A 2N3904 bipolar transistor switches a relay coil that draws when energised

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Semiconductor devices and analogue circuits

Electronics for Measurement, Ch Transistors: a current controlled by a third terminal

Problem

A 2N3904 bipolar transistor switches a relay coil that draws

I_C = 100\,\mathrm{mA}

when energised. To guarantee saturation regardless of device spread, the base drive current is chosen as a tenth of the load current,

I_B \ge I_C/10

(a standard forced-gain rule, well below the data sheet's minimum

\beta_F

). The base is driven through a series resistor

R_B

from a

5.0\,\mathrm{V}

logic output; take the base-emitter voltage in saturation from the constant-drop model,

0.6\,\mathrm{V}

. ((a)) Find the largest value of

R_B

that meets the forced-gain requirement. ((b)) Choose the nearest E12 value that still meets it, and state the actual base current it gives.

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