Watlow 989 Operating Manual

Addendum to:

Series 988 Family

1/8 DIN Microprocessor-Based
Temperature/Process Controllers

User's Manuals

(1796) Made in the U.S.A.

Addendum to: Printed on Recycled Paper
Series 988 Users Manual - 0600-0009-0001 Rev W 10% Postconsumer Waste
Series 982 Users Manual - 0600-0009-0008 Rev N
Series 98LF Users Manual - 0600-0009-0000 Rev A
Series 998 Users Manual - 0600-0010-0000 Rev F

June, 1999

Watlow Controls
1241 Bundy Boulevard
P.O. Box 5580
Winona, Minnesota
55987-5580
Phone: [phone]
Fax: [phone]

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CAUTION:
Refer to the
accompanying
document for wiring
and safety
information.

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 in­rush 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
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 customer 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.

988 Process

Input Control

4-20mA dc

Internal

Resistor

7Ω

1/8 Watt

+

—

R

100 to 400

ohms

x

Transmitter

—

+

+

24VÎ (dc)

Power Supply

—