Specifications

GENERAL INSTALLATION GUIDELINES
ADVANCED MICRO CONTROLS INC.
4
30
Surge (EMI) Suppression (continued)
Surge Suppression: DC Outputs
All inductive DC loads require a commutating, or “fly-back” diode across the load. Inductive DC loads
include relays, solenoids, and DC motors.
Unlike resistors, diodes have a polarity and only conduct current in one direction. Therefore, care must be
taken when installing diodes. As shown in the figure below, the cathode of the diode, which is denoted by the
white or black band on one end of the diode, must be installed on the positive side of the load. If you install
the diode backwards, it will most likely destroy itself as soon as you apply power to the load.
Figure 4.4 DC Output Surge Suppression
h
The diode must be sized to handle the inductive surge of the load when it turns off.
h
Some devices can be ordered with built in fly-back diodes, or the device manufacturer will offer sup-
pressors designed specifically for the device. These types of devices are strongly recommended.
Surge Suppression: AC Outputs
If you are also switching AC loads with hard contacts such as mechanical relays or contactors, then you must
install a suppression network on the load switched by these hard contacts. The two most common suppres-
sors for AC loads are varistors and R-C networks.
h AMCI strongly suggests R-C networks for all AC applications.
A varistor is a solid state device that turns on and conducts when the voltage across its terminals exceeds its
rated value. Herein lies the problem with using a varistor as an AC suppressor. The voltage (problem) must
be generated before the varistor responds. In our testing we have found that hard contacts will still arc when
a varistor is placed across an AC load. This arcing is due to the fact that the breakdown voltage of the air
between the contacts when they first open can be less than the rated voltage of the varistor. If the instanta-
neous AC voltage applied to the contacts is above the breakdown voltage of air, but less than the rated voltage
of the varistor, the contacts will arc.
On the other hand, an R-C network acts as a low-pass filter, instantaneously dampening fast transients when
they occur. The main drawback of R-C networks is that they are harder to correctly specify than varistors.
Varistors only require you to specify breakdown voltage and power dissipation ratings. R-C networks require
you to balance the need of suppression when the contacts open against the amount of surge current the relay
can tolerate when the contacts close. Table 4.1 shows the trade-offs you must be aware of when specifying
R-C networks.
Table 4.1 R-C Network Trade-offs
In general, capacitor values range from 0.1 to 1.0 µF and resistor values range from 150 to 680 ohms.
The easiest way to specify a R-C network is by following the recommendations of the load’s manufacturer.
Most manufacturers have tested and specify standard R-C networks, and many sell networks that are designed
to integrate with their products. If you cannot get help from your load’s manufacturer, feel free to contact
AMCI for assistance.
When Contacts Close When Contacts Open
Low Resistance,
High Capacitance
Higher surge current through relay contacts
to charge capacitor. (Negative)
Lower transient voltage spike. (Positive)
High Resistance,
Low Capacitance
Lower surge current through relay contacts
to charge capacitor. (Positive)
Higher transient voltage spike. (Negative)
Diode
n/o
Inductive
DeviceContact
Power In
Power Return
DC Load Connection
+ –
COLORED BAND