Author Topic: 12V/240V -> 12-20V laptop power supply with 12kHz whine  (Read 1297 times)

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Offline moz

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12V/240V -> 12-20V laptop power supply with 12kHz whine
« on: April 07, 2017, 07:23:57 am »
I bought a cheap UNIVERSAL100W 12V-24V 5A Max Laptop AC Adapter Power Supply Charger with Carplug  off eBay for $40 (did not come with car plug, but I didn't want that anyway). It actually works quite well for my intended purpose - running the laptop off a 100AH 12V SLA in my shed. Laptop draws about 20W so it's no big strain on anything, but it runs noticeably warm to the touch. Opening it up there are a few tear-down comments:

  • some of the end-cap screws are pre-stripped for your convenience
  • only the display driver chip has had the markings ground off, and that was done badly
  • there are isolation slots in somewhat random locations, and plastic-coated isolation barriers under the main board and between the 240V input and rest
  • the transformer seems really awful to me
  • there's a weird tiny contact between the inside heatsink and the case

The really annoying thing is that there's a 12.1kHz whine coming out of it, and I'm not entirely sure exactly where from. My inclination is to hot-snot or silicon the inductor and see if that helps, but I'd appreciate the expert opinions of anyone with an oar to poke in. Am I likely to wreck it by running amok with some silicone sealant/glue (construction adhesive), or with a hot glue gun (assuming I steer clear of the heatsinks)?

I will take better photos tomorrow with a proper setup, but for now here's some cellphone snaps to hopefully give you some ideas.
« Last Edit: April 07, 2017, 07:27:38 am by moz »
 

Offline blueskull

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Re: 12V/240V -> 12-20V laptop power supply with 12kHz whine
« Reply #1 on: April 07, 2017, 07:43:03 am »
Metal case without double insulation nor ground pin, fail.
Also, should the input cap vent, the electrified water vapor and cloud directly goes to secondary side. The input cap is also not doubly insulated, should the sleeve break, there is practically no insulation between capacitor's metal can to secondary.

Now, your question: what's the 12.1k whine. The particular PSU may use a technique called PFM in order to reduce light load power consumption, in this mode, switching frequency will go down, possibly fall into audible range.
Also, some PSUs use pulse skipping mode, which may also fall into audible range. Another mechanism is control loop's ringing falling into audible range.

Hot snoting magnetics will alleviate, but how well it does depends. If I were you, I will request a refund and get rid of this. For 99.9% of time it won't kill you, but I bet you don't want to take the chance.

This is a proper AC/DC input laptop PSU, and it is very affordable.
https://www.amazon.com/dp/B002VY6NRM
« Last Edit: April 07, 2017, 07:52:26 am by blueskull »
 
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Offline capt bullshot

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Re: 12V/240V -> 12-20V laptop power supply with 12kHz whine
« Reply #2 on: April 07, 2017, 07:50:27 am »
Don't use random silicone for electronics. Some of them realease acetic acid.
Safety devices hinder evolution
 
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Offline moz

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Re: 12V/240V -> 12-20V laptop power supply with 12kHz whine
« Reply #3 on: April 07, 2017, 08:07:41 am »
Metal case without double insulation nor ground pin, fail.  For 99.9% of time it won't kill you, but I bet you don't want to take the chance. This is a proper AC/DC input laptop PSU, and it is very affordable.

Sadly "This item does not ship to Australia.". But I can buy it here... for $90.

I do take your point about the lack of grounding, but I have no intention of plugging it into 240V ever, for that reason. I bought the supply because there's no mains in my shed, which helps me avoid that temptation. The mains cable supplied is also a cheap non-approved chinesium one that was surprisingly rigid for its diameter.

« Last Edit: April 07, 2017, 08:09:53 am by moz »
 

Offline amyk

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Re: 12V/240V -> 12-20V laptop power supply with 12kHz whine
« Reply #4 on: April 07, 2017, 12:06:23 pm »
You can use a drinking straw as a stethoscope to determine which of the magnetics is emitting the whine. It might be a ceramic cap too.
 

Offline moz

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Re: 12V/240V -> 12-20V laptop power supply with 12kHz whine
« Reply #5 on: April 16, 2017, 03:42:00 am »
The noise is worse, and I have a better spectrum, again using a simple spectrum analyser on my phone. But I'm also having (more important) issues with a new solar charge controller only giving about half the expected output, so I have not opened this thing up again yet.
 


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