Tuesday, 10 December 2019

Evaluation of the UNI-T UTP1306S 32V 6A 'linear' power supply

Due to some recent instability issues with my switched-mode bench PSU when subjected to a high sudden current demand spike, I decided to get a linear bench power supply. Also, by adding it to my existing bench PSU, I could get a +ve/-ve supply or I could get over 60V if I connect them in series (not quite sure how the CC would work though?).

In particular, I wanted a bench PSU with an Output Enable\Disable switch because I find it useful to be able to set a Voltage and Constant Current setting whilst my test circuit is already connected to the PSU terminals. Why all bench PSUs don't have this is totally beyond me!

One of the few suitable small (cheap) linear bench power supplies was the UNI-T UTP1306S (approx. $100 or £60). I looked at the review on YouTube (see below) before deciding to buy one. I figured that even if it failed on me, at least I would have a nice-looking project case for a future home-made bench PSU!

The first thing I did was to take it apart and inspect it carefully, check soldering, earth connections, tightness of fixings, loose components, etc. The transformer seems remarkably small for a 192W linear PSU...


Some notes:

1. The blue LEDs behind the 5 buttons (M1, M2, M3, OCP, OVP) are very, very dim when switched on (so dim that I didn't notice them until it got dark in the evening!) - you can only see them in a dark room. Luckily the intensity of the On\Off button LED is just right!
2. There was a loose piece of wire (looked like a bit off a snipped component lead) sitting inside the coils of the current shunt on the front panel which could have been shorting out a turn of the shunt or could have fallen into one of the PCBs. 
3. V\Amps LED readings are pretty accurate but I can't see how to tweak them (no trimmer pots)? 
4. No mains switch on the front panel - I must reach over the top and round the back to switch power on\off. 
5. If you press one of the Memory buttons whilst the output is enabled, it immediately changes the output voltage to the new value. It would be safer for it to switch off the output and allow you to enable the new setting after checking the voltage is what you want. You could accidentally press one of the Memory buttons whilst adjusting O/P or CC with disastrous results.
6. No 0V to Gnd terminal shorting link provided (would it break the bank to ship a 5 cent piece of metal?).
7. Ventilation holes - the fan pulls air in (when on) but they have put the outlet ventilation holes on both sides of the case. This means you can't put any other unit next to it (like another UNI-T power supply) or you will block the holes. Brilliant design guys! Why didn't they put the holes in the top of the case near the front and maybe with a grille to prevent metal wires, etc. falling in, like PC enclosures often have (heat rises you know!).
7. When you press the V or CC knobs, it defaults to the least significant digit. It would be more useful if it defaulted to the 2nd-least significant digit, you don't very often need to adjust the voltage by 10mV and the current by 1mA. So the default should be to select the 100mV and 10mA digits when the button is first pressed in, because currently I have to press it two or three times to move it off the 10mV\1mA digit every single time I want to adjust the V\CC.
8. Firmware bug - set voltage to 32V (for example) using pot, now press M1 (for example) to set voltage to a different setting - say 3.3V. Now you can use that voltage for your circuit. Now switch off the mains power - the next day you come back and switch it on again. The setting is now back to 32V! If you didn't notice this and just enabled the output you could fry your circuit! Basically it only remembers the last settings if you manually changed the pots or if you long-pressed on the M1 button to re-save the setting before you switched it off.
9. When the fan comes on (supplying 1A at 30V for 5 minutes is enough to get it on) it is quite noisy. Video review says its quiet but mine sounds like a small jet engine (just not as loud). I might have to change it if it gets annoying.

It does have one major issue however...

  1. Set O/P to 32V
  2. Connect small load e.g. 1K or a bit less or no load
  3. Enable output and connect scope probe
  4. Now switch off mains 230V AC power
The output will shoot up from 32V to 40V before drifting down slowly to 0V. If you set the O/P to 26V or below then there is no surge on power off. Also, if you connect a larger load (52R or more) then there is no surge.

Switching off mains creates 40V spike on output!

Apart from these niggles, the build quality seems good (electrolytics are not top brands though). I could find no other evidence of power glitches/surges when on/off testing so far.

I have not tested the ripple/noise on the output yet. The most annoying thing is those stupidly dim blue LEDs in the buttons.

Considering the price (you may find it cheaper elsewhere but bear in mind that you might have to send it back), overall I was pleasantly surprised.


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