Why this happens
A battery can deliver a very large current into a fault, which is how wires melt and parts burn. A bench supply with a current limit lets you choose the maximum current: if something is shorted or faulty, the supply drops its voltage and holds the current at your limit. That turns a potential burnout into a harmless reading.
What you need
Tools
- Adjustable bench power supply with constant-voltage and constant-current modes
- Digital multimeter
- Crocodile-clip test leads
Materials
- Inline fuse holder and fuses
- Test items such as a 12 V LED light, horn or small sensor module
- Fire-safe work mat
Before you start
- Do not use a bench supply to charge lithium cells or EV batteries; use only the charger supplied for that battery.
- Do not connect a bench supply to the vehicle's wiring or battery; test parts off the vehicle.
- Never touch the output terminals while the output is on, and keep the supply's mains side closed and untouched.
- Large electrolytic capacitors in some items stay charged after switching off; wait and measure before touching.
Step by step
- 1
Learn the two modes
A bench supply has a voltage knob and a current knob. In constant-voltage mode it holds the voltage you set, and the load draws what current it needs. If the load tries to draw more than the current limit, the supply switches to constant-current mode and lowers the voltage to hold the current at your limit. Indicator lights usually show CV or CC.
- 2
Set the voltage with no load
With nothing connected, switch the supply on and set the voltage to what the item expects, for example 12 V for a 12 V accessory. Check it with your multimeter at the output terminals, because front-panel displays are not always accurate. Switch the output off, or the supply off, before connecting anything.
- 3
Set the current limit with a short
Turn the current knob fully down. Short the output terminals with a thick test lead and turn the output on. The supply enters CC mode. Slowly turn the current knob up until the display shows a limit a little above what the item should draw, as shown on its label or datasheet. Turn the output off and remove the short.
Tip: Start with a limit well below the expected draw for unknown parts and raise it slowly.
- 4
Connect the load with a fuse
Fit an inline fuse in the positive lead, rated for the item. Connect positive to positive and negative to negative, double-checking polarity, because many small electronics are not protected against reversal. Lay the item on a fire-safe mat. Keep the output switched off until all connections are made and nothing can short against the bench.
- 5
Power up and watch the display
Turn the output on and watch the CV or CC light and the current reading. If the supply stays in CV mode and the current matches expectations, the item is drawing normally. If it jumps straight into CC mode at your limit, something is drawing too much: switch off and check for a short, wrong polarity or a faulty part before going further.
- 6
Test and measure
With the item running, check it works: the light is bright, the horn sounds, the sensor output changes. Measure voltage at the item itself with your multimeter to see any drop in the leads. Note the current drawn; this tells you what fuse rating and wire size an installation on the vehicle will need. Feel for nothing; use an IR thermometer for heat.
- 7
Switch off and record
Turn the output off before disconnecting. Record the item, voltage, current drawn and result in your notebook. Keep these records with the part, so if it later fails on the vehicle you know its normal draw. Never leave the supply powering an unattended test, and switch it off at the wall when finished.
How to tell it worked
You can bring up any low-voltage accessory on the bench with a known current limit, see straight away if it draws too much, and record its normal current for fuse and wire sizing.
Stop and call a technician if
- The item smokes, smells hot or the fuse blows at a current well below its rating.
- The supply itself makes unusual noises, gets very hot or its mains cable is damaged.
- The part you want to test is a high-voltage component such as a motor controller or DC-DC converter from a vehicle.
Questions
Why does the voltage drop when I connect my light?
The supply has reached its current limit and switched to constant-current mode. Either the limit is set too low for the light or the light has a fault.
Can I use the bench supply as a battery replacement for my scooter?
No. Vehicle controllers and motors draw large, sudden currents and are designed for the vehicle's battery. Keep bench supplies for testing small parts off the vehicle.