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Step-down voltage without transformer?

Discussion in 'General Electronics Chat' started by ym2k, Sep 22, 2004.

  1. ym2k

    ym2k New Member

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    Hi, just as the topic said, is there a way to step down voltage without using transformer? I came across some products that did 230V AC to 24~5V DC using only 2 capcitor and a rectifer. Maybe be i am wrong about the components, but at lease it looks like those components. Any suggession if i want to make one myself?
     
  2. JimB

    JimB Super Moderator Most Helpful Member

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    The best suggestion is DON'T.

    Something which converts 230volt mains to 5/24volts using "capacitors and a diode" will at best:

    Be very inefficient

    Run very hot if supplying more than a small amount of power

    Downright DANGEROUS, there will be no isolation from the 230volt mains.


    Transformers are big, heavy and expensive but they are used for a very good reason - safety.

    Jim GM3ZMA
     
  3. Optikon

    Optikon New Member

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    Next best thing to no transformer is a small transformer.
    Consider rectifying the 230V and then design a step down converter from that high DC voltage to 5/24 or whatever you like.

    PROs: Converter can run at may kHz or even MHz making this part both efficent and small (as can be for load requirements that is..) and you maintain the safety isolation from mains.

    CONs: More parts, maybe not more expensive (the big transformer that you got rid of is expensive!) more complicated design. And it will have more noise (from switching) on outputs but maybe you do not care.

    There are refernce designs of offline buck converters abound and some of them are what you are after (no large mains transformer)

    just a thought..
     
  4. dave

    Dave New Member

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  5. Sebi

    Sebi New Member

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    The trafoless power supply not always dangerous.If You need some small power (10..50mA) easy to build with a serial 100..470nF X2 type capacitor and a rectifier+stabilizer. I've built many small circuits for light control/switch with infrared, so nobody never touch it. Other cases maybe the safety functions expensiver then trafo...
     
  6. John Sorensen

    John Sorensen New Member

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    It's always dangerous, even if no one gets hurt.

    Best for safety = wall wart (cheap, too).

    j.
     
  7. ym2k

    ym2k New Member

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    Hm.. thanks! maybe i can try this out, but can i have a clearer information of how to do it?
     
  8. Sebi

    Sebi New Member

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  9. MALLESH

    MALLESH New Member

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    i need circuit daigram of step down rectifier without using transformer, i hav e made a circuit by using a 4 diodes(as bridge ) and capacitor s and resistors.
    please reply me soon is it possible to make rectifier with out using transformer.
     
  10. Hero999

    Hero999 Banned

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    [​IMG]
     
  11. dknguyen

    dknguyen Well-Known Member

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    If you don't know how to build what is essentially a 2 component circuit, you're going to kill yourself since there is no transformer for isolation.

    Also, a rectifier only changes AC-DC. There are a few ways to step down the voltage:

    -resisitive divider: no no for powering stuff since resistance sits in between your source and load. The more current you draw, the more heat is dissipated through the load and the more current gets shunted straight to ground
    -linear clamping of the voltage (linear regulator, zener diode)- clamps the voltage. Completely unreasonable when you want to step 180V down to 5V since all that extra voltage is dissipated as heat.
    -switching with transistors- complicated. difficult.
    -transformers- why wouldn't you just use these and then rectify the low voltage AC signal?

    WHy wouldn't you want to use a transformer? Honestly, I'm not sure how they stepped down the voltage with just resistors and capacitors. You'd need a transformer or switching. Everything else is unreasonable since they just dissipate too much energy.
     
    Last edited: Mar 15, 2007
  12. CheapSlider

    CheapSlider Member

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    I very strongly advise against experimenting with mains electricity! -Its definitely not something for a beginner. Any design work should only be done in a properly kitted out workshop which has isolated supplies and is fitted with appropriate safety devices. You should never do any experimenting alone - always have someone present who knows what to do in case of accident.

    These type of circuits are often used in light fittings which have a built in photocell or PIR sensor.
    Its definitely NOT suitable as a general purpose power supply. Its far too dangerous especially on 230V mains.

    For information only, have a look at sheet 4 of the data file attached in the thread "Energy Meter Design". (Most applications don't bother using a regulator chip).



    (I've deliberately not given any details or hints on component values)
     
    Last edited: Mar 15, 2007
  13. danishmehran

    danishmehran New Member

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    supply without step down transformer

    dear it is very easy , i have drawing latter i will send you
     
  14. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    As others have said, there are some safety issues that come up with using a
    transformerless power supply. If you can get hold of a small wall wart as one
    poster pointed out that would definitely be better if you dont have too much
    experience with electrical circuits. The wall wart has the added benefit in
    that it will be isolated from the line too, so you dont get a shock.

    It's also true that the single capacitor (or two cap for the filtered version)
    transformerless solution works, and it works well and for a long time, but
    it has to be designed with a lot more care, and it has to be designed with the
    load type in mind. If something isnt sized right it could burn up or even blow
    up violently sending hot parts flying into the eyes and what have you.
    At 120vac this is a problem and dangerous, but at 230vac it's about four times
    more dangerous because of the increase in energy vs time available.
    The first rule therefore is to make sure it is designed right so that it is safe and
    can run for a long time without overheating.

    There are a few things that have to be addressed with these offline power
    supplies too, and one of them is surge current. A resistor of the correct size
    has to be in series with the cap so that the surge doesnt blow anything up.
    A small fuse should also be used to protect against a common cap failure mode
    where it shorts out. Without the fuse the resistor would overheat and cause a
    fire.

    A practical unit i had designed several years ago was used to find out how practical
    this kind of power supply would be in running a small while LED as a night light.
    It ran just fine for a year or two, after which i started buying commercial units
    that were packaged nicely and had light sensors to turn off in the day time.
    The advantage in this kind of application over a single resistor is of course power
    dissipation. The single resistor dissipates lots of power (wasted) while the
    series cap solution uses about a half watt! Thus, it doesnt get hot at all,
    not even warm. As mentioned before though, these power circuits have to be
    designed just right or very serious safety issues come up.

    In short, if you dont have a lot of experience with electrical circuits you should
    go with a wall wart.
     
  15. dox2

    dox2 New Member

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    I want to make a rectifier circuit to convert 220 VAC to 220 VDC. I have tried using a diode bridge 600V/35A and filter capacitor 220uF/450V but output voltage is 300VDC. is there any way to step down the voltage 300VDC to 220VDC?? Thanks
     
  16. dox2

    dox2 New Member

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    I want to make a rectifier circuit to convert 220 VAC to 220 VDC. I have tried using a diode bridge 600V/35A and filter capacitor 220uF/450V but output voltage is 300VDC. is there any way to step down the voltage 300VDC to 220VDC?? Thanks
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,


    How much current do you need from this supply output?
     
  18. Gary B

    Gary B New Member

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    What the original circuit was doing was to use a series capacitive reactance instead of a resister to drop the voltage. The circuit can be balanced by using two caps with one in each leg and a diode forming a half wave rectifier. Otherwise, the second cap is a filter. The voltage drop will, of course be primarily affected by the total impedance of the load so you can’t design such a circuit without knowing the exact details of the connected load but, one thing I can say is that it is going to be a very high impedance load or the caps are going to be very large.
     
  19. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    The reason you are seeing about 300 VDC is because your capacitor is charging to Epk so 1.414 X 220 = 311. Once under a load that will drop. If you are curious then remove the cap and just measure the rectified voltage.

    Additionally I strongly suggest you be extremely careful messing with mains power, especially cahrged capacitor(s) that will hold a harmful charge after power is removed!

    Ron
     
  20. Hero999

    Hero999 Banned

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    What do you need the 220VDC for?

    If it's just a DC motor, omit the capacitor, the RMS voltage will still be 220V and power the motor from that. The motor doesn't care if the DC is steady or not. The only potential problem with pulsed DC is vibration but if the motor is large enough, it shouldn't be a problem.
     
    Last edited: Apr 12, 2010
  21. dox2

    dox2 New Member

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    i have add a Resistor 18ohm/20W and the output voltage drop into 240VDC. I need 220 VDC / 4A as input of IGBT module and then oscillate it into 220 VAC/50Hz/1000W
     

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