Author Topic: Improving small unregulated power supplies  (Read 5114 times)

0 Members and 1 Guest are viewing this topic.

Offline soldarTopic starter

  • Super Contributor
  • ***
  • Posts: 3595
  • Country: es
Re: Improving small unregulated power supplies
« Reply #25 on: October 15, 2019, 12:03:33 pm »
What about reliability?

I am not sure I understand. What about it? You think those regulators are not reliable?
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Improving small unregulated power supplies
« Reply #26 on: October 15, 2019, 12:31:05 pm »
What about reliability?

I am not sure I understand. What about it? You think those regulators are not reliable?
You previously said you had problems with switched mode power supplies being unreliable, so I'm surprised you want to add one to the output.

You might as well just use a reliable, decent quality, switched mode wall wart in the first place.
 

Offline soldarTopic starter

  • Super Contributor
  • ***
  • Posts: 3595
  • Country: es
Re: Improving small unregulated power supplies
« Reply #27 on: October 15, 2019, 01:43:42 pm »
The purpose of this is to use the unregulated PSUs I already have.

And I don't think I could find SMPS for the price of those regulators.

In the photo two Samsung PSUs which have failed within one year of each other. I keep saying I am going to look into them even if it is just to determine the cause of failure.


All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline soldarTopic starter

  • Super Contributor
  • ***
  • Posts: 3595
  • Country: es
Re: Improving small unregulated power supplies
« Reply #28 on: October 15, 2019, 01:51:44 pm »
Come to think about it the ideal regulator for an unregulated mains supply would be a buck-boost, SEPIC or Cuk which can output higher or lower than the input. That way there is no problem even if the input voltage sags a bit under the output voltage.

Otherwise I have to choose an output voltage which is lower than the input might sag.

E.T.A.: I had a look in Aliexpress and it is amusing how sellers throw in words just to attract views.

Buck boost buckboost stepup stepdown step up voltage converter module power supply ...

You have no idea what you might be buying, but, hey, for under a dollar what would expect?


« Last Edit: October 15, 2019, 02:12:51 pm by soldar »
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Gyro

  • Super Contributor
  • ***
  • Posts: 10174
  • Country: gb
Re: Improving small unregulated power supplies
« Reply #29 on: October 15, 2019, 03:22:58 pm »
Surely one of the two main reason for an unregulated brick is quietness (that and reliability). Once you've accepted the lower efficiency, aren't you most likely to want a nice quiet linear regulator?
Best Regards, Chris
 
The following users thanked this post: Ian.M

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Improving small unregulated power supplies
« Reply #30 on: October 16, 2019, 09:06:23 am »
Surely one of the two main reason for an unregulated brick is quietness (that and reliability). Once you've accepted the lower efficiency, aren't you most likely to want a nice quiet linear regulator?
Yes, it does seem daft to do that. You might as well go switched mode for the whole thing.

I haven't had any problems with noise from switched mode power supplies which weren't fixable, with a bit of filtering. I've just completed a project with an audio amplifier running off a switched mode power supply. It's for an intercom system that uses dynamic microphones. Firstly, the low level microphone pre-amplifier part was powered from a linear regulator, powered by the switched mode, which directly powered the power amplifier. The biggest problem was hum, due to leakage across the Y capacitor, which was solved by connecting both power supply rails to the chassis with 10µF capacitors and the squeaky noise was removed with a common mode choke and a big inductor in series with the power supply.
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13217
Re: Improving small unregulated power supplies
« Reply #31 on: October 16, 2019, 10:18:14 am »
OTOH if you need multiple rails for an item of testgear you are designing, and want it to be fully floating, high frequency DC-DC converters (for ease of filtering) running off a line frequency transformer wallwart may well be ideal if you need to minimize line frequency leakage current.
 
The following users thanked this post: soldar

Offline soldarTopic starter

  • Super Contributor
  • ***
  • Posts: 3595
  • Country: es
Re: Improving small unregulated power supplies
« Reply #32 on: October 16, 2019, 11:02:57 am »
Surely one of the two main reason for an unregulated brick is quietness (that and reliability). Once you've accepted the lower efficiency, aren't you most likely to want a nice quiet linear regulator?

Yes, I have done that in the past. a single transistor and a zener. The main issue is the headroom needed. But, yes, I have done it and will do it again as needed.

The thing with those switching regulators is (1) they are incredibly cheap, (2) more efficient and (3) very simple.

There is no way I can get SMPS for that price. And, at the risk of repeating myself, the purpose is to use the wall warts I already have.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Improving small unregulated power supplies
« Reply #33 on: October 16, 2019, 01:51:55 pm »
OTOH if you need multiple rails for an item of testgear you are designing, and want it to be fully floating, high frequency DC-DC converters (for ease of filtering) running off a line frequency transformer wallwart may well be ideal if you need to minimize line frequency leakage current.
But by the time you've done that, the money would have been better spent on a better switched mode power supply, with lower leakage current.
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13217
Re: Improving small unregulated power supplies
« Reply #34 on: October 16, 2019, 02:01:53 pm »
Availability!
You *TRY* finding a SMPSU that has a grounded inter-winding screen and absolutely no secondary side grounding at a half-way reasonable price.  Then see how much extra such a PSU with say three outputs is. e.g. +/-15V for your analog circuits and +5V for your logic.
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Improving small unregulated power supplies
« Reply #35 on: October 16, 2019, 03:37:29 pm »
Availability!
You *TRY* finding a SMPSU that has a grounded inter-winding screen and absolutely no secondary side grounding at a half-way reasonable price.  Then see how much extra such a PSU with say three outputs is. e.g. +/-15V for your analog circuits and +5V for your logic.
You're right that a wall wart with an AC output is the most convenient choice for +/-15V.

You don't need a grounded inter-winding screen, just a PSU with a decent Y capacitor between the secondary and earth, which is quite common with laptop power supplies.

Grounding the secondary is a good idea and doesn't cause any problems, as long as you don't connect it in series with another grounded PSU.
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13217
Re: Improving small unregulated power supplies
« Reply #36 on: October 16, 2019, 06:32:02 pm »
A 'Y' capacitor* from secondary to earth would be rather undesirable  for a floating item of test equipment.  When you initially make a connection from secondary side 0V to a ground referenced circuit, there will be a nice zap of current to charge/discharge that capacitor to the new potential.  Not exactly desirable for a supposedly floating instrument.

That's why I specified an inter-winding screen, as the only way to return the HF leakage current from the inter-winding capacitance between the secondary and the 'hot' (switched) end of the primary whence it came without compromises# is to intercept it with screening before it reaches the secondary.  Once you've got the screening, it might as well be grounded to minimize line frequency leakage current. Unfortunately inter-winding screens cost core window area so push up the size, weight and cost of the magnetics so are usually only seen in medical rated equipment, and purpose designed PSUs for precision equipment, where the extra cost is a minor factor.

* Is it a Y capacitor when it doesn't have to stand off line voltage, even if the part you use is rated as one?

# See above, & also, the leakage current causes bad things happen to sensitive devices under test if your suggested grounded secondary side, or grounded Y capacitor SMPSU ever looses its ground connection.


« Last Edit: October 16, 2019, 06:37:07 pm by Ian.M »
 

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 20363
  • Country: gb
  • 0999
Re: Improving small unregulated power supplies
« Reply #37 on: October 16, 2019, 10:02:08 pm »
A 'Y' capacitor* from secondary to earth would be rather undesirable  for a floating item of test equipment.  When you initially make a connection from secondary side 0V to a ground referenced circuit, there will be a nice zap of current to charge/discharge that capacitor to the new potential.  Not exactly desirable for a supposedly floating instrument.
The value of that capacitor is tiny (the order of a few nF) and there'll only be a zap if the capacitor is charged, otherwise nothing will happen. In normal conditions, the capacitor shouldn't be charged, as it's only coupling the secondary side to earth.

Quote
That's why I specified an inter-winding screen, as the only way to return the HF leakage current from the inter-winding capacitance between the secondary and the 'hot' (switched) end of the primary whence it came without compromises# is to intercept it with screening before it reaches the secondary.  Once you've got the screening, it might as well be grounded to minimize line frequency leakage current. Unfortunately inter-winding screens cost core window area so push up the size, weight and cost of the magnetics so are usually only seen in medical rated equipment, and purpose designed PSUs for precision equipment, where the extra cost is a minor factor.
Yes, that does sound expensive.

Quote
* Is it a Y capacitor when it doesn't have to stand off line voltage, even if the part you use is rated as one?
If it's just connecting the secondary to earth, then I suppose it doesn't have to be a safety rated capacitor, but the switched mode power supplies I've seen do use a Y capacitor. I suppose it's to ensure the secondary can float at high voltages, relative to earth, if necessary. I don't see why it's a problem. Any decent switched mode lab power supply will have a Y capacitor connecting the output to earth. It's never caused be any trouble and I've not heard any stories about it creating problems for anyone else.

Anyway, I forgot the original poster is only interested in this for hobby purposes, not a professionally designed project. To answer the question: yes, a switched mode converter can be added to the output of an unregulated wall wart.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf