Interesting (but disappointing) Mini Buck Boards

I needed some cheap little vregs recently and had run out of and/or lost all of my useful-value 780x linear parts, so I decided to look at what people in this century use. 

I found some little buck boards roughly the size of a TO-220 package that looked exciting. These particular ones are QSKJ Mini DC-DC Buck Step Down Module model “QS-1205CME-3A”, Vendor page here, mine were 5pcs/$9 from Amazon.

Upon analysis they have serious issues with regulating under load, so the hunt for something decent continues, but the form-factor and advertised feature set are really compelling.


  • High-efficiency high-frequency synchronous buck instead of a linear heater^H^H regulator.
  • Solder-jumpers for 1.8,2.5,3.3,5,9,12V or a default (fiddly, tiny) adjustment pot output so you only have to stock one device – one easy-to-cut trace to disable adjustable mode.
  • Tolerates 4.5-24V input as long as out < in or so.
  • Good stability to input voltage variation.
  • ~0.25V drop-out.
  • Does appear to have a cutoff for over-current.
  • No perceptible ripple under various load conditions.


  • Voltage regulation manages maybe 600mA at 5V before droop becomes unacceptable (<4.8v).

…and that makes it basically useless for most applications. Test data below the fold.

Maybe it could be resolved with appropriate external capacitors and/or offsetting the adjustable to regulate right at a known load or something, but not being drop-in really reduces their charm.

Anyone know of a similar offering that doesn’t suck at output regulation?
(Rel: Anyone know if any of the low-end electronic loads are worthwhile? I’m not looking to spend real-lab-instrument money, but it’s come up often enough lately that I want to be able to dissipate a couple 10s of Watts through a at least stepwise-controllable resistive load).


Input stability and drop-out test:
Set 5V fixed output jumper
Input hooked to a gigantic bench supply
Output voltage and stability from a Tektronix MDO3012 oscilloscope showing both the bare waveform and DMM mode.
Output current by running through a BK 5492B bench meter
50Ω of resistors as a load

Input – Output
20 – 5.013
12 – 5.013
9 – 5.013
5 – 4.762
(and it doesn’t boost, so much under 5 cuts off)

Large load test:
Input hooked to bench supply at 12V
Abomination USB AF-AF adapter to connect a USB to Alligator Clip cable to a “1A/2A” load for testing USB Power supplies. Hand DMMs in series and parallel for voltage/current.

12V input, dummy load for nominal 1A (apparently 5.33Ω) and 2A (apparently 2.75Ω) at 5v
(1A setting) 700mA@4.2V
(2A setting) 1A@2.3V (LOL)

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