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Old 9th November 2014, 10:33 PM   #1
tomchr is offline tomchr  United States
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Default Korad power supplies

Does anyone have first-hand experience with the Korad power supplies, such as the KA3005D-3S?

The supplies are rather inexpensive. For example, the 2x 0~30 V @ 0~5 A; 1x 5.0 V output version is only about $180. It features digital adjustment of the output voltage/current via a front panel knob, four memories, and USB connection.

EEVBlog has a tear-down review video of the single output version. I believe the single output is $120... You certainly get your money's worth it seems like. Granted, the build quality could be better, the boards could be washed, etc. but for $120... Not bad.

Has anyone here tried any of these supplies? I'm curious about what the output impedance is and how much residual ripple is on the output.

~Tom
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Old 27th November 2014, 10:11 PM   #2
tomchr is offline tomchr  United States
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I decided to answer my own question. I purchased an KA3005D-3S from SRA Soldering Products.

The KA3005D-3S is pretty obviously two KA3005Ds in one chassis. Fair enough. In addition, with the dual supply, you get an unmarked +5 V, 3 A supply as well.

If you have read any of the online reviews, you will know that the manual is a joke and that the fan is loud. I agree with those.
The fan measures about 50 dB(A) at idle and increases to about 60 dB(A) at full speed. These numbers measured about 30-40 cm from the supply using an SPL meter app from Studio Six Digital. This is loud enough to make me go, "ah...." when I turn the supply off. One could easily swap the fans out for some low-noise types, though. Of course, then you're adding $40 worth of fans to a $170 supply...
The manual is in Chinglish and is absolutely useless. Then, on the other hand, it's a power supply... It's not rocket science.

The supply section on the left, I'm sure, is an unmodified KA3005D single supply. The supply section on the right has the +5 V output added and has some of the buttons repurposed for additional functions: Series (SER) and parallel (PARA) modes. Hold in the SER button for a second or so, the supply goes beep, and the SER LED lights up. Then connect the load (+) to the red terminal on the left supply section and the load (-) to the black terminal on the right supply section. Dial in the voltage on the right supply section, but note that you get twice the voltage indicated on the display. Similarly, the PARA mode allows you to connect the two supply sections in parallel. Connect the load to the terminals on the right supply section and dial in the voltage and current there. Note that you get twice the indicated current. These features are a little clunky, but if you keep in mind that you're dealing with two supplies in one box, it sorta makes sense.

The beep sound is rather anemic. Not a big deal and you can turn it off ... but only on the left supply section as the BEEP OFF button has been repurposed on the right supply section. That's truly one of those, "really?!" kinds of moments.

User interface and fan noise issues aside, the supply is pretty damn solid. I ran it at 2x 24 V, 5 A for an hour and it never complained.
The indicated voltages and currents are not super precise, but probably within 1 % of the actual value (for a DC load anyway). That's certainly good enough.
The ripple voltage was low enough that I didn't bother to write it down. The specs say <1 mV RMS, which was definitely the case on my unit.

The output impedance is nice and low. It varies with the load current, as you'd expect, and shows good stability of the control loop (no high-Q peaking). Except for a hump at 5 kHz, the output impedance is below 10 mΩ for all the voltage/current combinations I tested (see attached plot).

An interesting quirk is that the ammeters in the supply are not RMS ammeters. It appears the current is sampled by the same micro controller that handles the user interface, so when presented with a pulsed load (such as a Class AB output stage), the ammeter will show currents anywhere from close to zero to somewhere around the peak value, depending on where in the cycle the current was sampled. This is a bit disappointing, but given the low cost of the unit, I can't complain too loudly.

The bottom line is that for $200 shipped, you get a lot of bang for your buck. To get something that's significantly better, you'd be looking at getting two HP/Agilent/Keysight E3632A supplies at $1200 each.

I did look at building my own supplies. I have the transformers for it and can probably dust up some output devices as well. Even so, I would be hard pressed to build a supply as good as the Korad for less than $200.

~Tom
Attached Images
File Type: png Korad_3005D_Zout.png (46.5 KB, 98 views)
File Type: jpg KoradKA3005D.jpg (68.2 KB, 92 views)
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Last edited by tomchr; 27th November 2014 at 10:18 PM.
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Old 1st December 2014, 06:52 AM   #3
tomchr is offline tomchr  United States
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Hmmm... Apparently, small-signal output impedance is one thing; large-signal output impedance quite another. Attached is a scope shot of the supply ripple when the Korad is loaded by a class AB output stage providing 1 kHz @ 40 W into a 4 Ω load (4.47 A peak; 1.58 A RMS). Pretty pathetic...

I think this one goes back to SRA...

~Tom
Attached Images
File Type: jpg Korad_KA3005_Ripple.jpg (220.0 KB, 75 views)
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Old 2nd December 2014, 09:11 AM   #4
PChi is offline PChi  United Kingdom
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Thanks for the information. The large scale output impedance is a bit of a shocker. In the UK TTi were and probably still are better quality but probably not as good or as expensive as Keysight.
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Old 2nd December 2014, 06:09 PM   #5
tomchr is offline tomchr  United States
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I was surprised too. I'm guessing the micro controller used for the display handles the voltage regulation as well and flat out runs out of bandwidth. The output voltage actually increases by about 0.5 V under these load conditions. I put a 1000 uF cap across each output, that helped some. Above scope shot was done with the 1000 uF in place.

I guess I'll be building my own supply. It's not hard. It's just time consuming.

~Tom
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Old 3rd December 2014, 04:32 PM   #6
PChi is offline PChi  United Kingdom
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My guess from the waveform is that the Korad has a slew rate limitation in the control loop. The downward slope may be the control loop failing to turn the pass transistor/MOSFET on fast enough as the class AB output stage current increases. Then the output stage current falls and the ouput recovers.
My guess is that they have used a power stage with gain such as a PNP transistor or P channel MOSFET to keep the drop out voltage to a minimum. Then used high value capacitors (but low current control circuit) to keep the voltage regulation loop stable.
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