS
168 Boulevard Camélinat
VecScan AB
Theres Svenssons Gata 9
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Application Note AN-IND-1-015
Using Inductive Loads with VT System
70499 Stuttgart
GERMANY
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+49 711-80670-111
E-mail: info@de.vector.com
92240 Malakoff
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Phone: +33 1 42 31 40 00
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+46 31 764 76 19
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China:
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UNITED KINGDOM
Phone: +44 121 50681-50
Fax:
+44 121 50681-69
Vector Informatik India Pvt. Ltd.
4/1/1/1, Sutar Icon, Sus Road,
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INDIA
E-mail: info@uk.vector.com
Vector Automotive Technology
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Room 1701, No.398 Jiangsu Road
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Phone: +86 21 6432 53530
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Korea:
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USA
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+81 3 5769 6975
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Application Note AN-IND-1-015