No-name switchmode power supplies

Discussion in 'General Electronics Chat' started by danrulz98, Apr 24, 2013.

  1. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    Hello there!

    I'm just curious if anyone's had experiance with these little switchmode power supplies you can get on ebay for quite cheap.
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=400470668693

    Some of these things are quite powerful, if the specs are true.

    I'm wondering how safe these are, or if I should toss it before something more valuable blows up (like my eyes or my ears..).
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Never used them, but if its 12V you need , why don't you use an old ATX psu.
     
  3. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    Most of the time 20A (typical 12V rail in one of those) or the other multiple voltages an ATX power supply puts out aren't needed. So why not use something small ;)
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    At least check if they have all the needed approvals. I noticed that your unit had a CE marking. In theory it should then be OK to use in the CE region (Europa). And it should follow certain regulations regarding safety and EMC.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    I used some of these. They work fine. Some 12V 1A, and one larger 24V 6A.

    But it can vary from brand to brand, had a 5V transformer here, after a few days, the bridge rectifier exploded. Replaced. After some weeks, just when I looked at it, it made "pop", and gave up it's useful service life.

    What do you expect? Normally these transformers work fine.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    It look like designed for the industrial uses.
    Normally this kinds of products is ok, unless you got the bad luck.
     
  8. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    The one that actually came seems to be the same power supply but with a different lable, one that doesn't have a brand name or any approval stamps. Just "Model FDPS-24W" and the input/output ratings.

    It is. I was thinking I could get a wallwort and dismantle it to go into a box with a project, then I saw these and they seemed easier and smaller.
     
  9. ScottWang

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    Aug 23, 2012
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    I'm selling the power supply related products, all almost we design that by ourself, but sometimes the engineer will buy the industrial psu to put into the box as you did, buying different types for the customers needed.

    In the most of cases that the psu need to modify to suit the order, it will depends on the tech abilities of engineer.
     
  10. takao21203

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    Apr 28, 2012
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    Isn't difficult at all, almost all of them have a 10% adjuster.
    Can be changed in a matter of minutes to allow more than 10%.
     
  11. ScottWang

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    Aug 23, 2012
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    Yes, you are right.
    But sometimes it's not like that easy, it should be changed to a adjustable psu and the adjustment range is from zero to the Vmax and Imax of the type of psu, I don't know about the method, do you know that?

    Sometimes even need to reduce the ripple voltage and interference and noise.
     
  12. takao21203

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    Apr 28, 2012
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    Ferrite transformers are flexible a bit. But of course if you grossly change parameters you won't be able to use max. current.

    Also capacitors and all semi's need to withstand max. voltages + a good margin.

    Zero to U Max. is a rare case.

    Sometimes you may need a few volts off from available standard voltages:

    5V, 12V very easy to find
    24V also easy
    48V somehow easy
    36V not so common but also available

    If you need more exotic voltages, consider a MOSFET DC/DC, 85% are quite normal for these.

    It is true these boxed supplies have only 10% adjustment, but it is not a big deal to change the resistor divider. As I say, check the parameters carefully.

    I changed a 12V/35V module to produce upto 100V, changed the capacitor to 100uF only, and using an old ferrite from a broken electronic transformer as filter choke.

    Later I used the same module to produce 60V for 2x 20W power LEDs from 20V, the module in no way designed for that, but no problem at all.
     
  13. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    Well, luckily 12V is exactly what I need, although probably only around 500ma; so the 2A this one puts out is slightly overkill.

    Pretty nice that they can be modded so much though!
     
  14. ScottWang

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    Aug 23, 2012
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    Many psu can't really fully output to the Vmax or Imax for working normally, so if you buy that psu then you should measure the current and how much Imax does it have?
     
    Last edited: Apr 25, 2013
  15. ScottWang

    Moderator

    Aug 23, 2012
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    Great, you mean that you can modify by yourself, so the meaning is that you know the theory of the psu, even that you can design the switching-mode psu?

    The parts that I'm used are under the rating of voltage and current about 80% and some are less and less, as the LEDs are using <=80%, resistors are <=25% watts, the capacitors are more complicate, as 16V rating is using with power <=10V, 25V rating is using with power <=16V, 35V rating is using with power <=25V, 50V rating is using with power <=35V, 63V rating is using with power <=50V.

    I found many PC's mainboards that the capacitors 6v rating is using with +5V power, that's why the capacitors so easy to damaged, also the switching-mode psu was using the rating voltage close to the power supply of the circuit, so it's the similar that capacitors of PC power are easy to damaged.

    All our products can be output to the Vmax and Imax, it means that they can be output fully Wmax and working fine as normally usage, considering the problem of repairing, so our products won't sale to other countries within a several years.
     
  16. danrulz98

    Thread Starter New Member

    Sep 17, 2012
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    Where do you sell yours?
     
  17. ScottWang

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    Aug 23, 2012
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    Present only in Taiwan.
     
  18. takao21203

    Distinguished Member

    Apr 28, 2012
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    Yes I have built dc/dc circuits discretely and also with ICs.
    I have some experience and I understand the theory.

    Most of these PSUs use a high side MOSFET IC, some use a power transistor on the primary side.

    And a feedback loop with a resistor divider and an optocoupler.

    Most ferrite transformers don't take it crude to use them for different voltage, but of course, you might not get the same amount of Watts before they turn hot.

    When I made ferrite transformer myself, I did not count windings or calculate material, just added larger number of turns for the primary, and smaller number for secondary with thick wire.

    Isn't a big problem normally, only heat will detoriate these. Most can withstand 10% or 20% higher. They don't just explode at 36V with 35V rating.

    Normally, you should have 25% reserve for caps, and if possible 100% reserve for MOSFET or BJT voltage as well current.

    Maxing out MOSFETs is a high risk, they are prone to explode or latch when they are hot and worked near the margin.
     
  19. ScottWang

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    Aug 23, 2012
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    I'm not design the power for my company, it's just for myself, the powers that what I selling are need to pass through a very strict standard procedures, it's complete different compared with my personal applications.

    I'm also got some ideas from the engineer, it's not a secret for a old engineer as you, all about rating of parts and power supply.

    The current of bjts that I used to design are about 20~30% of rating, and I will try to keep it around 20%, when I give the advice for some other people, it will be never using over 33%, if run 20% is better, I got the Idea from the kits of power that I bought it over 25 years ago, the normal power bjt 2N3055, rating 15A but it just designed for 2A, it's really shock and confused me when I don't understand how to design a power.

    You mean that I have to using rating of 50% for Mosfet?

    Although it won't damage the mosfet when I use it, but I'm not sure what is the appropriate percentage of rating for mosfet as what I know with bjt, but i will figure out someday.
     
  20. takao21203

    Distinguished Member

    Apr 28, 2012
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    You dont have to but I dont like to go higher if not neccessary.

    Especially if there can be inrush current, or supply spikes, be careful.

    The cooling system may not respond immediately, it may only be available at reduced degree and things like that, parts may age and so on.
     
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