ge
installations. There is no intent to foresee all possible variables
in individual situations, nor to provide all training needed to
perform these tasks. The installer is ultimately responsible
to assure that a particular installation will be and remain safe
and operable under the speci c conditions encountered.
Introduction
When an inductive device, such as a contactor coil, solenoid, relay coil, or motor
turns o , large voltage spikes known as back-EMF can be introduced in the circuit.
Parallel connection of inductive devices can substantially increase the amplitude
of the voltage spike. To avoid damage to a control panels D.O. (Digital Output)
switching circuit, or other circuit components, a means of reducing, or clamping the
back-EMF e ect should be used. This application note discusses available transient
limiting devices which can be used to provide that protection. The selection of speci c
components should be referred to a competent designer. Selection is usually based on
cost and mechanical considerations, since mounting, connecting and insulating the
selected parts will add cost to the job.
Methods
There are four types of transient suppression circuitry commonly in use: Metal Oxide
Varistors (MOV), Transient Voltage Suppressors (TVS), diodes (in DC circuits) and
resistor/capacitor (R/C) snubber combinations. Of these choices, the rst three o er
single component solutions which can often be connected along with the usual wiring
to the component needing suppression. The resistor/capacitor method essentially
creates a tuned circuit to reduce voltage excursions. Although potentially lowering
component cost, engineering and wiring considerations usually limit use to OEM
circuitry.
MOVs
MOVs o er easy selection based on clamping voltage and energy ratings and
low cost. The negatives are mechanical fragility, physical bulk and a characteristic
reduction in the clamp voltage after repeated absorption of high-energy spikes
eventually reaching the operating voltage. If the latter occurs, the device will heat and
potentially be a re hazard. Some form of current limiting, thermal fusing, or other
thermal isolation is advised by safety agencies when using these devices.
TVSs and diodes
TVSs and diodes are similar for DC applications. TVSs are designed to withstand higher
energy levels and can be obtained in bidirectional versions for AC circuits. Diodes
can be sized to protect the circuit without damage to themselves. Both are a single
component answer to the back-EMF problem and are mechanically robust, though
especially the TVSs are often a higher cost component.
Solutions
Usually, any one of the protection devices shown below is enough to solve the
problem. For small relays in DC circuits, a 1N4004 diode, or a P6KE30A TVS (one per
device) may do the trick, but remember that the requirements of your application
should be reviewed to select the correct components for the job.
HAZARD OF ELECTRIC SHOCK, BURN
OR EXPLOSION
Methods and suggestions described in this document
are intended for use ONLY by trained professionals.
Lethal voltages can exist within electrical enclosures
as discussed herein.
D A N G E R
!
Serious injury or death will result from misapplication
of the information provided in this document.
Veris Current Sensor
Relay Coil, e.g. H938
Aux. Relay Coil
Diode
(DC Only)
MOV
TVS
R
C
D.O.
COMMON
Select one of these
+ (if DC)