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Two regulators from one transformer

Discussion in 'General Electronics Chat' started by Andrew Borg, Aug 1, 2016.

  1. Andrew Borg

    Andrew Borg Member

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    Yes I tried that but maybe on tablet not compatible.

    It's not the same but looks similar.

    It sounds very good when I tried it with batteries to mixer.
     
  2. spec

    spec Well-Known Member Most Helpful Member

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    Hy kinarfi,

    Don't get me wrong, SMPS are used with both audio circuits and RF circuits. In fact one of the best audio amplifiers uses off-line switch mode power supplies, but the design, filtering and layout are pretty complex. http://www.chordelectronics.co.uk/files/Hi-Fi Choice SPM1200MkII HFC378 Nov 2013.pdf

    By the way SMPS not only generate conducted interference but also radiated interference which couples to equipment by both inductive and capacitive coupling. There is a good book, 'EMC for Product Design' which covers the arena of ElectroMagnetic Compatibility (EMC) without getting too heavy. https://www.amazon.co.uk/EMC-Product-Designers-Tim-Williams/dp/0750681705

    It is true that USB power is universally generated by SMPs but the designers of mobile phones, PCs, etc have many resources to design out any problems. Also USB supplies are not up to HiFi standards. Compared to HiFi, the sound on my Android phone is full of distortion and hash and the sound on my laptop is even worse.

    Audio, as you know, is generally taken to span the frequency range, 20Hz to 20Khz and RF covers the frequency range, 3KHz (VLF) to say 1TeraHz (microwave). Just in case you mistake 3Khz for an audio signal, remember that radio waves are electromagnetic and audio waves are compression and expansion of air.

    You make a very good point that if the switching frequency of a SMPS is higher than the highest audio frequency any interference would not be audible, but the fact that the switching signal can get into the circuitry is not good and can cause problems.

    Very often a high frequency signal will get rectified by the input semiconductors of an amplifier and produce an audio signal and a DC component, both of which can cause problems. For this reason audio amps often have a low pass filter on the input to prevent high frequencies from getting into the amplifier. These high frequency signals get picked up by the leads connecting to the amplifier for example. One of my home made audio amplifiers was an excellent broadband radio receiver.:eek:

    With RF circuits the situation is much more complex and difficult to sort.

    spec

    PS: another link which may be of interest: http://electronics.stackexchange.co...it-be-powered-by-a-switched-mode-power-supply
     
    Last edited: Aug 4, 2016
    • Thanks Thanks x 1
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    True amplifier power determines the maximum loudness, not the sound quality. I modified my clock radio so its output is 1W. It sounds great on the vented speaker I made with a 6.5" woofer and a dome tweeter.
     
  4. dave

    Dave New Member

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  5. Andrew Borg

    Andrew Borg Member

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    Hi Audioguru you're right but quality plays a good role too. I have an old amplifier Nad to my stereo and was expensive because it has very low distortion hum.
     
  6. kinarfi

    kinarfi Well-Known Member

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    Thank you
    Jeff
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    My pleasure. :)

    spec
     
  8. Andrew Borg

    Andrew Borg Member

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    Hi Spec I am building this. Those 330nf if polytene caps ok? Cause I don't have ceramic.
    Thanks for your help.
     
  9. spec

    spec Well-Known Member Most Helpful Member

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    Yes, any metal film capacitors should be OK too.

    spec
     
  10. Andrew Borg

    Andrew Borg Member

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    Today I finished the circuit +9v/-9v were ok but on the 12v side only 4.6v is coming.
    Is this maybe cause I used a 1000uf cap instead 4700uf?
    Any help please?
    Thanks
     
  11. spec

    spec Well-Known Member Most Helpful Member

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    Is that with the amplifier connected to the 12V supply?

    Does the 12V regulator produce 12V off load?

    spec
     
  12. Andrew Borg

    Andrew Borg Member

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    There is something in the transformer cause when I tested voltage only 2v was coming from red to blue wire and from 9v the center wire.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    Time for a different transformer I think. Get a nice toroid.:)

    spec
     
  14. Andrew Borg

    Andrew Borg Member

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    To correct myself from the rectifier 2v are coming out cause when I tried on trafo I read 16v ac . To connect one leg to br should give full voltage too?
     
  15. spec

    spec Well-Known Member Most Helpful Member

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    Hy Andrew,

    Can you give an exact definition of your transformer with a picture so we can establish exactly what is going on. When you mention colors of leads it does not mean much to us. :)

    spec
     
  16. Andrew Borg

    Andrew Borg Member

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    Today I connected the circuit as it should be and at one time it worked ok but after a while the 1000uf cap got very hot and got swallen releasing a very nasty smell.
     
  17. kinarfi

    kinarfi Well-Known Member

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    did you happen to check the temperature of you lm78xx chips
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    You need a high ripple current reservoir capacitor. Also change the value of the 12V capacitor to at least 4,700 uF and at least 28V. This would be a good choice: http://www.digikey.co.uk/product-de...onics-cde/SLPX682M035A7P3/338-1645-ND/1882122

    spec
     
    Last edited: Aug 8, 2016
  19. Andrew Borg

    Andrew Borg Member

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    Hi kinarfi I did not notice that 78xx were getting hot.

    I changed the cap to a 50v one but when I switched on no voltage was coming to 12v now maybe 7812 burned. I try to change the chip and as you told me put a 4700uf instead of 1000uf.
    Thanks
     
  20. spec

    spec Well-Known Member Most Helpful Member

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    As well as the working voltage, the ripple current rating of the 12V reservoir capacitor is also very important . Special heavy duty (big and heavy) capacitors are made to handle the high ripple currents found in power supplies. The ripple current rating of the -9V reservoir capacitor is not a problem because the current drain on the -9V supply is very low.

    I suspect that there may be a thermal fuse built into your transformer and that the fuse is cutting in and out. Is the transformer getting hot?

    The 7812 will not get damaged by over-temperature because it has built-in temperature protection. It will merely limit its output current. I would get the power supply working without a load to start with.

    In the final power supply the 12V regulator will need to be mounted on a substantial heatsink using a TO220 mounting kit (but use an aluminum oxide insulating washer rather than mica or plastic) to keep the 7812 cooler. You do not necessarily need a dedicated heatsink: often the metal case of an equipment can be used.

    Also, a power resistor may need to be connected between the positive terminal of the 12V reservoir capacitor and the input to the 7812.


    spec
    [​IMG]
     
    Last edited: Aug 9, 2016
  21. Andrew Borg

    Andrew Borg Member

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    You mean If I don't put a power resistor the readings would be false? What resistance and power should I put? Thanks
     

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