Author Topic: AC->DC Powersupply Advice  (Read 2533 times)

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Offline Tschuuuls

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AC->DC Powersupply Advice
« on: May 28, 2016, 04:02:56 pm »
Hi,

I own a Whirlpool/Jacuzzi which among other things has a built in Stereo with 2 Speakers and a passive Subwoofer.

The Pool Manufacturer fitted a Marine Style Radio (looks just like a Car single Din Radio, but is waterproof) which obviously runs on 12V DC and a AC->DC power supply to get 12V from my 230V wall power.

They designed this thing in a way that you can conveniently switch on your music with the included remote.
For this feature to work they simply didn't switch the 12V power supply (Millenia Spa Power8, Boars says:LG 100W 1.1).
So this poor power supply was on since 2008. 24/7/365. It died end of last year.

I disassembled it. The thing is enclosed in a rather large black container that is secured with those bloody secure flathead screws.

The supply looks just fine, it's an LG 100W model. No blown caps, no obvious heat discoloration or anything.
But the caps & semiconductors are both no name, it doesn't lack any output or input protection though.

I don't really fancy working on an power supply that is connected to the mains and I don't own an isolation transformer (got a Fluke 17B+ though).
And this thing has done quite some hours probably (8 Years of 24/7)...

So I quickly looked up 12V 100W power supplies on mouser. Found a delta that looks fairly good and is cheap (30€). Should fit in my black box.
http://www.mouser.com/ds/2/632/Datasheet_pmc-12v100w1aa-774692.pdf

My question is: Do I have to check for anything in this Audio application?
I guess the load I am applying will vary a lot. Is this Delta suited for this task?

Thanks a lot allready :)
 

Offline Ian.M

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Re: AC->DC Powersupply Advice
« Reply #1 on: May 28, 2016, 04:27:18 pm »
There are three issues - isolation, cooling and ripple in the audio spectrum.

Commercial grade isolation is probably OK if you power the PSU via a 10mA RCD, otherwise it needs to be a medical grade PSU.

Cooling is a significant problem if the enclosure is unvented.  If its plastic, you are FUBARed,   If its metal, you may be OK with a 12V fan in the enclosure to improve air circulation.   You may also need to look for a PSU with a higher ambient temperature rating. 
g
Audible ripple is only a problem if the PSU cotrol chip goes to discontinuous mode at low load currents.  If so, the cycle skipping can drop the effective switching frequency into the audio band.  Either get a supply that  doesn't do this or add a load resistor on its output to keep it out of discontinuous mode, but beware, this will worsen your thermal issues.
 

Offline Tschuuuls

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Re: AC->DC Powersupply Advice
« Reply #2 on: May 28, 2016, 05:28:04 pm »


Quote from: Ian.M on Today at 02:27:18 AM
There are three issues - isolation, cooling and ripple in the audio spectrum.

Commercial grade isolation is probably OK if you power the PSU via a 10mA RCD, otherwise it needs to be a medical grade PSU.

Cooling is a significant problem if the enclosure is unvented.  If its plastic, you are FUBARed,   If its metal, you may be OK with a 12V fan in the enclosure to improve air circulation.   You may also need to look for a PSU with a higher ambient temperature rating. 
g
Audible ripple is only a problem if the PSU cotrol chip goes to discontinuous mode at low load currents.  If so, the cycle skipping can drop the effective switching frequency into the audio band.  Either get a supply that  doesn't do this or add a load resistor on its output to keep it out of discontinuous mode, but beware, this will worsen your thermal issues.


The pool is connected properly to an RCD. The enclosure also has an O Ring protecting it from water ingress.

The enclosure is plastic.
I don't know how they designed the thermal envelope for the original supply, I think they just slapped it together, increased the price tag by 200% and sold it to the pool company.
The supply won't run on full load all the time, I ain't going to listen to music on full blast for too long in my pool  ;D

I could measure the "standby current", with an PC power supply. But I doubt consuming to less power is an issue, the supply has to drive a 1Din Radio, a Subwoofer, and the IR receiver circuit.

I just took some photos of the original power supply for you to check out.
 

Offline Ian.M

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Re: AC->DC Powersupply Advice
« Reply #3 on: May 28, 2016, 05:42:21 pm »
It looks like it hasn't blown the fuse, so chances are its bad caps or an O/C startup resistor to the  chopper control chip.  Personally, without an isolating transformer, I'd check likely resistors on the primary side and  shot-gun it by replacing the small electrolytics before giving up on it.  If that brings it back to life, go through it replacing *ALL* electrolytics with good quality 105 deg C ones, box it up and call it good for another five years.
 

Offline Tschuuuls

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Re: AC->DC Powersupply Advice
« Reply #4 on: May 28, 2016, 08:26:57 pm »
It has 105°C rated ones in there :D

I just thought it be cheaper to just replace the one that is in there, if a replacement is only 30 bucks.
Ordering caps and resistors from an supplier, paying shipping costs etc. I end up spending 20 bucks and I am not guaranteed, that it works afterwards  :-BROKE

I guess I could hook it up to some test gear in Uni next week and learn something in the process :P

Is this thing better designed than the 30€ Delta? Is this medical grade separation and transformers? (honestly have no idea)
 

Offline Ian.M

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Re: AC->DC Powersupply Advice
« Reply #5 on: May 28, 2016, 08:39:51 pm »
Its not medical grade, but the OEM reckoned it was good enough.  If you substitute, you are accepting liability, so you may well want a better isolation rating.   

85°C dec C caps wouldn't have survived eight years.  However you don't need 105°C ones for testing, and I would swap out the small ones, trace the circuit to find the startup resistor(s) (may use two in series) which will go from the +ve side of the bridge rectifier and +ve side of one of the big caps, to the IC and the +ve side of a little cap, probably with a 63V or less rating, test them and substitute something near enough out my junk box if open circuit (similar size +/-30% to original value).  That's at most a buck in parts.  If it runs I'd order the right parts, change *ALL* the electrolytics and put the correct startup resistors in.   If you find some other stuff you need you can probably top up the order till you qualify for free shipping.
 


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