The scariest moment when first powering a newly built circuit is a short from a wiring mistake. Batteries and adapters do not limit current, so a short burns parts in an instant, but a bench power supply with current limiting greatly reduces such accidents. This article covers the types and main specs of bench supplies, CC and CV modes and how to use current limiting, and alternatives (such as USB power).
Why you need a bench power supply
- You can raise the voltage continuously from 0 V and watch how the circuit behaves.
- With current limiting set, even if there is a short or a faulty part, the current does not exceed the set value, so the parts are protected.
- The voltage and current are shown on a display, so you can see right away how much current the circuit draws. It is the fastest way to judge whether the design is normal.
Types
| Type | Characteristics | Suited for |
|---|---|---|
| Linear bench supply | Low noise and stable. Heavy and large | Analog circuits, precision experiments |
| Switching bench supply | Light and efficient. Some switching noise | General digital circuits, portability |
| Small DC-DC adjustable module + adapter | Very cheap. Current limit and display vary by module | Simple beginner experiments |
| USB 5V power | Fixed 5 V, available anywhere | 5 V logic circuits (USB 5V Power Board) |
| Battery (AA, 9 V) | Fixed, silent | Small portable circuits |
Reading the main specs
- Output voltage and current range: for beginners, 0–30 V and 0–3 A or 5 A is typical. For mostly 5 V and 12 V circuits, 0–30 V is enough.
- Resolution: whether voltage can be adjusted and shown in 0.01 V steps, and current in 0.001 A (1 mA) steps. It matters when looking at low-current circuits (a few mA).
- Ripple and noise: noise mixed into the output (in mV). The smaller the better for analog and audio circuits.
- Number of channels: a single channel is enough, and two or more is handy for circuits that need + and − supplies (such as op-amps).
- Protection features: check whether it has overvoltage (OVP), overcurrent (OCP), and overtemperature (OTP) protection.
- Output terminals and ground: banana terminals are convenient. A product with a floating output (isolated from ground) is handy because either + or − can be grounded.
CV and CC modes
A bench supply operates in two ways.
- CV (constant voltage): holds the output voltage at the set value, and as much current flows as the load needs. This is the mode when the circuit is normal.
- CC (constant current): when the current reaches the set limit, it lowers the voltage to hold the current at that value. On a short or overcurrent, the supply switches to this mode automatically.
So if you set the current limit slightly above the circuit's expected current, it works in CV when normal and switches to CC when something is wrong, protecting the circuit.
The order for the first power-up
- With the output switched off, set the voltage to the value you want (for example, 5.00 V).
- Set the current limit. For a circuit being powered for the first time, set it low, around 50 to 100 mA.
- There is also a way to short the output terminals (output ON) and check that the current setting is the intended value. If you have never done it, follow the manual.
- Turn the output off and connect to the circuit. Red (+) goes to +V and black (−) to GND.
- Turn the output on and watch the current display. If it is larger than expected or the CC indicator lights, switch off immediately and check for a short (Power Rails, Grounding, and Decoupling Capacitor Placement).
- If it is normal, raise the current limit to 1.5 to 2 times the current the circuit uses.
Usage tips
- Judge state by current draw: if you know a circuit's normal current draw, you can tell right away of a problem when the current is larger than usual or near zero (It Is Finished but It Does Not Work: A Step-by-Step Checklist).
- Test at lower voltage: raising the voltage and seeing at which point it works tells you an IC's minimum operating voltage.
- Wire thickness: for large currents, use thick wire and solid terminals.
Common mistakes
| Mistake | Result | Prevention |
|---|---|---|
| Starting with the current limit at max | Parts damaged in a short | Always start low |
| Connecting the circuit with the voltage set high | An IC destroyed by momentary overvoltage | Set it with output OFF, then connect |
| Connecting + and − the wrong way round | Polarized parts fail | Color coding, check before connecting |
| Using several supplies without a shared ground | Signal references mismatch | Connect the GND of all supplies |
| Setting a voltage above a part's rating | Part destroyed | Check the absolute maximum ratings in the datasheet (How to Read a Datasheet) |
Alternatives
If a bench supply feels like a burden at the beginner stage, you can start with these combinations.
- Test 5 V circuits with a USB power board (with a fuse).
- Make 5 V from a 9 V adapter with a 7805 power board (7805 Power Board).
- Put a resistor of about 100 Ω or a fuse in series with the power line to limit current the first time you turn it on (temporary protection).
But if you keep experimenting, a supply with current limiting is the best protection, so we recommend it as your first equipment investment.
Summary
A bench supply is a tool that lets you adjust voltage and limit current to test a new circuit safely. Start with a low current limit, connect with the output off, and watch the current draw to catch problems early. It is even more effective together with measurement, so see also How to Choose a Multimeter.