We’ve all seen those cheap bench power supply units (PSUs) for sale online, promising specifications that would cost at least a hundred dollars or more if it were a name brand model. Just how much of a compromise are these (usually rebranded) PSUs, and should you trust them with your electronics? Recently [Denki Otaku] purchased a cheap unit off Amazon Japan for a closer look, and found it to be rather lacking.
Major compromises include the lack of an output power switch, no way to check the set current limit without shorting the output, very slow drop in output voltage while adjusting due to the lack of a discharge circuit, and other usability concerns. That’s when the electrical performance of the PSU got tested.
Right off the bat a major issue in this cheap switching mode PSU is clear, as it has 200 mV peak-to-peak noise on its output, meaning very little output filtering. The maximum power output rating was also far too optimistic, with a large voltage drop observed. Despite this, it generally worked well, and the internals – with a big aluminium plate as heatsink – look pretty clean with an interesting architecture.
The general advice is to get a bench PSU that has features like an output power button and an easy way to set the voltage and current limits. Also do not connect it to anything that cares about noise and ripple unless you know that it produces clean, filtered output voltages.
I have a similar one 30v 10A . Its ok for simple stuff and is better or just as good as most mid priced units .
all in all I would not use it certify anything but but its ok for mucking about semi seriously. If one is looking for low noise specs then maybe non switching power supplies would be better.
There are probably a bunch of rebranded ones that are essentially the same. I started with one that was good enough, then a couple months later it totally crapped out. Without too much to go on, and mains supplied power supply not exactly something I feel comfortable messing with, I bought … another one. Then I picked up a lab quality one at a ham fest for probably $50, maybe $100. It’s massive over kill for what I need so far, but definitely a personal case of buy twice, cried once (the second d cheapo still works) and was also proud of myself once. TL:DR for same $$ thereabouts get a used good one. About zero down side.
Used lab PSUs usually need replacement of most electrolytic caps, relays and pots, cleaning all connections and switches to work as intended. And they are hard-to-find things – I spent months to find one in decent condition. And then spent many hours to clean and service it properly. Fortunately I had all wiring diagrams and spare parts.
Honestly, today I would prefer to buy cheap Chineese one and add missing filters and features. This will give me much smaller, more effective and more powerful device.
Old stuff is nice and have it’s own aestetic value, but it’s not the best choice today. Especially if you buy some cool used lab stuff for cheap and then find out that to make it up and running, you have to find some unobtanium, or original spare part cost more than plane wing, or you have to buy another one for spare parts in hope it will not have the same problem. Of course you could win in that lottery, and get high grade equipment for cheap, but that’s not for certain.
Just adding filters won’t make a crappy supply into a good supply, usually the current control loop is useless, load regulation is horrible and gets worse when you add filters, and some have overshoots during power-on or power-off.
Good lab supplies regularly get replaced because of age, or because of simple defects like a broken knob. I recently got a really nice supply from TTI that blew fuses, it just needed 2 new diodes in the PFC rectifier.
I have a simple model second hand and yeah, all it needed was a new cord and some cleaning, no problems at all.
Don’t waste your time with those good-looking but half-baked switched “lab” supplies. Roll your own with a linear regulator, this isn’t rocket science and once upon a time this was a beginner’s project. The electronics magazines of the past 100 years are full of such constructional articles.
+1
you can also get the best of both worlds. hack the supply to be a pre-regulator for your linear supply.
that said, my bench psu is just a good old fashioned LM338K and it’s served me well for the past 15 years. it has a digital readout for voltage and current provided by a PIC16F877A and a preset knob with 2.5, 3.3, 5, and 12V presets. Last position is variable and it has a 30 turn pot
the 877 does one more trick: the transformer I used happened to be a 12+12V one. for voltages under 9V it uses one leg and the center tap (12Vac) over that it uses both legs (24Vac). so in low voltage I’m not burning so much power.
originally I meant to make a LM723 based PSU with remote sensing and more current but … things happened and this makeshift 338K became my daily driver
I have one of these rebranded units and use it more often than my bulkier rack mount $$$ power supply. It’s fine for quick prototyping and I like it’s small size but I wouldn’t use it supply power to a vfo I’m working on. Nice to see a review and tear down of it. It is a bit annoying to set the current though.
I have two switched mode power supplies. One, branded “Manson” is good for brute force but the output voltage walks all over the place and has a few hundred mV of noise. The other one is a Riden and it’s surprisingly good for the money. Feature rich, fairly stable and generally nice to use. It does still have 100mV of noise on the output.
I think I have the same Manson power supply. You can get the display more accurate by replacing the 5V regulator on the display with a more accurate one.
You can also disable the internal over-voltage trip which gets annoying and lets you get a couple of extra volts out of it. Been a long time, but I think it was snipping a trace at one of the comparators.
It has been a pretty solid brute force supply. Great for charging batteries.
I’m kinda surprised we haven’t seen a USB-C PD plus some control and display logic, sold as a “variable bench power supply” in one of these housings yet. Seems like, for lower voltage and current, they’d be trivial to make. Guess I’ll put that one on my build list.
but we have: https://hackaday.io/project/194295-pocketpd-usb-c-portable-bench-power-supply
We have, there are builds. I’ve also been eyeing the toolkitrc p200, which is not exactly the same, but a cool device anyway
I have a multi-USB charger with both USB-PD and QC3.0 that I use when I just need to power a device with a barrel jack, or when the main bench power supply is already in use. Though for the USB power supply, I only have fixed value trigger cables for it, nothing to take advantage of PD3.0.
My bench is getting a makeover soon, and I’m thinking that I might do something with a spare ATX power supply. There have are times I really could have used multiple voltages, despite lacking in current limiting or even metering. I have a 1980s/90s Motorola adjustable supply when I need that, but it’s single output.
I’ve watched too many DiodeGoneWild videos, I don’t really trust anything cheap that uses mains power.