Yihua 305DA Power supply negative voltage on output and calibration

[Damian1908]

Hello everyone i am new here, i am looking for some help with linear power supply Yihua 305DA. I have two problems with it first is calibration does anybody know which potentiometer is coresponding with certain settings. There are six potentiometers described as V1 V2 V3 V4 VR1 VR2 attached picture. Second problem is present of negative voltage -0,7V when output is turned off it is normal behavior? I will be very gratefull for any help, best regards.

On my first attempt to calibrate for me it looks like described below but i am not shure it will work like this

VR1 set maximum CURRENT value on display
VR2 set maximum VOLTAGE value on display
V1 set CURRENT on output
V3 set VOLTAGE on output

Still dont know whats it  V2 and V4.
Now on low voltages and current dislay is fine but on higher is not correct.

[jwet]

Welcome to the group.  Your best bet is to get a schematic/service manual from the manufacturer. 

Here are some educated guesses- you're likely right about the pots that you've found.  Watch for interactions between the adjustments- sometimes they aren't completely independent.  This is where a test procedure/service manual could help.

On the -.7v, when off, this is likely the charge on the output cap clamped by a reverse polarity diode on that little board on the front panel.  Load it with a 1k  R and see if goes away consistent with value of that Cap - T=RC.  A better instrument might have a 10k R across the C to deplete this- in your case, it gets down to .7 and has nowhere to go once the diode stops conducting at .7v.a

My guess for the mystery pots is switchover points that tap the transformer secondary at different voltages.  This is common in high current variable linear supplies.  At lower output voltages, a lower voltage tap is used to lower the voltage that the pass transistor has to take.  If you wanted to make 1v at 5 amp from a 30 volt, basic transformer and rectifier, the pass element would have to dissipate nearly 150W.  If you use and 8v tap for these lower voltages, you can limit this case to 35W, much more manageable.  Its likely there are three secondaries, a high voltage one- say 30V, a medium one, say 22v and a low around 8V.  The output pass element has to dissipate the drop in voltage times the output current.

Get a schematic somehow and have fun.  I've told vendors that due to being an ISO 9000 shop, all our equipment had to be fully documented.  They coughed up schematic from somewhere. Good luck.

Good luck.