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Help: Basic Full-Wave Rectification

Discussion in 'Electronic Basics' started by Ravendug, Dec 16, 2003.

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  1. Ravendug

    Ravendug Guest

    Hi, I am hoping someone here could help me with what I believe would be a
    simple problem
    for someone that knows what they are doing (not me), or even with a fairly
    basic knowledge of electronics.

    I have a 24V DC (6.7W) fan that I would like to power with a transformer
    that is rated as:
    Prim: 230V 50Hz
    Sec: 24V 0.85A

    The problem is I have no idea on how to go about rectifying the AC output
    from the transformer to be suitable for the DC fan.
    I am a 2nd yr Computer Engineering Student but seem to know very little of
    building any practical electronic circuits :(.
    I know some theory about bridge rectifiers, centre-tap transformers, ripple,
    etc not much else on this subject. Pathetic I

    I would appreciate any help on finding the correct components and building
    the circuit. I am located in the UK so it would be useful to have references
    on components from somebody like Maplin ( or
    RS-Components (

    Any other references or sources of information would be very welcome as
    If I have left any other required/useful bits of information please ask and
    I will try my best to provide
    a suitable answer.

    Your help will be greatly appreciated.

  2. "Ravendug
    A 4 diode bridge rectifier between the secondary and motor should work
    pretty well. No need for a filter capacitor.
  3. Looks like a common 'feature' of nowadays' universities. I'm a Computer
    Science student at the Technical University of Berlin and also in the 2nd
    yr. Taking a look at my class mates tells me, you're not the only one to
    whom the practical part of EE is something rather EErie. So welcome at
    S.E.B, you're right here.

    You will need a classic-style full-bridge rectifier (4 diodes) and a
    sufficiently large capacitor. See ASCII schematic below.

    .---|>|---x-----x------> + out
    | + - | |
    ------->|<-------x----|>|---' __|__ +
    mains <|> 24VAC || _____
    ------->|<--------x---|<|---. | -
    | - + | |
    '----|<|---x-----x------> - out

    The + and - out connectors is where the motor is to be connected. The cap
    should have a rated voltage of 50V and a capacity of (at least) 1000uF. Note
    that the output voltage will be higher than 24V. Under no load it will
    probably be over 35V (the transformer is rated to provide 24V when loaded,
    under no load the voltage increases and the rectifier will further multiply
    it by sqrt(2)). Taking into account that the motor will draw a about a third
    of the transformer's rated current and the transformer's resistance is
    sufficiently far from zero, I would estimate the output voltage somewhere a
    little above 30V. Make sure your motor can handle this or use a regulator or
    a series zener to cut off the excess voltage. In this case, make sure the
    regulator (or zener) does not overheat. P=U*I will reveal the power the
    regulator will have to dissipate. Of course, it will not account for the
    power dissipated at poweron as the motor is spinning up (changes U and I),
    but since the motor is rather small and will probably spin up quickly,
    there's no real use in calculating this power either. Do not however leave
    out the capacitor. The motor may be an electronic one requiring a constant
    voltage. Such a thingy will probably not work off unfiltered DC.

  4. Ross Mac

    Ross Mac Guest

    Your circuit will certainly do the job but....
    I have had some pretty good luck with fans, motors and solenoids on
    unfiltered DC....It would be worth a try...If it hums or overheats...filter
  5. Ravendug

    Ravendug Guest

    Thanks greatly for your help, I'll give it a go.
  6. Ravendug

    Ravendug Guest

    Another question for further clarification.
    Which diodes would be suitable/ (How would I determine if is suitable?)

    i.e. Would something like the 1N4001 be ok?
  7. Yes
  8. N. Thornton

    N. Thornton Guest

    dont, youll fry the fan. Your 24v transformer will give you about 35v
    on load, and even higher with a load less than its max rating. This is
    much too high for a 24v fan.

    Try a 16v transformer for about 24v dc.

    Regards, NT
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