Guide · Meters, supplies and scopes
How to use a bench power supply
Set the voltage, set a low current limit, then switch on and watch the current. How CV, CC, OVP and OCP work, and what to look for in a bench supply.
Short answer
Set the voltage with the output off, set a low current limit, connect the leads, then switch on and watch the current. If the supply jumps straight to its current limit, something on the board is drawing too much: switch off and look for a short before you raise the limit. Over-voltage and over-current protection (OVP and OCP), where a supply has them, add a second line of defense by switching the output off.[1]
How a bench supply behaves
A bench supply has two main settings: a voltage and a current limit. It always works in one of two modes:
- Constant voltage (CV): the supply holds your set voltage, and the board draws whatever current it needs, up to the limit.
- Constant current (CC): when the board tries to draw more than the limit, the supply holds the current at the limit and lets the voltage fall.
That switch from CV to CC is what protects a board with a fault. A short circuit gets your limit, not everything the supply can deliver. Look for a CV/CC indicator on the front panel; it tells you at a glance which mode you are in.
Current limit vs OCP. A current limit keeps the output on and holds the current. OVP and OCP switch the output off when voltage or current passes a threshold you set. Rigol's DP700 series, for example, has user-set OVP and OCP that switch the output off.[1]
Step by step: powering a board
- Find the right voltage. Use the device's input rating or its service documentation. Do not guess from the connector.
- Set the voltage with the output off.
- Set the current limit low. If you know roughly what the board draws at idle, set the limit a little above that. If not, start low and raise it in small steps.
- Set OVP and OCP if your supply has them, a little above your voltage and current settings.
- Connect with the output off: red to positive, black to negative or ground. Check the polarity twice; reversed polarity can damage a board.
- Switch the output on and watch the current. On a healthy board, the current rises and settles below the limit, with the supply in CV.
- Raise the limit only when you know why the board needs more.
Reading the display
| What you see | What it usually means | What to do |
|---|---|---|
| CV, current settles below the limit | Normal | Carry on |
| CC as soon as you switch on, voltage near zero | A short on the rail | Switch off; find the short with a multimeter |
| CC with the voltage partly sagged | An overload or partial short | Switch off and investigate before raising the limit |
| CV, zero current | No connection, an open fuse, or the device is switched off | Check the leads, fuses and power switch |
A supply's display shows what leaves its terminals. To confirm what actually reaches the board, measure at the board with a multimeter; multimeter CAT ratings explained covers what to check before that meter goes near mains.
Choosing a supply
| Feature | Why it matters | A published example |
|---|---|---|
| Voltage and current range | Covers the devices you repair | Rigol DP711: 30 V / 5 A; DP712: 50 V / 3 A; both 150 W[1] |
| OVP and OCP | Shut the output off on a fault | DP700: user-set, switch the output off[1] |
| Resolution | Lets you see small currents and set small steps | DP700: optional 1 mV / 1 mA[1] |
| Noise and regulation | How clean and steady the output is | DP700: noise under 500 µVrms / 3 mVpp; 0.01% line and load regulation[1] |
| Output on/off button | Lets you set everything before power reaches the board | Check the front panel |
| Mains input | Must suit your outlet | US outlets supply 110-120 V at 60 Hz[2] |
Note the trade-off in Rigol's range: no single DP700 model offers both 50 V and 5 A.[1] Choose the voltage and current your devices need rather than the biggest numbers on the box. If your supply has an input-voltage selector, check that it is set for your outlet before you plug it in.
Linear and switching designs both exist. Judge a supply by its published noise and protection figures rather than by the design label alone; a listing that publishes neither gives you nothing to compare.
Batteries and lithium
- Battery steps come first. Follow your device's repair guide before you power anything. In its ROG Ally battery guide, iFixit tells readers to let the battery drain below 25% before starting, because a charged lithium-ion battery can catch fire if it is damaged.[3]
- Charging is out of scope. This guide does not cover charging cells from a bench supply. Use the device's own charger.
- Disposal: EPA says lithium-ion batteries should not go in household garbage or recycling bins. Take them to battery recycling or household hazardous waste collection, with the terminals taped or each battery bagged.[4]
Next
Questions
What does current limiting do on a bench power supply?
It caps the current the supply will deliver. When a load tries to draw more, the supply holds the current at your limit and lets the voltage fall, which stops a shorted board from drawing everything the supply can give.
What is the difference between a current limit and OCP?
A current limit keeps the output on at the set current. OCP switches the output off when current passes a threshold; Rigol's DP700 series, for example, has user-set OVP and OCP that turn the output off.[1]
Is a 30 V 5 A bench supply enough for repair?
It depends on your devices, so check the highest voltage and current they need. As a reference, Rigol's DP711 is a 30 V / 5 A model, while its DP712 trades current for voltage at 50 V / 3 A.[1]
Can I power a phone board from a bench supply?
Follow the device's repair guide first, starting with its battery steps. In its ROG Ally battery guide, iFixit says to drain the battery below 25% before starting, because a charged lithium-ion battery can catch fire if damaged.[3]
Why does my supply go into CC mode as soon as I switch on?
The board is trying to draw more than your current limit. On a board that should draw little, that usually points to a short: switch off and investigate before you raise the limit.