b) A digital interfacing card (Labjack) is able to supply a maximum output curre

Question

b) A digital interfacing card (Labjack) is able to supply a maximum output current of 20mA. Using a digital output of (0V, 3V) the following circuit can drive the relay Given the following values: Relay: 12V, 200 Ohm., Transistor gain:he 250 +15V Relay:12V, 200 Ohm Transistor hfe- 250 i) Whatis the switching current of the relay? ii) Calculate the maximum value of the base resistor, R, which allows the relay to switch with a V input i What current does the Labjack supply with your chosen value of R? Note any assumptions made.

Explanation / Answer

(i) Switching current of relay (IC) = (VCC-VOUT)/Rrelay

IC = (15-3)/200 = 60mA

We know that transistor gain hfe = IC/IB

(iii) IB = IC/hfe = 60m/250 = 240uA

Considering VBE(sat) = 0.7V,

(ii) Base resistor (R) = (Vin-VBE(sat))/IB

R = (3-0.7)/240u = 9.583 kilo ohms

R =10k ohms (approx)

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