Korad KA3005P power supply faulty/repair

[electricar]

Hi folks,

does the newest revision of the PSU have those overshoot issues too? I bought the Korad KA3005P recently and have overshoots when enabling the output. Unfortunately, I didn’t look up the revision of the PSU yet.

[milamber]

Right. But they could also be level shifters/buffers for the R2R DAC. IMHO the 100ms transient response (no opamp is THAT slow) and the steps in the voltage after turn-on (watch Dave's video) speak a clear language.
Unfortunately Dave didn't do a transient test. Anyone who owns one of these want to do one? All you need to do is to connect a big resistor to the output and switch it with a MOSFET at ~100 Hz and trace the output voltage on a scope.

Is the Atmel clocked with a 12 MHz quarz? Did someone already try to exchange it with a 16MHz or 20MHz to tweak the transient response to 75 or 60ms?

[Andreas]

Hi folks,

does the newest revision of the PSU have those overshoot issues too? I bought the Korad KA3005P recently and have overshoots when enabling the output. Unfortunately, I didn’t look up the revision of the PSU yet.

Hello,

I have around 8% overshoot on the 5V setting.

With best regards

Andreas

[oger]

Hi everyone,

I also have this Powersupply. I have a later date, so I don't need to deal with a lot of the issues discussed here.m But I did a mod that prevents the fan from spinning permanently.
I do have however two things that bother me.
First is that the supply seems to have a capacitor at the output. I could not source a schematic so I'm not sure.
The problem is that when I connect an led for example and have the output voltage set to anything higher than 7 or 8v it will be destroyed because
current limiting did not kick in quick enough even though I set it for less than 10mA.
This might be a common problem among PSU though. I'm not sure.

Secondly, what bothers me the most is that the rotary knob for setting voltage and current behave erratically.
When I turn it, it might jump to 25v right away. Very dangerous when the output is on.
So I wonder if I could just replace the digital "pot" or is it the logic behind it at fault?
This gets progressively worse.

Thanks for info!

Stefan

[Dominic-V]

Just disassemble the rotary knob and clean it with isopropyl alcohol, then lube it and put it back in. Should work as new, at least mine did and never missed a digit after.

As for the current limit, just connect the led before turning the output ON.

[oger]

That did it!
Just had to clean the encoder. No more steps missed.

[Andreas]

in a german forum I have read that you can replace the encoder by ALPS STEC12E07 available at Conrad / Reichelt.

With best regards

Andreas

[oger]

I t does not not seem to be a special encoder.
but cleaning seems to remedy the problem quite good.
No need to buy a new one. Especially mine was hardly used.

[uv3afl]

Hello,

As this is my first post here I want to thank this forum and Dave Jones for plenty of constructive things I knew from EEVblog forum and videos!

I have recently purchased this device as I knew from several youtube videos to be not bad price/performance value. It seemed to be in working condition but out of calibration. It gave about 1,5% setting voltage error. Thanks to this theme I managed to calibrate it and it turned at last into some usable device! Anyway in couple of days playing with it I have run into some sort of a bug or what ever. I haven't seen anyone mentioned it. Thus I registered here and want to describe it and ask if anybody of you had such situation.
So. I attempted to charge a 18650 battery. The process went on normally in CC mode indicating 200mA and something about 3.45 volts on battery. At that moment I thought that there was no need to give it full charge and decided to limit charge voltage to some 'long term storage' value - 3,6 volts. I have set it up and surprisingly found out that it started indicating 3.6 immediately after setting. Multimeter attached showed that actual output voltage was still 3.47V... While indicated was already 3.6.

Some further experiments showed that when in CC there is a difference in set voltage and actual output voltage greater than 0.2 volts - everything is fine. Otherwise you will see limiting voltage instead of actual.

That's the story. Any comments?

[Kleinstein]

Still showing the set voltage is a kind of cheating - the voltage display should always show the actual voltage. The only good exception could be with the output disabled and than with a clear marking that it is the set voltage and not the measured.

So it might be worth do a few more tests on this.

[Andreas]

Hello,

that is the behaviour of the device. (also on mine).
When the difference of current / voltage is below a limit the "set" instead of "actual" value is displayed.
perhaps to supress excessive flicker near cc/cv limit.

With best regards

Andreas

[JackM]

I attempted to charge a 18650 battery. The process went on normally in CC mode indicating 200mA and something about 3.45 volts on battery. At that moment I thought that there was no need to give it full charge and decided to limit charge voltage to some 'long term storage' value - 3,6 volts. I have set it up and surprisingly found out that it started indicating 3.6 immediately after setting. Multimeter attached showed that actual output voltage was still 3.47V... While indicated was already 3.6.

Does the same thing occur when you attach a static load (ie. a resistor) and then put it in constant current mode?

[uv3afl]

Thanks for answers.

Does the same thing occur when you attach a static load (ie. a resistor) and then put it in constant current mode?

I've tested this. It behaves completely the same at the static dummy resistor load.
1. Set 2V as a limit, 0,5 Amps as current limit, attach 3,3 ohm resistor

2. Load on - everything is fine

3. Change the voltage limit to 1,9 volts
4. Good bye! 1,9 instead of actual output.


Now I really interested if it is a software bug or is it a hardware design feature. And I definitely don't think that it can be normal power source behavour...

[Ton]

It's a SW "feature" of the Korad power supplies (and the rebranded Tenma units), when ever the actual voltage output is within 0.2 V from the setpoint it will show the set-point instead of the actual value. 

already discussed here: https://www.eevblog.com/forum/testgear/korad-display-bug/msg585763/#msg585763

this "feature" is also seen on some cheap Chinese PID controllers, the speculation is that it gives the user a feeling of greater precision than the device actually gives. 

It is annoying but it is how these supplies have behaved, basically since they were released from Korad some 3-4 years ago.

Still I find them good value for the money even if this "feature" is irritating

Br Ton

ps. I have 3 of the Tenma branded version
pps A forum member have started the design of a alternative display PCB where this "feature" can be removed, but I dont know the current status of this, see the tread if interested.
https://www.eevblog.com/forum/projects/korad-ka3003d-redesign-and-upgrade/msg1011712/#msg1011712

[uv3afl]

It's a SW "feature" of the Korad power supplies (and the rebranded Tenma units), when ever the actual voltage output is within 0.2 V from the setpoint it will show the set-point instead of the actual value. 

already discussed here:
...
pps A forum member have started the design of a alternative display PCB where this "feature" can be removed, but I dont know the current status of this, see the tread if interested.
...

Oh, thanks a lot, Ton for proper directions! Now all further questions will be in 'right' places. And theme with alternative solution is of course of great interest! I will study that!

[uv3afl]

this "feature" is also seen on some cheap Chinese PID controllers, the speculation is that it gives the user a feeling of greater precision than the device actually gives. 

One more question if you please - why?     How does it work for getting the illusion of higher precision in this case?  I don't believe in flicker or noise Andreas tells about, as it will be also 'noisy' and flashing in say CV mode and also 'noisy' being far from voltage limit in CC mode... And if they care about CC/CV - 'noisy' mode switches, they will have it independently of displayed value , isn't it so?

[Ton]

Hi uv3afl

I guess that the theory, if you can call it that. Are that for a user with a pid controller set to keep for example a temperature at say 30 degC it will look more stable if it always show 30.0 degC than if it is sliding between 29.9 to 30.1 - it is all about the perceived stability.

I should probably have used stability in my first post instead of precision.

if you know stuff regarding process control, you are expecting some sliding around, not a static number.
Or your process is very stable with very small noise input.
The task to minimise the sliding around is sometimes a quite challenging one if your plant is not simple.

Br Ton

[tombi]

Hello,

With the overshoot problem, I wonder if putting a small cap in parallel with D1 might help here? I've seen some Agilent schematics that do this.

Tom

[Andreas]

What is D1?

if you mean the output: how much compared to the already used 330uF across the output?

Normally you would use a feedback capacitor across the voltage/current regulator.
But adjustment is critical: if too large the load regulation suffers. If too small you get overshoots.
And the largest issue gives a change from voltage <-> current mode.

with best regards

Andreas

[mos6502]

the already used 330uF across the output?

Are you saying this PSU has 330uF across the output? That's total junk ...

[tombi]

Sorry,

Way back in the thread somebody posted reverse engineered schematics and D1/D2 were between the voltage control and current control opamp and the pass transistors.

I was thinking like 1nF - nothing major.

Tom

[mos6502]

Way back in the thread somebody posted reverse engineered schematics and D1/D2 were between the voltage control and current control opamp and the pass transistors.

That would slow down the regulation even more. The reason a designer would put a ridiculous amount of capacitance like 330uF on the output of a linear power supply is because he's trying to make up for the slow regulation. Sorry guys, but everything points towards the fact that this PSU is garbage. I wouldn't consider it using for any electronics work because of the 330uF at the output alone. Maybe it's OK for charging car batteries. 

[uv3afl]

Way back in the thread somebody posted reverse engineered schematics...

I have found some better schematics for the device. And as the support for the device is far from being called 'support', I think this must be here. Attaching to the message. Maybe someone will stick it up to some 'documents' theme.

[uv3afl]

Now couple more pics about 'garbage' )) One of them:

 - garbage on output terminals when the load is soft-off and you hard-powered off the PS. The second one:

- garbage on output terminals when you momentarily press/depress power-on hardware button. How about killing something attached to PS with that? Hopefully too short, but... Anyway, do know.

[uv3afl]

Some more news!
My dispute with the seller at Aliexpress has just finished successfully. They agreed that the 'feature' of not displaying output voltage when delta between Set and Out voltages is less than 0,2volts is a constructive defect and will refund me half-price of the PS.