Understanding Power Sources in Electronics Labs

EP215 Lecture 2 Power Sources: Voltage, Current EP215 Electronics Lab 1 Slide 1

Review of Lab 1 (A) Benchtop power supply : DC power CV CC DT 5V +V0-V +V 0 5V 0 CV mode : Adjust +V w.r.t 0 CC mode : Set +V, adjust current Fixed 5V,0 Useful for Digital ckts Dual Tracking: One knob sets |V| EP215 Electronics Lab 1 Slide 2

We used bench PS in CV mode Set Voltage output to value Vset As the load RL = R1 + R2changes, the voltage across R1, R2 each change based on resistor voltage divider: ?2 ?1+ ?2 ???? ??2= ???? ?? Current also changes based on Ohm s law: ? = For any load RL and any current, the source supplies constant voltage Vset: Voltage Source EP215 Electronics Lab 1 Slide 3

Is the Function Generator a Voltage or Current Source? Really, it s a voltage source (of AC voltage). Notice the 50 output resistance label! We demonstrated how to make it act like a current source FG 50 Follows definition of a voltage source: Small internal R in series EP215 Electronics Lab 1 Slide 4

How to convert a Voltage source into a current source? First guess: current source has a Large internal resistance in parallel So how about adding 100 k in parallel ? FG FG 50 Doesn t work because load Still sees ~ 50 100k 100k 50 50||100k ~ 50 EP215 Electronics Lab 1 Slide 5

How to convert a Voltage source into a current source? Second guess: Add 100 k in series ? FG FG 100k 100k Works Load sees 50 k (Note that just adding the 100 k series would also work!) 50 100k 100k 50 100k||100k =50k EP215 Electronics Lab 1 Slide 6

How good is our current source? Current Source: Large internal resistance in parallel FG 50k Works RL only if RL << 50 k EP215 Electronics Lab 1 Slide 7

RL is important in determining source characteristics Current Source: Large internal resistance in parallel FG 50k Works If RL becomes large, say RL=50 k , Current will be split in half between internal 50 k and RL EP215 Electronics Lab 1 Slide 8

Source behavior is dependent on RL In general, a voltage source likes a large RL In general, a current source likes a small RL EP215 Electronics Lab 1 Slide 9