# Wall Wart has a hissing sound and crapped out

Discussion in 'Troubleshooting and Repair' started by Woodman, Apr 13, 2013.

1. ### Woodman

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0
Dec 3, 2012
When I plugged in charger for my older HP digital camera last week it made noticeable hissing sound, then I think hissing sound was not as loud but still detectable. I pried wallwart apart and used my wife's stethoscope to find that noise is closest to the large power Cap. so I'd like to replace since I don't have a tester. Don't have a 33uf 400 volt, but do have a 1000uf @ 180 volt and a 44uf @ 250 volts. Using the cap formula for series combinations, I show this will be 44uf @ 430 volts. Question is my calculation correct and is 44uf close enough to 33uf to work?

Woody

2. ### davennModerator

13,837
1,952
Sep 5, 2009
No neither cap is sufficiently voltage rated. Get the correctly rated 400V cap.

Dave

3. ### Woodman

47
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Dec 3, 2012
Are you sure? I read that when CAPs are connected in series the voltage capacity increases, and will 44uf work for a 33uf?

4. ### Woodman

47
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Dec 3, 2012
CAPs in series increase Max voltage, in theory at least. By the way, I found a pair of CAPS that are closer in size to the original CAP, and also closer to original CAPS tolerance of 33uf @ 400volts:

A series combination of two CAPS (100uf @ 160 volts + 47uf @ 250 volts = new specs of: 32uf @ 410volts).

I know how to connect the two caps in series to increase MAX voltage to 410 volts. Apparently I may have to add resistors somehow to level out 410 voltage across the caps. Any advice on wiring resistors in circuit (size, watage, parallel/series, etc. would be appreciated.

I understand that these CAPs could explode violently in overvolted, fail, etc. so full frontal warfare garb including eye shields will be implemented.

Woody

Last edited by a moderator: Apr 14, 2013
5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
The problem, as with resistors in series, with capacitors in series, the voltage across the capacitor depends on it's impedance.

In this case the 47uF capacitor will have a much higher impedance and will have almost the full 400V across it.

The other issue us that the 1000uF 160V cap is likely *Huge* compared with the original capacitor.

6. ### Woodman

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Dec 3, 2012
YOU OBVIOUSLY DID NOT READ CORRECTLY MY LAST POST THAT SAYS I FOUND TWO THAT ARE ABOUT THE SAME PHYSICAL SIZE AS ORIGINAL (actually both replacements are the same size, and are about 33% larger that original), ALSO FARADS OF BOTH ARE AT LEAST CLOSER (100 AND 47) THOUGH VOLTAGES ARE STILL DIFFERENT - 160 volts and 250 volts.

ANY SUGGESTIONS ON WIRING RESISTORS TO EVEN OUT VOLTAGES WOULD BE APPRECIATED. SERIES CAPS CAN BE USED AS A SUBSTITUTE, BUT NOT WITHOUT PROPER KNOWLEDGE WHICH OBVIOUSLY I DON'T HAVE. I HAVE EIGHT HIGH VOLTAGE CAPS WITH DIFFERENT SIZES, VOLTAGES AND FARADS -SOME COMBINATION MUST WORK?

Woody

Last edited: Apr 14, 2013
7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
1) Don't shout.

2) Your original post says 1000uF

The capacitor runs at mains voltages. The device is designed to keep certain clearances around the component. Replacing the capacitor with a larger one or two of them that will have to somehow squeeze in, possibly compounded by having extra resistors, is going to compromise your safety.

My suggestion is that unless you have a 400V (or higher) capacitor with a capacity between about 22uF and 47uF having the same or higher temperature rating and that physically fits -- that you go out and buy one.

If this were the 5 volt rail or if it were battery powered, then you could do this with little danger and risk damaging only the equipment concerned. But it's not.

8. ### Woodman

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Dec 3, 2012
gee, thanks again superSteve.

9. ### Woodman

47
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Dec 3, 2012
IT LIVES ANOTHER DAY!

The two caps were wired in series, and replaced original cap on the circuit board. Calculations are as follows - series wired combination of two CAPS --> (100uf @ 160 volts) + (47uf @ 250 volts) = new specs of: 32uf @ 410volts). Or mathematically 1/ (1/47 +1/100) = new 32uf spec. Voltage is simply additive, and regarding voltage distribution, the math is a simple ratio. In this case 100/147 for the lower 47uf cap or about 2/3 of voltage allocates to the 47uf @ 250 volts cap, so then 1/3 of voltages will be at the higher 100uf rated cap. Note, use of resistors to further even out voltages should be done. I did not bother with that so this is more of a quick and dirty replacement.

As it turns out I got lucky, in that these two salvaged caps are actually a very close match to the original single cap specs of 33uf @ 400 volts. The new 32uf rating is almost spot on, and votage drops across each cap are with specs. The higher voltage rated cap could have to take about 10% more that ideal, but only if maximum voltages are exceeded. Not a perfect volt ratio, but dam close, and did I mention that it charged my two batteries - no problem!!
Will post an update in a few weeks or next time I charge batteries because I'm really not sure how much longer it will work -- the faint high pitch hissing noise, or original problem that I noticed when it stopped charging still exists so there may be another part failing that is causing caps to fail. Anyone venture to guess as to what part is causing the noise?
As usual, I did not follow Steve's usually practical advice. And his answers are sometimes not very supportive for us weekend electrically simple minded hackers; however, I did heed some of his warnings by not cramming both caps inside but instead used a coping saw to cut an opening for the new caps to protrude out the side, and am using protective eyewear and covering wal-wart with heavy insulated glass lid when charging.

Woody

Last edited: Apr 20, 2013
10. ### Woodman

47
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Dec 3, 2012
Update: the noise is coming from a coil, not a very replaceable part?? Any suggestions other than buying another wart? As mentioned, charger does work now, possibly because caps are bigger or maybe they will fail soon, or not??

Last edited: Apr 20, 2013
11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
The first solution to a noisy inductor is to make sure it can't vibrate. Is it held down with some cable ties? Are the broken? Does it have some "goo" holding it down?

12. ### Woodman

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Dec 3, 2012
Yes, there was some "goo" holding it down. Maybe I should encase it in goo? It is an annoying high pitched sound, but it only occurs when charger is plug into wall but not plugged into camera (when also plugged into camera, then noise it absent). If you recall this inductor's high pitched whine preceded the capacitor going bad. Don't know if inductor whine had anything to do with cap going bad or not.

A few months ago I said that I would update how this capacitor in series substitute worked. Well, I have charged batteries about 20 times since last post so it is still working great.

Woody

Last edited: Jul 20, 2013
13. ### eKretz

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Apr 8, 2013
These chargers always make that noise, it's high frequency switching. It gets louder if they have to work harder (or, as in your case too hard). Normally you can only hear it when you're within a foot or so of the charger. Now that you have modified the charger and cut holes in it, you will just have to live with the noise or put it in a larger sound insulating enclosure if it bothers you. If it is loose now it will only be louder yet.

Steve did nothing wrong, BTW, if he had given you the info (or perhaps confirmation) you were looking for and something had gone wrong, someone unethical could hold or try to hold him liable. He gave the same advice anyone would give to a person they didn't know with a not very advanced knowledge level. Do the safe thing and just buy the proper replacement part for a few bucks.

Last edited: Jul 20, 2013
14. ### Woodman

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Dec 3, 2012

Sorry eKretz, but as stated the noise only occurs when charger is not working (plugged in wall only), when plugged in and charging or working hard there is no noise. And no I will not waste my time and money buying the "proper" part and removing mods with new junk. I needed a quick replacement, learned something while fixing with parts that I had on hand, and had a little fun in the process.

Without Steve this website would be crap, and I sometimes forget to give him the respect he deserves. He is an electrical genius who I think selflessly helps out us electronic knobs. Just wish there were more like him. He seems overworked since he is the first to reply to most questions. In a perfect world (dreamer ) there would be more experts here, then advice would be more descriptive, and alternative advanced info could be given come hell or high water. Disclaimers on this website abound, unsafe posts are removed, though I don't have a clue about website's liabiltiy potential (owner of website care to comment?). Anyhow, we are all lucky to have Steve here, for us hackers he is our obi wan kenobi of electronics. I have Private Messaged Steve in the past telling him the same. Utopia will never exist, but the Web and people like Steve get us a little closer. My 2 cents - like it or lump it!

Woody

Last edited: Jul 20, 2013
15. ### duke37

5,364
772
Jan 9, 2011
Woody.

Steve is excellent and I like his humour but I think you are very hard on the rest of us. I try to answer simple questions within my limited knowledge. There are several other very good contributors but we sometimes get an odd ball who tends to get weeded out.

The power supply you have probably runs at a very high frequency, well above the range of human hearing. When it has no or very little load, the switching will be turned on and off to maintain the correct output. This low frequency may be in the audio range.

The transformer will vibrate due to magnetostriction.

16. ### eKretz

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Apr 8, 2013
Yep exactly, it is always making noise, it's just outside your audible range when under load. Forgot about that simple explanation. And yeah there are quite a few good contributors here IMO. I am still pretty much noob level for the most part. Also, I found a similar question elsewhere the other day and it had a good explanation slightly more in depth regarding the varying switching frequencies:

Quote:
"Maybe it's worth mentioning that it is also common and normal for unloaded or very lightly loaded switching supplies to do that because there might be a lower limit on the duty cycle forcing the supply into a pulse-skipping mode creating a lower effective switching frequency. As pulse skipping/burst mode operation can be beneficial for efficiency at light loads there can be reasons to want to introduce it on purpose."

Last edited: Jul 20, 2013