Part 2

Plug-In Assembly Configuration 487Designing External Contact Protection NetworksAfter establishing that the load is type 3 and thus requires additional contact protection,the next step is to design an appropriate contact protection network.Although many types of networks can be used, guidelines for design of RC, diodeand Zener diode protection networks are described.RC NetworkIn designing RC networks, the protection resistor Rp is selected as a compromisebetween two values. The minimum value of Rp is determined by the maximum acceptablerelay contact current (lo) of 4 amps. Thus, the mimimum value of Rp isVl4, where V is the peak value of the supply voltage. The maximum value is Rp isusually made equal to the load resistance, RL. So, the limits on Rp are: Vl4 < Rp< RL.To select a value for the protection network capacitor, Cp, first compute the totalcircuit capacitance C desired when the protection network is added. Then, Cp canbe computed from C = Cact + Cwiring + Cp. For the high voltage actuator, C mustbe such that the peak voltage across the relay contacts does not exceed 353 V.To compute the value of C required, use:where L = load inductance, lo is the current through L just before the relay contactsopen and C is the total circuit capacitance.For example, let's take another look at the type 3 load example discussedpreviously and see what the values of a typical RC protection networkmight be. The original circuit without contact protection is shown as (a).Recall that even with the snubber network the peak contact voltage Vc of425 V exceeded 353V and thus additional contact protection was required.To keep the maximum contact current below 4 amps, set Rp fmin) = V/4= 17014 = 42.5 ohms. Set Rp (max) equal to the load resistance of 400ohms. To keep the peak contact voltage below 353V peak, the total circuitcapacitance C is calculated from:Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com