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Review: Puctec QJ3005C Bench PSU



here's a small review and, naturally, a teardown of a PucTec QJ3005C bench power supply. If the model number looks familiar it's because it is shared with hundreds of manufacturers such as Mastech (HY3005D), Instek (GPS3030), and other chinese "tec(h)" companies. Rebranding as usual...

Anyway, it's a single output 0-30V 0-5A regulated power supply with some nice LCD displays's cheap! Paid just a little bit over 100€ for it so I was a bit sceptical about the performance and build quality. I came home, took a screwdriver and the rest, as they say it, is history! <evil laugh>

Front panel is business as usual - pots for adjustment, LCDs for current and voltage, two indicators for CC/CV mode and a power button. Nothing fancy and just the way I like it. Both current and voltage have "coarse" and "fine" pots for a more precise adjustment which is a nice touch since usually only the voltage adjustment has dual pots. LCDs are big enough and have a nice green uniform backlight. They look great but the vertical viewing angle is just terrible (horizonal angle is much better) and 7-segment LED displays would definately be a better choice.

Case is all metal without the top handle which limits the portability a bit but I can live with that. Heatsink is tucked away inside the unit and has an 80mm fan. I was afraid that the fan would be on all the time and noisy but it's actually temperature controlled and therefore off most of the time. Haven't had a chance yet to push the PSU to the limits and activate the fan...yet!

Now for the interesting part!
Upon removing the case I was pleasantly surprised by the build quality, layout and...well...everything! For a price tag of 100€ I was expecting through-hole components, PCBs of different quality, wiring mayhem and stuff like that. It turns out that it's really well built! There's a hefty transformer in the middle, power board with output devices and heatsink on the back and control/display boards on the front. Wires are all neatly arranged and separated (mains wiring on the left and output/control wiring on the right side). Power board is 100% through-hole components while control boards are 99,9% SMD components. Might be a trouble to fix if something blows up but if anything blows up it will most likely be on the power board and that one can be easily serviced.

Components are all well known; nothing esoteric. LM324 opamp, ICL7106 DMM IC, 7805 regulators and a bunch of output transistors in TO3P package. Soldering looks consistent and of reasonable quality although the "cleaning crew" could have done a better job.

Time to power it up and do some testing...
Stay tuned!

And now for some quick testing...

As you can see from the pictures, meters have good accuracy although the specs say +/- 1% for voltage and +/- 2,5% for current. OK, so I don't have a Fluke to test with...sue me if you want ;D Voltage goes from -0,03V to 31,4V and current from 0A to 4,85A. The lowest setting I could achieve (apart from 0A) is 10mA which is great for testing LEDs although a bit tricky to adjust. Not that I will use it for testing LEDs but it's nice to know how low it can go.

I tried to measure ripple but my handheld scope is not really up to the task. Looking at the screen it would seem like a pefect flat voltage without any ripple...which is obviously a lie! Turn-on curve looks good, without any nasty spikes/overvoltage but I will have to repeat this once I get a proper scope (hopefully next week). Until then I will have to trust the specs which say that the ripple is under 3mV in CV mode and under 6mV in CC mode.

Conclusion: excellent value for money!
I know that a lot of people try to build their own bench PSUs (myself included) which is great for experience but a unit like this is definately a "must have"!

Great review and thanks for taking the time to make the measurements and photos of the tests.

My guess is this is a QJE made linear PSU, they are the original makers versus Mastech; you can see it on the PCB.   Some handsoldering is evident from R37 onwards below the big QJE labels, but it shouldn't impact performance, see photo: P1010141.JPG.

Your Uni-T scope/DMM can be up to the task of measuring the ripple, you just need to load it so the PSU outputs its maximum capacity: ~ 30+V 4.8 A.  That's also a better test of its onboard volt and ammeter when loaded to maximum.

If it doesn't fly offscreen on your scope, you can AC couple it, worse case.

Did another test with a 12V 55W light bulb in CV and CC mode (don't have any power resistors to load it fully @ 30V).
In CV mode @ 14V, ripple is around 1mV (picture with the flat line) while in CC mode @ 4,5A the ripple is evident and around 3mV which is within spec. I'm not sure should I trust my eyes and a low-res screen or the reading on the meter. I'm still waiting for a proper scope :)

What I've found interesting is that the same PSU in Slovenia costs 50% more than in Croatia.
So...a note for my Slovenian colleagues: your suppliers are ripping you off! ;D

Great work!  Good review; see that Uni-t scope meters can be useful, and now you can see the ripple with load, that's ingenuity using the light bulb.  Harder to find these days we we move to CFL and LED.


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