Electronics > Beginners
Improving small unregulated power supplies
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james_s:
Most of the small consumer grade switching wall warts are not what id call "properly designed", they work ok but they rarely last me more than a couple years in 24/7 service and they tend to be RF bombs. Yes some are ok, but far from all of them.

I have had probably a dozen of them fail over the last decade compared to zero failures of the iron transformer type in my lifetime. These days I save all the transformer ones because they're hard to get anymore.
soldar:

--- Quote from: Zero999 on September 23, 2019, 08:30:02 pm --- A properly designed switched mode power supply can be more reliable than the old transformer type, because it won't run as hot. It will also have over current protection/limiting, so won't overheat, like the old transformer types do resulting in the single shot thermal fuse failing.
--- End quote ---

I disagree. The old type are simpler and more rugged. I have linear power supplies that have been going for decades. It seems your experience is unusual.

I have no doubt SMPS can be built to last but the small ones are not built like that, they are built to minimize size and budget. The public prefers a tiny charger even though unreliable to a bulkier but reliable one.


--- Quote from: Zero999 on September 23, 2019, 08:30:02 pm --- That sounds very strange. I thought all modern phones just worked at 5V?
--- End quote ---

Um, no. https://www.digitaltrends.com/mobile/how-does-fast-charging-work/

Now, build a complex SMPS and make it this size and it is a recipe for unreliability.


--- Quote from: Zero999 on September 23, 2019, 08:30:02 pm --- Just stick to decent branded USB chargers and you should be fine.
--- End quote ---

The ones that failed were original Samsung and Apple. Again, look at the design and size of the thing.

Your experience that SMPS warts are more reliable seems to be contrary to everybody elses's, inclusing mine.
Zero999:

--- Quote from: Gyro on September 23, 2019, 09:32:25 pm ---
--- Quote from: Zero999 on September 23, 2019, 08:30:02 pm ---One issue I've found is the smoothing capacitor is almost always far too small. It's often something stupid like 2200μF in for a 1A unit, which will result in a huge ripple. I slap in a 4700μF capacitor before the regulator in anything run from one of those old transformer PSUs and the capacitor should be rated to 25V, for reliability's sake.

--- End quote ---

Oh yes, I place a premium on the ones that have case fixing screws rather than being ultrasonically welded shut. Quite a surprising number (of UK ones) do. Always worth checking when scanning ebay.

--- End quote ---
I'm talking about the unregulated type. No need to open it up, just put the capacitor on the PCB of the device you're designing.


--- Quote from: soldar on September 23, 2019, 10:44:36 pm ---Your experience that SMPS warts are more reliable seems to be contrary to everybody elses's, inclusing mine.
--- End quote ---
I'd hardly call a thread with a few people commenting concrete proof either type of PSU being more reliable. It's anecdotal evidence which means nothing. To prove one way or another, we need hard, reliable, mean time before failure data.


--- Quote ---The public prefers a tiny charger even though unreliable to a bulkier but reliable one.
--- End quote ---
It's probably more to do with energy saving legislation. The old transformer PSUs have too higher idle current to meet the current EU regulations, hence why switched mode power supply power supplies are now the norm.

--- Quote ---The ones that failed were original Samsung and Apple. Again, look at the design and size of the thing.
--- End quote ---
I've no experience with Samsung, but with Apple it tends to be the cable which fails, nothing to do with the actual power supply; I've had that happen to me several times.

--- Quote ---Your experience that SMPS warts are more reliable seems to be contrary to everybody elses's, inclusing mine.

--- End quote ---
No, I didn't say that. I've found modern switched mode power supplies to be no better, nor no worse, than the old transformer type. In my experience they are equal, reliability wise.
soldar:

--- Quote from: Zero999 on September 24, 2019, 11:26:14 am --- I'd hardly call a thread with a few people commenting concrete proof either type of PSU being more reliable. It's anecdotal evidence which means nothing. To prove one way or another, we need hard, reliable, mean time before failure data.
--- End quote ---

Right now I got in my hand something hard, reliable and mean telling me all I need to know.


--- Quote from: Zero999 on September 24, 2019, 11:26:14 am ---

--- Quote ---The public prefers a tiny charger even though unreliable to a bulkier but reliable one.
--- End quote ---
It's probably more to do with energy saving legislation. The old transformer PSUs have too higher idle current to meet the current EU regulations, hence why switched mode power supply power supplies are now the norm.
--- End quote ---

I was not comparing SMPS with old style. What I meant is that I have no doubt they could design and build a SMPS for a phone which would last for many years. What is apparent is they cannot do that and make it fit in a one inch cube and between reliability and miniaturization they choose the one that sells.



--- Quote from: Zero999 on September 24, 2019, 11:26:14 am --- I've no experience with Samsung, but with Apple it tends to be the cable which fails, nothing to do with the actual power supply; I've had that happen to me several times.
--- End quote ---

Nope, I have fixed many cables but I have a few "one inch cube" SMPS which failed. I don't know why I keep them because I am pretty sure I will never attempt to repair them. 
james_s:
Efficiency regulations is the reason the iron transformers have gone away. The small iron wall warts are not very efficient, only about 50% IIRC, they are "impedance protected" by having thin wire windings that cannot supply enough energy to catch fire if there is a short but this also leads to high losses and significant sag under load.
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