A transistor switches the coil of a relay, which has a resistance of 360 Ω, on a supply of 9.0 V…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A transistor switches the coil of a relay, which has a resistance of , on a supply of V. The collector current is the current in the coil. The lowest value of in the batch is . The control voltage from a logic circuit is V. Take V and V. Find the current in the coil when the transistor is saturated, the least base current that saturates it and the largest base resistor that does it.
Need a reminder? The key ideaShow
The voltage gain of a common-emitter amplifier is , the current gain is and the power gain is . The output stays between V (saturation) and (cut-off). A transistor switch saturates when .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The saturated current. mA.
Step 2. The least base current. mA.
Step 3. The largest base resistor. .
Answer: mA, mA and .
Watch out. A designer picks a resistor a good deal smaller than the limit, such as , so that saturation is sure. A coil also needs a diode across it, because the coil makes a large voltage when the current stops.
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