Korad KA3005P power supply faulty/repair

[cavlovic]

This is Krissy from KORAD TECHNOLOGY. Would you please kindly send me your e-mail address? I want to solve it soonest. Thank you in advance!

What is your policy regarding schematics? Can you give them out or not?

She was very prompt with email and very kind. She asked for pictures of the display fault, but in return will provide and ship new board to me. I just made a short video which I will share here also, when done uploading.

The support is really good and means a lot to me, especially for future purchases.

They also sent me latest version of software in English (currently downloading - will report later), without me even asking! I guess they took time to read through all posts. I will share it with others once I check with them that it is OK thing to do.

I'm very satisfied with response so far!

[Flump]

This is Krissy from KORAD TECHNOLOGY. Would you please kindly send me your e-mail address? I want to solve it soonest. Thank you in advance!

This is really nice to hear, I need to get a new power supply soon.
I may well consider a Korad unit now.

big thumbs up from me  O0

[cavlovic]

This is Krissy from KORAD TECHNOLOGY. Would you please kindly send me your e-mail address? I want to solve it soonest. Thank you in advance!

This is really nice to hear, I need to get a new power supply soon.
I may well consider a Korad unit now.

big thumbs up from me  O0

Yes, very nice to hear they take care of customers after sales, even though they sell cheap power supplies.

In last email I asked her about some firmware upgrades, that could improve usability, and will let you all know about the response. I also asked her about schematics and publishing of it.

[van-c]

Back in June I purchased a KA3005P from SRA Soldering.  It has been a reliable power supply that is easily calibrated to within Korad's published specs using the procedure I posted in reply #7.  However, there is a bit of an issue that I've seen on mine (and observed on others in videos) that causes the outputs to produce a very low negative voltage when the output switch (not the power switch) is set to the OFF position.  With no load on the output terminals I measure between -107 mV and -115 mV until the output switch is pressed ON, at which point the measured output becomes about equal to the displayed output voltage.

I've discussed this with SRA soldering, who have swapped out an entire unit for me, and recently sent a replacement board, to attempt to fix the issue.  After I replaced the microcontroller board, the no-load output changed slightly, from -108 mV to -115 mV.  But basically, the issue still remains.  Under loaded conditions, I measured the following voltages : 2200 Ohms: -73 mV; 1000 Ohms: -53 mV; 100 Ohms: -10 mV.  This is obviously not a serious problem, but I would expect a properly designed switched output to produce zero volts across the output terminals whenever the output switch is OFF.

Although the issue is still present, I must say that once again, SRA Soldering has done a superb job of customer support.  They paid for shipping my original unit back to them, sent me a new replacement unit without charge, and finally, sent a replacement board all in an effort to fix the issue.  I told them I am willing to just consider this issue a quirk of this particular supply, but they have continued to show an interest in resolving it.

Now that Korad has popped up on this thread, I thought they may want to take the opportunity to chime in on this.  Is Korad aware that the KA3005P produces over 100mV on the output terminals even when the output switch is turned off?  Is this considered normal behavior for this particular model?

--Van

[pomonabill221]

I did rev eng the power supply and have drawn (hand drawn) the schematic.
I can scan it and create a pdf for your dining and dancing pleasure, but I want to make sure that I won't get in trouble with Korad for publishing it here.
If Korad reads this please let me know if I can post, OR better yet, post the official schematic for us!

[AllanMN]

I'd like to see the schematic. And fast. Because I have to buy a supply soon. I was about decided on this one, but using the microcontroller as the feedback regulation loop.... the transient response. Lack of repairability that entails. Sort of. Have me looking elsewhere.

[pomonabill221]

I'd like to see the schematic. And fast. Because I have to buy a supply soon. I was about decided on this one, but using the microcontroller as the feedback regulation loop.... the transient response. Lack of repairability that entails. Sort of. Have me looking elsewhere.

Well I will scan and upload my hand drawn schematic, but for right now I can tell you that the uP is NOT used in the feedback loop.
All it is used for is setting the R/2R latches which sets the reference levels for voltage and current.
There ARE opamps that actually compare the V/I and control the output stage.
Give me a few minutes and I will scan the drawings.....
Bill

[pomonabill221]

OK here we go!
Notice that this is a PARTIAL schematic where I tried to draw ONLY the important parts... I left out decoupling (not that there was alot) and I tried to note the connectors.

My notations are:

J1-1 means connector 1, pin 1

Components with only a number are the component number on the board, like a resistor with a 7 means R7 and the value like 5.1 is 5.1 ohms.  A diode/transistor should be obvious.

An op amp I have noted the component number (like U6) with the number inside the symbol and the pin numbers are on the outside of the symbol.

I did mod my supply to try and reduce the turn on overshoot by paralleling a 1uF cap across the cap labled 20 on page 3.
  This effectively slows down the Vref turn on ramp.  It works ok but the response of the loop seems to be rather slow on turn on possibly due to an amp being at it's rail and the recovery time of the amp is slow, so the output peaks before the loop can "catch it".  IE, an opamp is saturated and typically this results in long recovery times.
A larger cap makes things worse and a 1uF seemed to be the best value.

What I am going to try, rather than use a relay on the output, is to delay the turn on of the shift registers in the Vref by disabling the registers for a short time.  What I think is happening is that when the supply is turned on, the uP load the registers, but the outputs are active so the Vref is not stable because of clocking glitches on their outputs, and the voltage loop is trying to follow the stepping/glitching.

There is another trick that I have used in the past to slow down turn on, but not affect operating response time, and I will post my findings when I do (try) it.

If you have questions about my drawings... ASK!
Remember, this took HOURS to do because of all the plate throughs/traces under components.  I have reversed engineered many things before, so I have experience doing this.... hence if you don't like my schematics... DO IT YOURSELF!
It ain't real easy!
Enjoy!

[pomonabill221]

Another thing...
Note that the POWER -V is NOT the same as the CONTROL -V!!!!!!

The control -V is connected to the POWER +V  this is NOT a mistake!!!  That is why I used different symbols for the two supply returns.
Make SURE that you pay attention when metering/scoping.

[commongrounder]

@pomonabill221    Thanks so much for your efforts!  This is very useful information for us Korad KA3005 owners.  Will look forward to hearing about further results of your experimenting.  It seems strange that the firmware could not be written to load the registers before turning on the outputs, to give things time to settle.  Probably is some other tradeoff.

[pomonabill221]

Yes, they could have controlled the latch's output enable pin rather than just enable it all the time...
I will keep updates coming as I find out more about the turn on overshoot... I still believe it is due to an op amp sitting at it's rail when off, and the recovery time when turned on, as these overshoots aren't present when changing voltage while the supply is on.

I really hope the schematics help others, and others share what they find as well.  I like my supply and want to improve on a few things.

It ain't an HP/Agilent, but it didn't cost like on either!

[AllanMN]

@pomonabill221 Yes, thanks indeed. I ordered one.

[pomonabill221]

Now I just need to find out why the software doesn't work on win7 64 bit.
It will start and kind of controls the supply but keep complaining about labview which I do not have.
Why does it need labview and where do I get it (free)?
also, I cannot change the path in the box where it is indicating d:\koradka3005p save data... I don't have a D drive (well I do but it is a cd drive).
VERY strange!

[commongrounder]

@pomonabill221:  I have the D version (non-USB/serial) of the power supply, so I can't check this for you, but I do know that some interface software uses the National Instruments drivers to talk to hardware.  My Rigol scope would not communicate properly with the capture software until I installed the driver specific to that model.  There is a Third Party driver repository on the NI web site.  I don't believe you need to install the Labview software itself (many $$$$).  The problem with the default save directory sounds like lousy software coding to me....
Hopefully, someone else using Win7 64bit can relate their experience with the Korad software/hardware.

[pomonabill221]

I know what you mean about the Rigol and the NI drivers... I went through that and got the Rigol to talk just fine.
The Korad is a different story and is frustrating to use, AND is pretty lousy as well.
I just wanted to try the new version, and it sucks! (just like others have found).
The graphics are pretty, but also pretty useless!
Thanks though....

[AllanMN]

The KA3005P arrived today from SRA Soldering. QUick.

Some testing... and.... the negative isn't floating. It surely IS suppose to float on this supply, right? Because if I connect negative to ground it draws over the CC setting (i.e. set to 0.1 it draws 0.2) with nothing on the positive. I noticed it when I tried to scope the output with scope ground to negative.

Allan

[TopLoser]

I've recently heard the phrase 'Single use only' being applied to these power supplies. Korad, Tenma, whatever the name that's stuck on them.

[AllanMN]

'Single use only.... lol.

Yes, I think of them as more of a kit/project. You start with what you get and go from there. So, this thread, and a partial schematic makes for a just more interesting project. I guess.

Allan

[baljemmett]

Some testing... and.... the negative isn't floating. It surely IS suppose to float on this supply, right? Because if I connect negative to ground it draws over the CC setting (i.e. set to 0.1 it draws 0.2) with nothing on the positive. I noticed it when I tried to scope the output with scope ground to negative.

Now, that rings a bell; something about a chassis screw in a really stupid position...  *rummages*

Ah, unfortunately that was on a Mastech supply, rather than one of the Korad threads -- so it's not an already-documented issue with this model.  But just in case it's of any use, https://www.eevblog.com/forum/reviews/bench-supply-ground/ has the gory details.

[pomonabill221]

The KA3005P arrived today from SRA Soldering. QUick.

Some testing... and.... the negative isn't floating. It surely IS suppose to float on this supply, right? Because if I connect negative to ground it draws over the CC setting (i.e. set to 0.1 it draws 0.2) with nothing on the positive. I noticed it when I tried to scope the output with scope ground to negative.

Allan

I checked both the positive output and the negative relative to chassis ground and they ARE floating.... Your supply may have a problem.

[tony3d]

The KA3005P arrived today from SRA Soldering. QUick.

Some testing... and.... the negative isn't floating. It surely IS suppose to float on this supply, right? Because if I connect negative to ground it draws over the CC setting (i.e. set to 0.1 it draws 0.2) with nothing on the positive. I noticed it when I tried to scope the output with scope ground to negative.

Allan

I checked both the positive output and the negative relative to chassis ground and they ARE floating.... Your supply may have a problem.

Yup! Mines floating as well. Call SRA Solder, speak with Doug, and tell him you want another one sent out! They are really good to deal with.

[AllanMN]

Thanks Tony. I'm in touch with Doug. But this is perhaps an easy fix. I think something is touching ground and since the supply floats, that signal on ground is pushing plus and minus around from there. Here's what I see on the scope on the positive output lug (attached image):

5ms/div, 5v/div vertical, 0v is at top of scope screen so you're looking at -10v to -40v about.

I think that's the unregulated plus rail (about -43v) with pulses to the regulated voltage (set at 10, so -10v). It's inverted since I'm looking at it reversed if that makes sense (plus rail is grounded so positive out is pushed negative).

There isn't a lot that's at ground in the supply. I was about the disassemble the front panel assembly but it's plastic (not grounded metal). There's just the ground from the front panel lug coming though.

My question is where would such a signal be on the schematic, and then, circuit board (Pomona Bill). There's where to look for a short to ground I think. Careful inspection didn't reveal any solder bridges or physical damage.

An aside, learning to repair this supply would seem to be a handy trick for the future.

Allan

[tony3d]

Thanks Tony. I'm in touch with Doug. But this is perhaps an easy fix. I think something is touching ground and since the supply floats, that signal on ground is pushing plus and minus around from there. Here's what I see on the scope on the positive output lug (attached image):

5ms/div, 5v/div vertical, 0v is at top of scope screen so you're looking at -10v to -40v about.

I think that's the unregulated plus rail (about -43v) with pulses to the regulated voltage (set at 10, so -10v). It's inverted since I'm looking at it reversed if that makes sense (plus rail is grounded so positive out is pushed negative).

There isn't a lot that's at ground in the supply. I was about the disassemble the front panel assembly but it's plastic (not grounded metal). There's just the ground from the front panel lug coming though.

My question is where would such a signal be on the schematic, and then, circuit board (Pomona Bill). There's where to look for a short to ground I think. Careful inspection didn't reveal any solder bridges or physical damage.

An aside, learning to repair this supply would seem to be a handy trick for the future.

Allan

Let me know how you make out. When the supply is working properly, I really like it. I like the size. Fits perfect on my small bench. Was SRA willing to exchange it?

[AllanMN]

Ah. Found it. The fan wire wasn't routed through the slot in the fan housing and was then crushed when assembled, shorting to case/ground. What I saw on the scope was the PWM of the fan return,  pulled low by U12, an On Semiconductor  ULN2003A High Voltage, High Current Darlington Transistor Array. Splice or replace to repair.

A comment I saw on the video EEVblog #315 - Korad KA3005P Review/FAIL :

" The biggest problem with these 1HL brands is the counterfeit pass transistors. They can barely supply 1/3 their rated current due to a severely reduced SOA. I routinely buy them - they're a steal - and replace the pass transistors with the genuine parts. I never, EVER have problems again. I am very hard on power supplies and do everything I can to blow them up. "

Any thoughts if this mod is warranted? And details? If say, you are inclined to charge SLA batteries at 5 amps for a few hours which I would not be? Cavlovic killed his doing this I believe. Did you ever figure out how, or get a repair/replacement?

I wonder how hot the sink gets at a full dissipation. I don't have a thermocouple to check but will in a few weeks.

The overshoot isn't such a concern to me if it only happens at 3-5A load ranges as in Dave's video. Do you see that too, Bill?

All my voltage outputs are very accurate. I guess I like this PS for the money. Even if I have to fix it!

Allan

[pomonabill221]

Hey Allan... GREAT that you found the problem!  Your scope picture does look like a PWM signal with AC ripple on it, although since the supply for the fan is unregulated, that could be where the ripple is coming from... just glad you found the problem.

Yes, the overshoot does change depending on the load.  I do see overshoot at even low loads (although I don't remember what the loads were and what the overshoot was.. will have to try it again).

I have (I think) the same "version" that you have as far as pass xsistors used and the assembly of the heat sink.  Don't know if they are real or fake though.
I was charging a 24V battery once, and drawing max current from the supply, and it did shut down due to over temp on the heatsink (or thermostat in the xfrmr... really don't know which tripped), but after a while of cool down (about 10 minutes), it powered back up ok.
It was running pretty hot and the thermal overtemp seemed to do it's job.

The heatsink is a little small if you are going to push it, and I may mod it with a larger heatsink (maybe even a larger fan), but for now it is working great for a cheapie!

I know alot of people whine about how cheap it is, and all it's problems, but like I have said, it ain't an HP/Agilent/Lambda/Fluke/etc., and I know that.

One thing on my supply... the FWB was NOT bolted to the heatsink and they relied on the stiff leads of the bridge!  It was NOT laying flat on the heatsink (the lead side was not touching the aluminum), so I desoldered the leads, bolted the bridge on the sink, and resoldered.  How cheap was that!
I would suggest that you check the mounting of the bridge to make sure that at least it is making good mechanical contact to the heatsink, and maybe even bolt it down... it probably runs pretty hot when under high load and will not last too long.

Here are some pics of the bridge (notice the slight space on the lead side), the xsistors and bridge (notice how the goop doesn't look like it was making contact on the bridge), and the contact area on the heatsink (only partial contact on the bridge).