Electronics > Repair
Korad KA3005P power supply faulty/repair
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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:

--- Quote from: AllanMN on January 18, 2014, 05:51:40 pm ---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.

--- End quote ---
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.
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