ITT - Index of

SELENIUM CONTACT PROTECTORS
The voltage induced across the
coil is expressed by the equation
di Rl +r
V = -L-= (Rl+r) IE--L-t
dt
protected con'tacts, it is very appreciably
shorter than the decay
time of a circuit using the halfwave
configuration,
The current values listed in the
column "Coil Current for 300-V
Max, Rise" are representative
values, based on the observation
of typical relays and solenoids,
The maximum value of voltage V APPLICATION NOTES The specific value of current that
occurs when t = a V max, =
will limit the voltage at the con-
I(R + r) = E + Ir Voltage Range tacts to 300 volts is a function of
In an inductive circuit with un- The words "voltage range" are 'the coil design, the size of the coil,
protected switch contacts, the coil used because the maximum volt- the number of turns of Wire, etc"
current decays through a path that age rating increases in discrete and it is possi~le for a contact proincludes
the widening air gap be- steps, according to the number of tector to furnish adequat,e spa,rk
tween the moving contacts, The blocking cells in series, Each pro- suppression when used with cOils
high resistance of this gap results tector has a maximum permissible carrying currents greater, than
in a very short current decay time, voltage rating which is the low- those listed; but the. maximum
However, the voltage induced in voltage end of the range for the current rating, should not ,be ~xthe
coil at the instant of switch- next higher voltage group, ceeded, For this rea~on, It IS WIS~
i ng must rea c h a va I ue high,
enough to force the current across Max, COil, Current - m~ ,
to observe the action and effl-
,ciency of the protector before
the air gap, This leads to the elec- The ma~lmum permissible cOil freezing a design, Samples and
trical breakdown of the air gap curr~nt ,IS based on ,th~ thermal engineering assistance are availwith
attendant arcing, contact dissipation characterIStics of the able upon written request to the
damage and electrical noise, contac~ protec~or when operated Engineering Department.
at maximum cOil voltage and repe-
'
In the half-wave configuration the tition rate,
In selecting a suitable Contact
resistance of the protector is low Coil Current for 300-V Max. Protector from the list in this data
when it is acting as a suppressor, Rise _ mA
sheet, it should be noted:
since the coil current flows in the At the instant of switching, the Ratings are based upon operation
low-resistance forward direction th h th t t . in typical inductive circuits at
of the device, The forward resist- current roug e pro ec or IS
the same as the coil current, and temperatures of 50 e C or lower.
ance of the half-wave suppressor the induced voltage at the coil Current ratings given in the colwill
usually be only a small frac- terminals will rise to the value umn headed "Coil Steady State
tion of the coil resistance and, for required to drive this current Current for 300 volt max. rise"
practical purposes, may be re- through the protector, This volt- represent the rating of the progarded
as a short circuit around 'II dd t th I It tector to limit the voltage rise
the coil, The low value of protector age WI a 0 e supp y vo age
and appear across the contacts. across the contacts to approxiresistance
prevents the induced Sparking at the contacts will be mately 300 volts. This rating may
voltage from rising more than a minimized if the sum of these two be exceeded if a rise in excess of
volt or two over the battery volt- voltages does not exceed 300 volts, 300 volts is permissible, but the
age, and increases the current de- and this requirement is met if the maximum peak current rating
cay time compared to the decay . d d It' t th should not be exceeded, The voltt
'lme of a coil with unprotected In uce vo age IS no grea er an
the difference between 300 and age rating should not be exceeded
contacts.
the supply voltage. This condition under any circumstances,
In the back-to-back configuration is achieved by reducing, for a
the reverse resistance of the pro- given cell size, the permissible Fast Contact Action
tector is inversely proportional to coil current as the supply voltage
the induced voltage. This resist- increases. Thus, it will be obance
will be the smallest at the served that the coil current for
instant of switching and will in- 300-volt rise will be less than the
crease in value as the current de- maximum coil current for many of
creases. This n'onlinear resistance the higher-voltage units, Concharacteristic
causes the induced versely, the coil current for 300-
, voltage to rise to a value higher volt rise is so great that it exceeds
than it would reach in a circuit the maximum coil current for the
using the half-wave configuration, low-voltage units. These protecbut
it prevents the induced volt- tors carry the note in the rating
age from attaining the contact- table "MA for 300-Volt Rise Exdamaging
values existing in a cir- ceeds Max. Current Rating"; this
cuit with unprotected contacts, means that operation at the listed
Although the current decay time is maximum coil current will give a
increased somewhat compared to voltage rise of less than 300 volts
the decay time of a coil with un- - at the contacts.
12-6
Rate of operation of contacts in
the range 10 to 40 breaks per second.
The back - to - back arrangement
of the ITT Contact Protector
provides adequate arc suppression
with a negl igible effect upon the
release time of the relay, The protectors
are fated for applications
where the rate of operation of the
contacts is in the range of 10 to 40
breaks per second. ITT should be
consulted for recommendations if
the rate of operation exceeds 40
breaks per second.