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Inside the new Korad KA3305P linear PSU

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torch:

--- Quote from: Kleinstein on September 07, 2015, 07:32:57 pm ---The reaction to this very rough test looks OK, but not very good - there is one spike going noticable higher than normal voltage.
--- End quote ---

Yes, the worst spike appears to be about 5v (8%), bearing in mind the low resolution of the screen on my hobbyist-level Rigol scope. There's also two lesser spikes, apparently about 2-2.5v.

Frankly, I suspect there were more that just weren't captured given the time base.


--- Quote ---The funny spike at the beginning seems to be part of the normal way of switching from CC to CV. 
--- End quote ---

That makes sense. I didn't think of that.

Maybe over the winter I can try your three stability tests. At the moment I don't have an ideal load nor do I have any really large capacitors sitting around. Out of curiosity, how much more stable would one expect a professional grade PSU to be, and how important is it to a hobbyist or student on a limited budget?

Salas:

--- Quote from: Kleinstein on September 07, 2015, 07:32:57 pm ---The linear regultor is done analog, the µC just gives the setpoint and might give a controled ramp after power on. A µC might be used to controll the relays and the fan.

The reaction to this very rough test looks OK, but not very good - there is one spike going noticable higher than normal voltage. The funny spike at the beginning seems to be part of the normal way of switching from CC to CV.  So having short pulse of high current might be a problem for this regulator. This might even leed to overheating of the transistors, as the relays are not very effective with such dynamik loads.

--- End quote ---

Could the MCU be controlling the CC/CV transition also and its speed having something to do with the magnitude of such a spike in your opinion?

torch:

--- Quote from: Kleinstein on September 07, 2015, 07:32:57 pm ---The funny spike at the beginning seems to be part of the normal way of switching from CC to CV. 
--- End quote ---

If that was it, then the sudden in-rush plays a role too. Turning down the current 10mA to force the switch from CV to CC or vise-versa results in a smooth transition. Here's a sample that switched it from CV to CC and triggered a relay to click at the same time:




The transitions where the relays kick in are quite smooth going up too. This is increasing the voltage by 0.1V, triggering a relay:



(both under load -- my 5 parallel lightbulbs)

Kleinstein:
Stability (that is protection from oscillation and well behaved reaction to load changes) is something every PSU shoud offer. This a mainly a question of following basic rules. This about finding the rigth size for the nF range caps nothing that costs a lot of money per unit. It gets more difficult if the PSU is trying to regulate very fast. So there a cheap PSU can be very good in this respect, and a high end PSU that tries to push the limits speed wise is likely to be worse in this respect.

Switching from CC to CV mode is a slightly different thing. This includes nonlinear effects. In control theory the problem is called wind up - easy to solve in a digital regulator, but quite complicated analog. So here well testet high qualitiy PSUs can be expected to be better. At least there should be not complete desaster.  As comlete tests are difficult, however there still is a chance to find a bad one from HP and good chinese one.
Somehow the shape of the peak when changing CC to CV looks very much as I expect from a standard circuit. So it might even be a hint that stability is likely good.

The real drawbacks of these cheap chinese PSUs is the thermal design: they are made to deliver much power at low cost. Here less safty marging saves costs and thus things get hot right to the limit. Similar main filter caps are rather small as they are expensive parts. So some units that get the smaller ones show rippel at the upper limit.  For me this PSU looks good up to about 3 A. A more reputable brand might sell such a supply with just the upper current limit reduced and some fuses added.  So if you know and respect the limitations such a cheap PSU is not hat much inferior to brand named one.

One other point the chinese save some money is to pots / DACs. This might be the reason for the strange jigged shape on startup. Instead of a good DAC there may be a ADC / DAC loop to set the voltage.

Besides problems with constant high power, the other type of overload, might appear with dynamic loads. This is because the relays are slow, and some ealier stages might get hot as well. Here more reserve helps, but using relays to switch taps can not cope well with highly dynamic loads. So even a HP unit might fail or at least stop with overtemperature error. So the file test is allready pushing the limits of such a supply.

Salas:
Solid state relays are out of the question?

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