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An investigation about ripple current of Kunkin KP184
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vladant:
Hi, is not that black wire on the photo bypasses current shunt ?
graybeard:

This video should start on the portion about load current noise.

I have seen similar noise in my Beich CH9720CU Electronic Load.   After doing a comprehensive test and tear down I believe it is due the digital feedback loop controlling the load.

In the Beich load I see two distinct almost periodic events; a faster one I think is do to the single A/D sampling the Voltage and current, and a slower one that I believe is feedback sending the current level update to the D/A driving the op-amp/MOSFET load cells.  The fact that it is not perfectly periodic is due to the regulation/measurement MCU having other duties like exchanging information with the control/GUIe interface MCU.

In the Beich load the circuit is laid out on two-sided PCBs (I assume to reduce cost) and lack controlled impedance transmission lines.   Thus coupling from the fast digital signals into the analog portion are happening as well as reflections from the fast edges of the digital signals.  I think the designer got around the reflections by giving the data time to settle before clocking into the next stage.

From your photos it appears the Kunkin load also uses two sided PCBs, so the issues may be similar.
vladant:
I measured voltage on CD7/12 capacitors and just curious if it is a design "feature" to have 19V on 16V caps :).

So far I have found out that KP184 design has 5 major flaws:
1) +-12V supply and its ripple and noise.
2) +5V supply, its (capacitance) impact on +-12V and in turn the front LED panel impact on +5V (which is devastating).
3) FAN supply circuit and it noise.
4) Oscillation in U16 TL074 opamp #4 in Constant Voltage (CV) mode with an input resistor > 5Ohm.
5) It exists.

Resolution for flaw #:
1) Cut off +5V from main transformer by removing diodes D15/16. Now +5V rail is not powered.
2) Install another 7-12V 5W transformer to power up +5V rail (fortunately the case has plenty of room). Connect its output via diode bridge to CD12. Insert 50-100 Ohm resistor in J12 line 1 (+5V to front panel).
3) Install unpopulated C34 100nF (if you have it on your PCB version, near R25) OR install LDO with 12-13.5V output instead of existing switching DC-DC.
4) Connect input+ to R88/R9 by 1MOhm resistor and 500-1000pF capacitor (>200V) in series.
5) Cannot be helped.

Please share your results.
PaWill68:
I contacted the developers. C34 must be installed without fail. Rated 220nF 0805 MLCC. Can be used from position C17. When changing the fan power circuit, they forgot to install it in the first batch of products.
aristarchus:
Can this C34 position on pcb be pinpointed on a photo ?
Tried for some time to find it and it was not easy.
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