Watlow Series 982 - Manual (Page 15)

WATLOW Series 982 User’s Manual 2 . 5 Installation and Wiring, Chapter 2
Wiring
Wiring 0-20 and 4-20mA Process Inputs
Certain “transmitters” used in process input applications are producing
internal 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 transmitter 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 recommended. 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 figure 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 formula to the example below produces the follow-
ing: 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
customer applications; the external resistor (Rx) is only for certain trans-
ducers/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
Figure 2.5 -
Process wiring
example.