Bench Power Supply Constant Current EZ-SET


Here is a nice hack you may find very useful if you have a cheaper bench power supply that supports constant current limit protection (CC mode) and the only way to set or check your max current limit is to disconnect your circuit, short the power supply outputs and then check or set your limit. Yes, what a pain! [Ian Johnson] was enduring this pain with a couple of Circuit Specialist bench power supplies and decided to do something about it. After finding a download of the circuit diagram for his CSI3003X-5 supply he was able to reverse engineer a hack that lets you press a new button and dial-in the max current setting. Your first guess is that he simply added a momentary button to short the power supply outputs, but you would be wrong. [Ian’s] solution does not require you to remove the load, plus the load can continue running while you set your current limit. He does this by switching the current display readout from using 0–3 volts off an output shunt resistor to using the 0-3 volts output from a digital potentiometer which is normally used to set the power supplies’ constant current limit anyway. So simple it’s baffling why the designers didn’t include this feature.

Granted this is a simple modification anybody can implement, however [Ian] still wasn’t happy. A comment by [Gerry Sweeney] set him on the path to eliminate the tedious multi-button pressing by implementing a 555 momentary signal to switch the circuit from current load readout to current set readout. This 2nd mod means you just start pressing your up-down CC set buttons and it momentarily switches over the display to read your chosen max current and a few moments later the display switches back to reading actual load current. Brilliant! Just like the expensive big boy toys.

[Ian] doesn’t stop with a simple one-off hack job either. He designed up a proper PCB with cabling and connectors, making an easy to install kit that’s almost a plug-in conversion kit for Circuit Specialist bench power supplies (CSI3003X-5, CSI3005X5, CSI3003X3, CSI3005XIII). It is not a 100% plug-in kit because you do have to solder 3 wires to existing circuit points for signal and ground, but the video covering that task seemed trivial.

This hack could very well work with many other power supplies on the market being Circuit Specialist is just rebadging these units. For now, only the models listed after the break are known to work with this hack. If you find others please list in the comments.

After the break we will link to all three progressive mod videos incase you want to learn how to mod your own power supply or you could just order a prebuilt kit from [Ian].

Adding off-load constant current setting.

Replacing the front panel push-button switch with a 555 chip & relay

New pcb for the CC modification.

Single variable output 0-30VDC 0-3A Bench Power Supply #CSI3003X-5
Single variable output 0-30VDC 0-5A Bench Power Supply #CSI3005X5
Dual variable output 0-30VDC 0-3A Bench Power Supply #CSI3003X3
Dual variable output 0-30VDC 0-5A Bench Power Supply #CSI3005XIII

17 thoughts on “Bench Power Supply Constant Current EZ-SET

    1. who cares about warranty, it has probably expired anyway. Why would this be “a major fire hazard”? Just because he modified something? In that case, what the hell are you doing here anyway, since you can’t possibly have any use for a bench power supply.

    2. @oigja: Please explain to the rest of us how this has become “a major fire hazard”, obviously we lack your extreme intelligence on the subjects of power supplies and fire hazards.

      Clearly I must not have learned how to spot electrical fire hazards in the 15 years I spent as a commercial electrician.

    3. yeah a 5V power source with 0.3v running threw it inside of a metal case with automatic power off features is such a major fire hazard~
      and if your on this site and worried about voiding a warranty than you should find another site

    4. oigja,
      I get the feeling you think my kit shorts out the output terminals in order to set the current. This is not the case at all, I don’t go near the outputs at all. It’s a perfectly safe modification to make.
      In regards to invalidating the warranty……’s up the the individual I guess, but for me the art of electronics is all about taking the lid off to look, tweak or repair the internals and PSU’s are no different in my workshop anyways.

  1. The hack could also be purely software, since the PSU is digitally controlled on current limitation side, it could be made to just show the current limit at a first push of the current adjust buttons.
    Why it does not work like that is a good question to whoever designed it…

    1. It might have something to do with the fact that there isn’t any software in the PSU; the up/down buttons control the digital potentiometer through some discrete logic, and the actual control loop is entirely analog. There is no microcontroller to be found, which you would have known if you had actually watched the video.

          1. It’s not, that’s why it’s expensive. A digitally controlled potentiometer connected between a fixed voltage and ground is functionally a DAC though … and that’s all you need for a current limiter.

  2. ya know, its funny, I’m building a new bench supply for myself, and hell if It didn’t occur to me that I’d have to do that to work out the current limit….

  3. Of all the PSUs you can get Thurlby got it right first time years ago and though I do have a couple of the ones in this article they almost never get used and certainly do not get used for anything critical. The ones I don’t use were bought for a specific task a few years ago but having looked at the on / off transients it produces I stopped ever powering it on connected to anything and though setting the current like this is a good idea I think it would be much better to have an output (on use) / (off set) physical switch.

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