WEP/Yihua PS-305D Power Supply Current Sense Circuit

[vk2amv]

Hi all,

I am after a bit of advice understanding what is going on in the current sense/display of one of the Chinese PS-305D Power supplies.

Long story short I have been doing some mods to try and improve the power supply a little bit, (And before anyone says, my dream is a Rigol DP832 but this Chinese one has to do the job until finds allow for something better) anyway I decided to calibrate the Voltage and Current display as they were quite a bit out.

Voltage display adjustment was easy, just a single potentiometer on the back of the display board.

However I found 3 seperate pot's on the display board that effects the current display.

I have managed to get it to acceptable accuracy (for a Chinese PSU) after a couple of hours screwing around. (Now reads within 5mA of my BM235 multimeter when in the milliamps range, and within 0.08 amps when in the amps range) however I still dont fully understand what I was doing.

I have sketched out quickly the main parts of the current display circuit, and I am hoping I can get some advice with what is going on.

I have figured out that VR10 is the zero reference/setting, (At least that is the effect it was having was changing where it read zero)

Now VR11 and VR1 effect both the current reading, as well as the linearity of the current reading from low load to high load.

However I couldnt manage to make sense of the reading changes between the two when I was adjusting one then the other.

I would adjust them so it read accurate at one current draw, then when current draw dropped or increased the reading got further and further out and was not linear.

I have managed through pure blind luck to get it adjusted to a point I am happy with and it now close enough and reasonable linear, however it is annoying me that I dont understand what I did, and I would like to learn what is actually happening in the circuit.

I would very muchly appreciate it if someone can explain what is going on and give me some pointers.

Sorry for the roughness of the circuit sketch, it was fueled by caffiene, I have omitted some of the bypass capacitors and other things that didnt seem to do anything, but I think I got the important parts of the circuit.

Thanks in advance

[Kleinstein]

Form the circuit plan VR10 is adjusting the offset, this should be adjusted first.
VR11 adjusts the gain / full scale for the high currents (lower gain setting) - this should be adjusted next.
VR1 adjusts the extra gain for the low currents - so adjust only after VR11 is set.

Using a transistor (BJT) to switch the gain is a poor decision, as it is slightly nonlinear at low voltages and should give a slightly different offset for the high and low range. It might be possible to get better performance with a MOSFET like 2N7000 instead of the BJT.

[vk2amv]

Thank you so much for that. 

It has now clicked for me exactly how the circuit works.

I was close but it had just not quite clicked yet, and you have now given me enough that it has all fallen into place.

About the range switch transistor, it is only a basic 2n3904, so you can tell they went all out on usint expensive parts haha. 

[Kleinstein]

The 2N3904 is not even the cheapest transistor - for large quantities there are lower grades, that might save you a few 1/10 of a cent. Depending on the display chip they might have been even better of going without the transistors (e.g. directly use an open drain µC IO pin if the display chip is a µc) - can't beat the price of a transistor eliminated.

[delamoure]

I have schematic of the YiHUA PS-305D power supply display pcb.

[henryinsydney]

your "schematics" are useless... and you know it

[delamoure]

Btw I spent near 4 hours to make this schematic accurately like on original PS305D. Much more than you spent on writing useless comment.

[perieanuo]

Kleinstein nailed it

[coromonadalix]

if the mcu is multiplexing the display, it can handle them correctly, and you can save parts count ...

attached is some psu schematics,  yours should be similar to this one

hope it helps