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Type TD-5 Class 1E Time Delay Relay
2.8
OPERATION OF TD-5 RELAY
Operation of the TD-5 relay occurs when the silicon
controlled rectified (SCR) switches from a non-
conducting state to a conducting state. In the non-
conducting state, the SCR acts as an opened switch to
prevent energization of the TR relay, but in the conducting
state it acts as a closed switch to connect the TR relay
to the dc source. To switch the SCR from a non-
conducting state to a conducting state requires that a
maximum of 20 microamperes flow in the gate of the
SCR. This current is produced by the difference in voltage
across the capacitor (C) and brush of the potentiometer
(P).
When dc voltage is first applied to the relay, voltage
instantaneously appears across the potentiometer brush
but is delayed in building up across the capacitor in
accordance with the R-C time constant of the circuit.
As long as the capacitor voltage is less than the
pontentiometer brush voltage, a reverse voltage appears
across the diode D2 and the SCR to keep the silicon
controlled rectifier (SCR) biased off.
When the capacitor voltage reaches the potentiometer
brush voltage plus approximately one volt (forward voltage
drops across SCR and D2), gate current will flow to the
silicon controlled rectifier SCR. This current switches
the SCR to a conducting state to allow the TR relay to
pickup.
The SCR latches on when it switches and can be reset
only by removing voltage from terminals 8 and 9 of the
relay. If a trip coil supervisory indicator lamp is used
when the timer is used and with a fault detector to trip a
breaker, the breaker "a" contact must be connected
between terminal 8 of the relay and negative.
The rate at which the capacitor charges is determined
by the rheostat (T) setting. The charging rate is not a
linear function of the rheostat setting, since RP gives a
parallel resistive path. This has the effect of expanding
the scale for short times and thereby permitting more
accurate settings.
3.0 CHARACTERISTICS
The TD-5 relay is available in the following timing ranges:
0.05 - 0.4 seconds
0.05 - 1.0 seconds
0.2 - 4.0 seconds
1.5 - 30 seconds
The relay voltage rating can be 24/32 vdc, 48/125 vdc,
or 250 vdc.
3.1
VOLTAGE RATING OVER THE TEMPERATURE
RANGE
The relay can stand 110% of rated voltage continuously
over a temperature range of -40° C to +70° C.
3.2
REVERSE POLARITY
Diode (D1) limits reverse voltage of the static components
to less than one volt dc, so that no damage is done to
the circuit by connecting the relay with reverse polarity.
However, the relay will, of course, not operate under
this condition, and series resistor (RS) may overheat if
reverse voltage is applied for approximately 15 minutes
or more.
3.3
RESET TIME
TR dropout time = 0.1 sec. or less. TR dropout time of
TD-5 Relay with TX contact is an additional 15 msec.
3.3.1 Discharge of timing capacitor:
The discharge of C is essentially instantaneous, the
RC time constant through P being less than 20
milliseconds, in most cases. However, the discharge
path through P is limited by silicon voltage drops through
SCR and D2, totaling approximately one volt. Therefore,
C discharges rapidly through P down to about one volt
and then more slowly through RP down to zero volts.
41-579.11B
7
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