Litmux Studio

LessonPower sources and safety · 14 min

Bench supplies, adapters and the current limit

A bench power supply is the electronics workshop's adjustable battery - with one feature that saves many circuits: a current limit.

Constant voltage and constant current

  • You set a voltage (say 12 V) and a current limit (say 100 mA).
  • While the load takes less than the limit, the supply holds the voltage: CV (constant voltage) mode, like a perfect battery.
  • If the load tries to take more, the supply lowers its voltage just enough to hold the current at the limit: CC (constant current) mode, and the CC light comes on.
12 V into 47 Ω would be 255 mA. The limit is 100 mA, so the supply goes into CC: 100 mA flows and the voltage settles at 0.1 A × 47 Ω = 4.7 V.
12 V into 47 Ω would be 255 mA. The limit is 100 mA, so the supply goes into CC: 100 mA flows and the voltage settles at 0.1 A × 47 Ω = 4.7 V.

Adapters and chargers

A wall adapter marked "12 V 2 A" gives 12 V and can supply up to 2 A. The load takes what it needs. So the rules for choosing one are simple:

  • The voltage must match what the device needs.
  • The current rating must be at least what the device draws (more is fine).
  • The polarity of the plug must match (look for the centre-positive or centre-negative symbol).

Making one voltage from another: regulators (a preview)

  • Linear regulator (7805, LM317, AMS1117): simple and quiet, but it burns the extra voltage as heat: P = (Vin − Vout) × I.
  • Switching (buck) converter (LM2596 modules, phone chargers): 85 - 95 % efficient, so it stays cool even when stepping 24 V down to 5 V.
Worked example: a hot 7805

A 7805 makes 5 V at 0.5 A from a 12 V battery. How much heat does it make?

  1. It drops 12 − 5 = 7 V.
  2. P = 7 V × 0.5 A = 3.5 W.

Answer: 3.5 W - it needs a heatsink. A buck module would waste well under 0.5 W.

Where you meet it

Repair benches

A current-limited supply powers a suspect board without letting a fault burn it.

USB

5 V by default; USB Power Delivery chargers negotiate 9, 15 or 20 V with the device first.

Solar charge controllers

Limit the charging current into the battery - CC, then CV, like a bench supply.

Practice 1A supply set to 10 V with a 200 mA limit feeds 100 Ω. Which mode is it in, and what current flows?

  1. CV, 100 mA
  2. CC, 200 mA
  3. CV, 200 mA
  4. CC, 100 mA
Show the answer

Answer: A. CV, 100 mA

10 / 100 = 100 mA, under the limit: CV.

Practice 2The same supply (10 V, 200 mA limit) feeds 20 Ω. What voltage does it show?

Show the answer

Answer: 4 V

20 Ω would need 500 mA: CC at 200 mA. V = 0.2 × 20 = 4 V.

Practice 3A router needs 12 V 1.5 A. Which adapter is right?

  1. 9 V 2 A
  2. 12 V 1 A
  3. 12 V 2 A
  4. 19 V 3 A
Show the answer

Answer: C. 12 V 2 A

Same voltage, at least the current.

Challenge 4A buck converter makes 5 V 2 A from a 12 V battery at 90 % efficiency. What current does it take from the battery?

Hint

Output power / efficiency = input power.

Show the answer

Answer: 0.926 A

Pout = 10 W; Pin = 10 / 0.9 = 11.1 W; I = 11.1 / 12 = 0.93 A - less than the 2 A coming out. (A 7805 would take the full 2 A from the battery.)