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220V AC transformer

  1. Jul 13, 2011 #1
    Hello EE,

    I currently have a mini project that Studiot provided me with. Project is a kind of a multi-functional tester. But I want to add another feature to that tester. That tester has a 9V DC output, which gave me an idea.

    I always find myself searching for batteries and other power supplies. I have those DC adapters 24V 12V, but is there a way where I can build my own transformer, but with variable voltage outputs?

    Something like from 3 6 9 12 15 ... V(DC). Does anybody have schematics for this? What will I need to make this happen? I have some old DC adapters, old VCR , I even have old AC 220V to AC 24V...

    I will probably have to use a lot of Safety fuses and all.
    Last edited: Jul 13, 2011
  2. jcsd
  3. Jul 13, 2011 #2
    If your aim is to make variable Voltage power supply, then using ICs such as LM317 may be lot easier.
    However, If you can find the right core and then its not such a difficult thing to wind your own multi-tap transformer, especially when the power rating is moderate.
  4. Jul 13, 2011 #3
    Hmm I googled those terms that you mentioned, and I found some circuits. I find them very complicated. I will stick to building just tester thing. Thank you.
  5. Jul 13, 2011 #4
    Can't remember if your list included any power transistors but you could use one to build a simple step down from 12 volts to 9.
    (you need a power transistor for this)

    Alternatively 7809 fixed voltage regulators are common as muck and much cheaper. That would give you a four component step down box.

    The 2N39xx transistors you have would survive the project supply being upped to 12 volts.
  6. Jul 13, 2011 #5
    I see, I asked this mainly because I needed a power supply for that multi-tester anyway. I only learned power transistors in theory and I must admit I didn't understand them very well. Nor IGBTs. I will have to dig up some old DC adapters. Tomorrow I will be buying ceramic capacitors for that project along with some potentiometers. I will let you know at which stage I am.

    Thank you.
  7. Jul 13, 2011 #6
  8. Jul 13, 2011 #7
    Is this Zener diode? Or something more complex?
  9. Jul 13, 2011 #8
    The 78XX series are complex devices containing voltage reference, amplifier, power transistor etc.

    The provide a fixed positive output voltage (ie they are not variable without special extra circuitry).
    there is a whole family, the main ones are
    7805 = 5 volts
    7809 = 9 volts
    7812 = 12 volts
    7815 = 15 volts

    There are several encapsulations for different currents from 0.1 amps to 1 amp.

    There is a corresponding series of negative regs the 79xx series

    Google is your friend.
  10. Jul 13, 2011 #9


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    If you look up the LM317 suggested by I_am_learning, there's a reference schematic for an adjustable voltage power supply:
    http://www.national.com/mpf/LM/LM317.html [Broken]

    You can also use the LM1084 if you're looking for a beefier supply (5A output), however, you'll need to attach a correspondingly beefy heat sink to deal with all the heat (being linear regulators, the current input is basically the same as the current output--the voltage difference, I*Vdiff, is then all dissipated across the regulator). It's pricier, but the bigger TO-3 metal can package is probably worth it for either of these.

    Note that the difference between input and output voltages (the drop out voltage) is 1.2V on both (again, if i recall correctly).

    These are not merely Zeners (you can find simplified schematics online, IIRC) but help regulate the voltage output throughout its current range, and some also provide features like short-circuit cut-off and transient suppression.
    Last edited by a moderator: May 5, 2017
  11. Jul 13, 2011 #10
    Hey man, the OP is talking about adapting a simple wall wart to replace a PP3 battery.
  12. Jul 13, 2011 #11
    Thank you. I will sure check them out. I am going tomorrow to a local electronic store. Maybe they will have them.
  13. Jul 13, 2011 #12
    One more thing Studiot. Since this is still AC currents in the background we are talking about, will something bad happen if I accidentally short circuit that 9V DC voltage regulated source? Will the filter capacitor blow or something?
  14. Jul 13, 2011 #13
    Originally I was interned just to regulate a stable 9V, but the circuit you provided me with looks very simple. I might just add that to my project ! Thanks
    Last edited by a moderator: May 5, 2017
  15. Jul 13, 2011 #14
    These regulators have short, thermal and safe operating area (if you have come across this) protection.

    More modern ones have lower 'dropout' whcih means the difference between input and output.

    The 7809 will just work with q 12 volt supply, some will take a 24 volt input - ask your supplier there are many manufacturers of these.
  16. Jul 13, 2011 #15


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    More power! <Tim Taylor Grunt>

    What could possibly go wrong?
  17. Jul 13, 2011 #16
    hahahahahahahahah I literally CRIED ahahahahahhahahah
  18. Jul 13, 2011 #17
    I will ask them about it. Very looking forward to starting this. Thank you!!!!:wink:
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