The current limit
A bench supply set to 12 V with a 1 A limit, into ten ohms: the lamp says CC, the output sits at 10 V, and the reason is the whole lesson. There is no microcontroller in it: the simulator solves the 3 parts as a circuit, so it runs the moment you press Run, and nothing needs compiling.
Teaching? Assign it to your class in one click.
Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $5.05. On Mokxi they're free.
What you'll learn
- Constant voltage versus constant current
- Ohm's law as a way to predict the output
- Why a current limit protects a circuit
- Why the load resistor must be a power part
- Difficulty: Beginner
- Time: About 10 minutes
- Parts: 2
How it works
A bench supply has two jobs: hold a voltage steady, or hold a current steady. This one is set to 12 V with a 1 A current limit, and it feeds a 10 ohm resistor.
Ohm's law says 12 V across 10 ohms would draw 1.2 A. The supply is not allowed to give that, so it switches to constant current. It holds 1 A and lets the voltage fall to 1 A times 10 ohms, which is 10 V.
The CC lamp lights instead of CV, and the multimeter across the resistor reads 10 V, not 12. The voltage you dialed in is only a ceiling.
Parts list
2 parts, plus the jumper wires and 2 bench instruments. The parts retail for about $5.05; on Mokxi they're free, and so are the instruments.
- 1 × Full-size breadboard
- 1 × Resistor, 10 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Bench power supplyIn Pinball Table Kit.
- 1 × Multimeter
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Resistor, 10 Ω pin 2 | Ground |
| Bench power supply pin V- | Ground |
| Multimeter pin COM | Ground |
| Bench power supply pin V+ | Resistor, 10 Ω pin 1 |
| Multimeter pin V | Resistor, 10 Ω pin 1 |
The notes that come with it
There is no microcontroller in this one, so nothing is compiled: the simulator solves the circuit itself. The file in the editor is the notes that come with it.
// The current limit, and what CV and CC mean.
//
// The supply is set to 12 V with the current knob at 1.00 A, and there are
// ten ohms across it. Ohm's law wants
//
// I = 12 V / 10 ohm = 1.2 A
//
// and the supply is not allowed to give that. So it does the other thing it
// can do: it holds 1.00 A and lets the voltage fall to whatever ten ohms
// makes of it.
//
// V = 1.00 A x 10 ohm = 10.0 V
//
// Which is what both displays say, and what the multimeter across the load
// says as well. The CC lamp is lit, not CV, and that is the whole reading:
// the supply is in constant current, so the number you set on the voltage
// knob is not what is on the terminals.
//
// Things to try, all of them with the circuit still running:
//
// CURRENT + turn the limit up past 1.2 A and the lamp flips to CV, the
// voltage climbs to the 12 V you asked for and the current
// settles where Ohm's law puts it. The crossover is the thing
// to watch: it happens at exactly 1.2 A.
// CURRENT - turn it down to 0.50 A and the output halves to 5 V. In
// constant current the supply IS the resistor.
// OUTPUT takes the terminals off the circuit. The meter reads zero,
// which is different from the supply reading zero.
//
// The load is a 10 W resistor, and it has to be: one amp through ten ohms is
// ten watts of heat, and an ordinary quarter-watt one would burn out.
//
// Why this matters: a current limit is what lets you power something you are
// not sure about. Set the limit to what the circuit should draw, and a short
// or a part in backwards costs you a lamp on the front panel instead of a
// part. It is the single most useful thing on a bench supply and the one
// beginners leave wound up to maximum.
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The meter reads 10 V and I expected 12 V This project
That is the lesson. The supply is in constant current at its 1 A limit. Raise the current limit above 1.2 A and the lamp flips to CV. - The resistor gets hot or burns out This project
One amp through 10 ohms is 10 watts of heat. Use a 10 W resistor, not a small one. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure. - A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground. - The output voltage looks wrong or collapses under load because the current limit is set too low and the supply is in constant current mode (CC). Bench power supply
Set the voltage first, then set the current limit above what the circuit needs. Watch the CV and CC lamps. CV means the voltage is held, CC means the limit is working. - Too much voltage burns parts that expect 3.3 V or 5 V. Bench power supply
Set the voltage with the output off and nothing connected, check it, then connect. Start the current limit low for a new circuit, like 100 mA, and raise it if needed. - The reading is zero, negative or nonsense when measuring voltage. Multimeter
Set the dial to volts first, then touch the probes across the part, in parallel. Red goes to the more positive point. A negative number just means the probes are swapped. - Measuring current gives zero or blows the meter's fuse. Multimeter
Current must flow through the meter, so break the circuit and put the meter in series. Never put an amps setting straight across a supply. Start with the highest current range.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.