It was a cheap power supply and never worked particularly well. Probably should have paid a little more and bought a TENMA (which I eventually did) but hey - live & learn.
Degradation
I calibrate my power supplies every 2 years and found that the current display was getting progressively inaccurate. In 2019 the offset was +40mA overreading and 2021 this rose to +200mA. Voltage reading was always good but the knobs became increasingly unreliable making it difficult to select a voltage. The voltage setting drifts over time too. All in all, this little PSU isn't in a happy place.
Regulation
The power supply itself is fine, and from a software engineer's point of view the 240VAC to 30VDC is the scary bit. I decided to buy a ZK-4KX Buck Regulator on eBay for £13.30 - it was from a UK seller which costs a couple of pounds more than direct from China but then the import duties are somebody else's problem; no VAT receipt was provided. No manual either.
Buck Boost
The first experiment was to power the ZK-4KX with around 8V at ~500mA current limit and see what it did. I also have an Array 3710A DC load which is essential if you're testing power supplies. The first pleasant surprise is that the ZK-4KX powered-up generating 12V from the 8V input - so this really is a Buck Boost regulator capable of raising its input voltage. Nice. Slightly disappointed that its default is to start in the ON state but pressing the rotary knob dropped the output to 0V pretty quickly.
The Plan
- Select the highest voltage/current setting available from CSI3003SM - this is around 30.8V and 3.1A on my unit, or 95W.
- Wire the CSI3003SM output into the ZK-4KX and set its power limit to 91W so as not to trigger the CSI3003SM current limit.
- Remove the CSI3003SM voltage/current display board and insert the ZK-4KZ into the slightly enlarged hole.
- Remove the CSI3003SM voltage/current knobs and wire the ZK-4KX output to new 4mm sockets on the front panel.
- Add an earth socket - I like floating power supplies but also value having a common ground across my workbench. I usually pull GND to Earth with a ~100k resistor.
DANGER
Modifying a PSU is dangerous. Proceed at your own risk. Be aware that current kills, not voltage, and you're playing with over 3A. If you don't understand how voltage and current are related then you're not ready to take apart your PSU. If you accidentally put 90W through a metal watch or wedding ring you're going to have a really bad day.
Make a mess
Unplug your PSU from everything and undo 4 screws on each side. Remove the lid.
Undo 2 screws at the front underside. Unplug the display and adjustment boards. Remove the front panel.
Remove knobs and unscrew potentiometers from front panel. Unscrew both boards.
Modification
Enlarge the display aperture to fit the ZK-4KX module. I use an old 0.5mm soldering iron to melt the plastic and scalpel to clean the edges. Doesn't have to be super neat because ZK-4KX has a generous bezel that will cover the hole.
I also decided to put new 4mm plugs in the holes where the potentiometers used to be. You'll need low-profile ones to fit. Alternatively I suppose you could re-use the existing 4mm sockets.
De-solder the potentiometers from their board and replace with 4x wire links and 2x 10kOhm resistors. This has the same effect as turning all pots fully clockwise.
SEE IMPROVED MODIFICATION LATER BEFORE TRYING THIS!
Reassemble
Performance
Well it works! One surprise is that the ZK-4KX output power is limited to 35W by default and the maximum is 50W. I should have read the description more carefully (you can find sort-of datasheets of this thing online). It's only going to support 35W output without a cooling fan on the heatsink and even with a fan it'll only support 50W output. So you have 0-30V, 0-4A output but not at the same time.
The side screws foul slightly on the plastic casing of the ZK-4KX. Not a major problem but they tend to push it out a little.
The hardwiring of the CSI3003SM appears to select 31.7V and at least 3A. That triggers the ZK-4KV overvoltage so I modified the adjustment board to select high coarse and low fine potentiometer settings, replacing two links with 1kOhm resistors:
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