The short version
When the same bench power supply, multimeter or electronic load comes in two versions - one with a USB, RS-232 or LAN interface and one without - buy the one with the interface. The extra cost is small. The software the manufacturer ships with it is often poor, Windows-only and unsupported, but that does not matter any more: the interface speaks a documented protocol, and free, open-source tools such as BenchMesh use it on Windows, Linux and macOS. And when the front panel wears out - as the rotary encoder on my Tenma did - the interface keeps the instrument useful.
Connectivity costs less than you think
The parts that give an instrument a computer interface are among the cheapest on its board. A USB-to-serial bridge chip costs well under a few dollars in volume; an Ethernet controller with its connector costs a few dollars more. Most instruments already have a microcontroller that knows every setpoint and every reading - the interface only lets it answer a computer as well as the front panel.
A good example is the most cloned bench supply of the last decade, the Korad KA3005P, sold also as the Tenma 72-2540, Velleman PS3005D and RND 320-KA3005P. Korad also made a KA3005D. Retailers describe the two as the same supply, the only difference being that the P is programmable over USB and RS-232 and the D is not. Prices move and differ from country to country, but the premium for the P has been a modest part of the price of the supply - one owner put the local difference at around 20 percent.
Mid-range instruments from Rigol, Siglent, OWON and others now have USB as standard, often with LAN on the same box or one model up. So the real question is rarely whether you can afford the port. It is whether you will regret not having it - and on a bench you keep for ten years, you probably will.
What counts as connectivity
Not every port on the back of an instrument is remote control. These are the ones that are:
| Interface | What you see on the computer | Typical on |
|---|---|---|
| USB (virtual serial port) | A COM port on Windows, /dev/ttyUSB0 or /dev/ttyACM0 on Linux, /dev/tty.usbserial-* on macOS | Budget power supplies (Korad, Tenma, RND), multimeters, electronic loads |
| USB-TMC | A USB test and measurement device, addressed as USB0::...::INSTR in VISA | Rigol, Siglent, Keysight, OWON, most SCPI instruments |
| RS-232 | A serial port, often through a USB adapter | Older and industrial instruments |
| LAN (Ethernet) | An IP address; SCPI on TCP port 5025, VXI-11 or LXI | Mid-range and better instruments |
| GPIB | A GPIB adapter and VISA | Older lab instruments |
A USB port that only updates firmware or a socket for a USB stick that saves screenshots is not connectivity in this sense. What you want is remote control: the computer can read every value and set every setting the front panel can.
The software in the box
Here is the uncomfortable part. Much of the software that comes with budget and mid-range instruments is poor. In my experience it tends to be:
- Tied to one operating system. Almost always Windows, sometimes a particular version of it. On Linux it does not exist, and on a Mac you are lucky if it runs in a virtual machine.
- Unstable. It crashes, loses the connection when the instrument restarts, and needs a specific USB driver that a later Windows update no longer accepts.
- Unsupported. It was written once for the launch of the instrument and never updated. Bugs stay; questions to the vendor go unanswered.
- A demo, not a tool. It proves that the connection works - a few numbers on screen, a button for the output - and little more. No sensible logging, no export you would want to use, no automation.
- One instrument at a time. Each maker has its own program. A power supply from one vendor and a multimeter from another never share a screen, let alone a timeline.
It is easy to conclude from this that the interface is not worth having. That is the wrong conclusion. The interface is hardware with a protocol - usually SCPI, or a small documented command set like the Korad one - and the protocol outlives the software. The community documents it (the sigrok wiki lists the Korad clones and their quirks), command-line tools grow around it, and VISA libraries such as PyVISA can talk to it.
That is why I build BenchMesh: one free, open-source app for Windows, Linux and macOS that detects the instruments on USB, serial and LAN, gives every one the same kind of panel, and records them all on one shared timeline. On top of the interface you get automation with flows, virtual SCPI ports for MATLAB, LabVIEW and PyVISA - also for instruments that speak no SCPI - a Python package and AI assistants over MCP. None of this needs the vendor software. All of it needs the port.
My Tenma and its worn rotary encoder
This is the reason that turned the argument from "nice to have" into "do not buy without" for me.
I have a Tenma 72-series bench power supply - the Korad KA3005P design that Tenma, Velleman and RND sell under their own names. Voltage and current are set with a single knob: a rotary encoder, with the arrow buttons choosing the digit. Over time the encoder started to lose contact. Turning the knob sometimes did nothing, sometimes changed the wrong way, sometimes skipped several steps at once. In the end it was close to inoperative: setting 5.00 V became a small fight with the front panel.
I am far from alone. Search for "KA3005P encoder" or "Tenma power supply knob" and you will find the same story again and again:
- On the EEVblog forum a KA3005P owner described a knob that "behaves erratically" and that turning it "might jump to 25v right away" - and that it got progressively worse.
- In another EEVblog thread a Tenma supply only a couple of years old could no longer have its voltage set because the encoder had worn; the answer was the usual one: encoders wear out, especially cheap ones whose wipers cut into the track.
- On element14 a Tenma 72-13360 owner could not set the target voltage accurately, and repeated calibration made it worse.
The fixes people describe are all hands-on. Take the supply apart, remove the encoder, clean it with isopropyl alcohol or contact cleaner and lubricate it - several owners report that this works, at least for a while. Or desolder it and fit a compatible part; an ALPS STEC12E07 has been suggested for the KA3005P. Either way you open the case of a mains-powered instrument, unplug the front panel and hope the knob shaft and detents match.
A worn encoder on a power supply is more than an annoyance. A knob that skips or jumps can put 25 V on a circuit that wanted 3.3 V. That is exactly the moment you want to stop trusting the knob.
The fix that needs no screwdriver
My supply has a USB port. With the Tenma 72-2540 in BenchMesh (and the Korad KA3005P it is based on) I do not use the encoder any more:
- I type the voltage and current I want - 3.300 V, 0.250 A - and the supply takes exactly that, with no digit selection and no skipped steps.
- I set over-voltage and over-current protection (OVP, OCP) the same way, so a mistake cannot reach the circuit.
- Switching the output on is a deliberate action in the app, not a nudge of a worn knob.
- I watch output voltage, current, power and the CV / CC state live, and record them alongside the multimeter and the load.
- I can sweep the voltage in steps, or set it from Python, without touching the front panel at all.
The supply went from "needs repair" to the most comfortable one on my bench. The same applies to any instrument with a fragile front panel: a stuck button, a cracked display or a worn knob does not end the life of an instrument that can be controlled over USB or LAN. On one without an interface, it does.
Advantages and disadvantages
To be fair to both sides:
| With USB / RS-232 / LAN | Without | |
|---|---|---|
| Price | A small premium on budget models; standard on most mid-range ones | Cheapest |
| Setting values | Front panel or exact values from a computer | Front panel only |
| Logging and recording | Every reading, on a timeline, exported to CSV | Pen and paper, or a camera pointed at the display |
| Automation | Sweeps, soak tests, battery discharge, unattended runs | None |
| Worn knob, button or display | Still fully usable from the computer | Repair or replace |
| Resale value | Higher; programmable models are sought after | Lower |
| Ground and isolation | USB ties the instrument to the computer ground unless the interface is isolated | Floats freely |
| Network exposure | A LAN instrument is a device on your network; keep it on a trusted one | None |
| Software | Vendor software is often poor - use open tools instead | Nothing to install |
The two genuine disadvantages are electrical and network ones, and both are manageable. For floating measurements prefer LAN or an isolated USB adapter, and check whether a multimeter's interface is optically isolated (many are). Keep LAN instruments on a trusted network; BenchMesh's own remote access is off until you set a password and switch it on.
A checklist before you buy
- Is the port for remote control? The manual should say that every setting can be read and set from a computer, not only that firmware can be updated.
- Is the protocol documented? Look for a programming manual or SCPI command reference. If the vendor publishes it, others can support the instrument.
- Which interface? USB is enough for one bench; LAN is better for isolation, long cables and several computers.
- Is it isolated? Matters for multimeters and anything measuring floating circuits.
- Does open software support it? Check the supported instruments list; if your model is listed as awaiting confirmation, confirming it takes a minute.
- Ignore the bundled software. Judge the instrument by its hardware and its protocol, not by the CD in the box.
Questions
Is it worth paying more for a bench power supply with USB?
Usually yes. On budget supplies the version with USB or RS-232 (for example the Korad KA3005P against the KA3005D) is the same instrument with a port and remote control added, for a premium that is small next to the price of the instrument. With free software such as BenchMesh you get logging, automation and a second way to set the instrument when the front panel fails.
My Tenma 72-2540 or Korad KA3005P knob jumps or skips. What can I do?
The rotary encoder behind the knob wears or gets dirty. Owners report fixing it by taking the encoder apart and cleaning it with isopropyl alcohol, or by fitting a compatible encoder (an ALPS STEC12E07 has been suggested on the EEVblog forum). Without opening the case, you can set voltage, current, OVP and OCP over USB from BenchMesh and leave the knob alone.
Does the Tenma or Korad power supply software work on Linux or macOS?
The software that ships with these supplies is written for Windows. BenchMesh runs on Windows, Linux and macOS and controls the Tenma 72-series and Korad KA series over their USB virtual serial port.
Does a USB connection affect measurements?
It can: USB connects the instrument to the ground of your computer, which can create ground loops when you measure floating circuits. Check whether the interface is isolated, use an isolated USB adapter, or use LAN, which is isolated by design.
Buy the port
A connected instrument costs a little more and gives you logging, automation, a choice of operating system and a second way in when the front panel fails. The vendor software is not the reason to buy it - the interface is. If your bench already has instruments with USB, RS-232 or LAN, download BenchMesh and see what they can do without the software that came in the box.
Sources
- EEVblog forum: Korad KA3005P power supply faulty / repair - erratic encoder, cleaning with isopropyl alcohol, ALPS STEC12E07 (October 2016)
- EEVblog forum: Encoder repair problems - worn encoder in a Tenma supply, why cheap encoders wear (June 2023)
- element14 Community: Tenma 72-13360, not able to set the target voltage accurately
- sigrok wiki: Korad KAxxxxP series - the KA3005P and its rebrands (Tenma 72-2540, Velleman PS3005D, RND 320-KA3005P)
- EEVblog forum: Korad KA3005D - the non-programmable version
- ka3005p - a command-line tool for the same family, another sign of a documented protocol