17IPS17-2 Power Supply - Replacing SMD Capacitor

[JonathanAnon]

Hi,

Link To 17IPS17-2 PDF schematic

Trying to fix this power supply, which is power cycling. I'm having trouble with an SMD capacitor that is on the primary side, connected to pin 6 of the combined PWM/MOSFET (STRW-6253)... NB: Pin 6 is marked as "Frequency jittering and External Latch Protection terminal" in the chip's PDF.



I was testing the highlighted capacitor C835 and did the following:

- Tested in circuit with my ESR60. There is no marking on the capacitor and my ESR meter says "open circuit or low capacitance".
- The cap looked slightly odd so I took it off circuit... Tested with ESR Meter off circuit and same "open circuit or low capacitance"...

I want to test replace it (without having to go to the hassle of ordering/waiting on a replacement), but I dont have exactly the same capacitor.. So what could I use instead... It's an SMD, and it's shunt, so I assume that it's just decoupling capacitor to remove A/C voltage of particular frequencies?.. Could I just put in any 50V SMD capacitor of relatively similar capacitance and expect it to work..

[budm]

Re: 17IPS17-2 Power Supply - Replacing SMD Capacitor

That is small value cap, 0.0047uF, I do not believe the ESR60 supports capacitance that LOW, you do not really use ESR meter for testing small cap less than 1uF, especially the non-electrolytic type. See the spec of the ESR60.
"Supports capacitors from 1uF to 22000uF. "
http://www.peakelec.co.uk/acatalog/jz_esr60.html

[tom66]

Re: 17IPS17-2 Power Supply - Replacing SMD Capacitor

I can confirm small ceramics never measure well on the ESR60, because of the testing method it uses which is optimised for large capacitor ESR measurement. It's inaccurate below about 50uF on ESR, due to the Xc, so imagine how significant it would be at 4.7nF/

[JonathanAnon]

Re: 17IPS17-2 Power Supply - Replacing SMD Capacitor

I'm kind of disappointed in the meter tbh. I paid 100 euro for it, and would have expected it to be able to measure any capacitor in common circuits.. In this scenario I usually just set the MM to resistance test and see if the resistance increases as the cap charges.. But this is far from conclusive..

I need to rule this out as being the cause of the power cycle.. I suspect that the PWM / MOSFET could be the cause but it's testing okay with the MM, and I want to rule out everything else before buying a replacement (cos it's quite expensive). The capacitor is definitely not shorted, I suppose removing it altogether would be the best option for the moment?