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Chapter 7 Plug-in Modules
Protection Networks
7
For example, we will determine the typical values for an RC protection
network where the load is a small ac motor running on the 120 V ac line
(170 V peak). This motor draws a maximum of 2 amps.
Using Equation 1 we can find the minimum value for R
p
:
R
p
= V/Io = 170/2 = 85 Ω
The maximum value for R
p
would be equal to the load resistance or
400 Ω. Therefore, any resistor (preferably at least 1 watt) between 85
and 400 Ω will suffice.
To keep the peak contact voltage below 353V peak, use equation 3 to
determine Io:
Io = V/R1 = 170/400 = 0.425 A
Now use equation 4 to determine C:
C ≥ (Io/353)
2L
= (0.425/353)
2(0.1)
= 0.144 µF
Since R
p
can vary between 85 Ω and 400 Ω, an appropriate protection
network to be connected to this circuit is R
p
= 220 Ω and Cp = 0.15 µF.
Using Varistors
Just as contact protection circuits are important to suppress noise while
relay contacts are opening and closing, transient protection should be
provided while the relays are open. This is the purpose of the varistor.
When selecting a varistor, make certain that it has a voltage rating
sufficient for your application. A typical 250 VAC varistor can be
purchased with the Agilent part number 0873-0227.
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