Watlow Series 988 - Manual (Page 15)

WATLOW Series 988 User’s Manual 2.5 Installation and Wiring, Chapter 2
Wiring
RTD (100 Ω) input: Each 1Ω of lead wire resistance can cause a +2°C
error when using a two-wire RTD. A three-wire RTD sensor overcomes this
problem. All three wires must have the same electrical resistance (i.e.,
same gauge, same length, multi-stranded or solid, same metal).
Process input: Isolation must be maintained between input 1 and input
2. If both input 1 and input 2 are used as process inputs, a separate
power supply and transmitter must be used for each input. Output option
T (external signal conditioner power supply) can be used to supply power
for only one input.
Wiring 0-20 and 4-20mA Process Inputs
Certain “transmitters” used in process input applications are producing inter-
nal resistor failures in the Watlow Series 988 family of controllers. This is only
apparent with the Series 988 family 1/8 DIN units with Process Inputs selected
(0-20mA or 4-20mA dc only).
We are noticing that an external resistor is required to prevent a high in-rush
current which burns out the Series 988 family controllers’ 7-ohm internal
resistor. This high in-rush current occurs initially on “power-up.” If the trans-
mitter turns full on for a split second during power-up, the available current
weakens or damages the internal resistor.
Example: 20V / 7 ohms = 2,857mA (too much!).
The wiring diagram example below shows an application where a customer is
using a 4-20mA dc transmitter and power supply to feed the input of a Series
988 controller. The Rx range (100 to 400 ohms) for the external resistor is rec-
ommended. We suggest starting with 250 ohms.
Example: Customer is using a 24VÎ (dc) power supply to power up the 4-20mA
dc transmitter that inputs to the Series 988 terminals 8 (-) and 10 (+). To fig-
ure out what the internal Series 988’s handling current is for the 0-20mA or 4-
20mA dc input to the Series 988 controllers, we need to apply Ohm’s Law: The
square root of Watts divided by Resistance equals Current. Applying that for-
mula to the example below produces the following: Square Root of (0.125 Watts
/ 7 ohms) = 134 mA dc (handling input current). This is the acceptable input
current for the Series 988 universal input board.
Reminder, the input impedance of 7 ohms handles the majority of our cus-
tomer applications; the external resistor (Rx) is only for certain
transducers/transmitters that spike on power-up or power-down. Please make
sure your customer’s transmitter / transducer fall within our Series 988 family
(1/8 DIN) of controllers’ Process Input specification of 7 ohms input impedance.
Internal
Resistor
7 Ω
1/8 Watt
+
+ +
—
—
988 Process
Input Control
4 -20mA dc
Transmitter
100 to 400
ohms
24VÎ (dc)
Power Supply
—
R x
ç
CAUTION:
The Series 988 will
not function with
two grounded ther-
mocouple inputs.
Avoid using a
grounded thermo-
couple for both
input 1 and input 2.
Failure to follow this
guideline could
result in damage to
equipment.
NOTE:
Input-to-output iso-
lation is defeated
when the external
signal conditioner
power supply is
used to power a
transmitter connect-
ed to input 1 or
input 2.