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Improving small unregulated power supplies

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soldar:
I have crates full of these old wall warts and similar PSUs. While I have many SMPS fail the old transformer-diode bridge - capacitor type seem to last forever.

The main objection is that they are unregulated and they are a pain to match to the load. If the load is not stable the voltage can be all over the place.

I often end up using a PSU with a lower nominal voltage because at the current I need the voltage it outputs is higher than the nominal voltage. Testing each time I need one is a pain.

Series regulation is not easy or convenient because there is really not enough headroom.

I have used in the past parallel regulation with a zener diode or zener plus transistor. It is a waste of energy but the amount is very small and the addition can be placed in the output cable or receptor without needing to open the enclosure which sometimes cannot be done without breaking it and there is no space anyway.

I am playing with a small transformer which I guess we could call "12 V , 200 mA". The following table gives the voltages at different currents.


--- Code: ---mA V
0 19.4
10 18.9
15 18.6
20 18.4
30 17.9
50 17.0
75 16.0
100 15.0
125 14.1
150 13.3
175 12.6
200 11.9
225 11.3
250 10.8

--- End code ---

So, let's say I want to limit the voltage to 14 V max. I would need to sink 125 mA @ 14 V which is a bit under 2 W.  A zener and transistor can handle that easily.

This prevents up to 20 V building up in the capacitor and being dumped suddenly into whatever device is connected.

I'd like to hear thoughts and ideas on this topic.

BTW, I actually measured a number of points but then I put them into Excel and I obtained the following equation

V = 19.359 - 0.0492 I(mA) + 0.00006 I(mA)^2

So the table is an idealized table.

james_s:
I typically just build a switching regulator into the project and then use whatever old wall wart is reasonably suitable. My broadband router for example has a switching regulator inside it and originally came with a 12V switching type wall wart. I think I went through 2 or 3 of those before I replaced it with an old unregulated transformer type and it has been fine for years. They are not the most efficient things but they are much more reliable than the low cost switchmode ones.

Zero999:
I've not noticed a significant difference in reliability. I've hardly had any problem with switched mode wall warts failing and I've also had the old transformer type fail.

My advice would be to simply ditch the old transformer type of supply, especially in equipment which is left on all the time. Even if it is slightly less reliable, it will probably work out cheaper, due to the amount of power saved. If you keep having problems with switched mode power supplies failing, then it's a problem with high voltage spikes.

Ian.M:
I really wouldn't advise shunt regulation - if the mains supply voltage rises, something is likely to burn up and if the PSU doesn't have a thermal fuse in series with its primary, it could literally catch fire.
Needing a few volts extra headroom is the cost of doing business if you need to stabilise the output from a small unregulated linear PSU.


That nominal 12V unregulated PSU would make a decent feed for a 9V linear regulator.  At its nominal max load current you've got 2.9V headroom, and should still have 1.7V headroom if the mains supply is 10% low. 

soldar:

--- Quote from: Zero999 on September 23, 2019, 03:09:53 pm --- I've not noticed a significant difference in reliability. I've hardly had any problem with switched mode wall warts failing and I've also had the old transformer type fail.
--- End quote ---


My experience is quite different and it makes sense. SMPS are much more complex and the components are much more stressed.

Just a few days ago my wife's Samsung's phone charger stopped working. It was the second one to stop working. They are tiny and have some complex charging system with two voltages. Since she spends a lot of time in the car she's just decided to charge it while she's driving and not buy a new charger for the time being.  Maybe a non-Samsung, non-fancy fast charging, just regular more bulky USB would last longer. She had the same experience with her earlier tiny Apple chargers. Did not last.  I have them in a box with the idea of looking into repairing them but they are tiny and it's not worth it.

I wish they would make things more substantial and not try to miniaturize everything to the point where it has to fail. One of these small SMPS should be eternal.

About a year ago I was woken up in the middle of the night by a small explosion in my room.  A SMPS wall wart had exploded. No fire, nothing. Just a small explosion and the thing was dead.

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