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Inside the new Korad KA3305P linear PSU
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Kleinstein:
Solid state relais have a relatively large voltage drop. So they are OK at 110 V or 230V, but not that good at 12 V. Switching at AC also has a second problem: the filter caps have to charge, and this gives quite some current spikes.

If mechanical relays are not wanted SCRs are a real alternative. Old school, but less voltage drop than solid state relays, as the can also replace a diode.

The other alternative is doing switching an the DC side  in a class G like output stage. Slightly more diode drop, but as changeover can be very fast there is no need for a worst case reserve for ripple etc.
torch:
I have found a serious bug:

If the M4 button is held down for about 2 seconds, the PSU turns on Channel 1 and Channel 2 and starts randomly changing the voltage of one or the other channel (usually Channel 2). While this is going on, the other channel displays 0.000 volts and 0.000 amps, but may be outputting power despite the display. The only way to get out of this is to power down the PSU.

The on/off button will turn off the outputs, and the voltages will drop to 0.000v but the random voltage changes will resume as soon as the outputs are turned back on. Also, the outputs may or may not be "on" when the PSU is powered back up the next time (outputs should be "off" when the PSU is powered up), however all controls will function normally.

This behaviour will occur regardless of the keypad lock. No other keys will function with the keypad locked, but leaning on M4 for 2 seconds will still result in this bizarre behaviour.
torch:
Speaking of bugs, the programming software is full of them.

First, the manual gives specific directions about what cable should be used for serial communications and how to use a terminal program to send commands. As I mentioned earlier, it also comes with a virtual panel and scripting software to control it via PC. It can execute a script and log the results. What came with it installed without any hassle and at first glance,, appeared to be working correctly with the exception of a screen resolution issue when running on an old XP machine. (the work around for that on my machine is to set the resolution to 800x600, apply, then restore the resolution. That resizes the window properly thereafter)

Well, things sort of work. The pretty graphical interface works, except it does not change the memory storage location on the PSU, it simply lets you have a second set of independent memory buttons, resident in the computer. Voltage changes are truncated at the decimal point. For example, 7.77v sets the PSU to 7.00v. Also, despite the real-time graphs of voltage and current for each channel, it seems to be logging only one of the channels. I suspect #1 but haven't verified that.

So I decided to try the terminal route. I still haven't figured out the serial port connection but either it's dead or maybe requires a null modem cable (manual shows straight pass-through). I did finally successfully communicate through the USB cable to the virtual Serial-USB adapter installed in the PSU itself.

The manual suggests MTTTY is suitable. It is not. Neither is  Terminal, Hyperterminal  and a host of others that do not send data until "enter" is pushed. The only way I could communicate manually is with RealTerm. Fortunately its open source, available at Source Forge. After selecting the correct port, and opening it, I could send commands via RealTerm's Send tab. 

The keyboard locks automagically when the USB cable is connected. The keyboard lock can be overidden manually at the keypad, but not in software that I could see.

Most of the commands will work despite the keyboard lock. Except the memory recall and memory store commands. That explains why the memory buttons are independently duplicated in software.

What is needed here is some decent software to replace the crap that came in the box. The capabilities are there in the machine, but accessing them is not.
torch:
Here is a short video showing the M4 bug:

http://picturehosting.verhey.org/korad/Korad_M4_bug_conv.flv

(Sorry for the separate link. I'm not sure how to embed a video here.)
torch:

--- Quote from: torch on September 09, 2015, 06:05:07 am --- Voltage changes are truncated at the decimal point. For example, 7.77v sets the PSU to 7.00v.
--- End quote ---

I confirmed this behaviour on a  Win7 laptop this morning, so it's not because I have an old XP machine on the bench.
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