12vDC fan from mains and other transformer question.

Discussion in 'The Projects Forum' started by Russell_AH, Mar 29, 2013.

  1. Russell_AH

    Thread Starter New Member

    Mar 29, 2013
    Hi everyone, I'd like your advice on powering a 12vDC computer fan from mains power (to act as a small fume extractor). I've had a quick look, but all the sources just attach the fan to a 12vDC power supply. However I'd like to accomplish this with bits I have lying around. My plan is to use 16V AC transformer (a Hornby C912 transformer), and I have a couple of questions.

    Firstly (more a general transformer question): The transformer I want to use is rated at 16V (13VA) output. Am I correct in assuming that 16V is the RMS value, so once the voltage is rectified using a diode bridge (1n4001, so assume a 2V drop across the bridge) and a filtering capacitor, the voltage will be approximately 20.5v ([16V x √2] -2V)?

    I've attached a diagram of my proposed circuit: it's the first time I've done something like this so it's a bit of a stab in the dark.

    Capacitor values:
    I usually see the 7812 used with C1=470μf and C2=0.1μf. I would guess that C1 smooths the rectified AC wave, so a bigger value is better (470μf seems to be a fairly common "large" value). C2 I assume filters the output of the regulator, and given the use of this circuit I would think it may be possible to omit it?

    The transistor is there because I'd like to have a switch in this circuit, but the only one I have is a tiny one scavenged from a solar light, so I'm not sure what current it can take (and it seems I should minimise the current flowing through components I will be routinely touching). The value of resistor would probably be around 1-10K, giving a current into the base of around 1-10mA to give a base-collector current of a few 100mA. The transistors I have are BC337, which can take 800mA, although can only dissipate 625mW, not sure if this may be a problem (I think I have a higher power one lying around, I'm not sure what its part number is though).

    Finally, a lot of the example circuits for the 7812 include a ground, but I'm not sure how I would incorporate that into this.

    So what are your thoughts? I'm I going about this in a reasonable way, and are my assumptions correct?
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    Welcome to the forum!

    Yes, your design looks OK. A rule of thumb for sizing filter capacitors on 60hz (or 50 hz) full wave rectifier circuits is 1000uf per amp of supplied current.
    You could add a 0.1 uf capacitor in parallel with the 470 uf cap to filter noise spikes.
    It would be good to add a fuse on the mains side of the transformer, and use proper safety when working with the high voltage mains. If you are unsure about the mains side of the wiring I would find someone familiar with it to help.

    What is the current rating of your motor? It may be that the switch can handle it without the extra transistor.
  3. vk6zgo

    Active Member

    Jul 21, 2012
    It's quite complex---the motor might be quite happy if you just use a halfwave rectifier straight off the transformer secondary.
    The 50/60Hz ripple may be a problem though.

    If all is OK,you may still have a few too many volts,so you could just use a series resistor to limit the current through the motor.
  4. Russell_AH

    Thread Starter New Member

    Mar 29, 2013
    Thanks for the replies.

    tubeguy: I wasn't going to touch the mains side, I was just going to connect the transformer via a barrel jack to the bridge.

    I'm not sure of the current rating, the fan was from Ebay a while ago. A quick search suggests these fans are rated for a few 100mA. I won't have the stuff with me for a couple of weeks to double check though.

    vk6zgo: I might give that a go too. I assume this wouldn't use the 7812?
    Also, I'm not too sure on how the current limiting works: the implication I got from the sites talking about a 12vDC supply was that the motor will only draw the current it needs (as long as the supply has a higher current rating than the device you're fine). I'm not sure if this will be the same with the rectifier and voltage regulator though.
  5. Metalmann

    Active Member

    Dec 8, 2012
    Why not use a 115V computer fan?

    I think it was DigiKey where I saw them.


    or, MCM.
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
    I see the transformer is a wall-wart, that simplifies the mains side.

    The calculation for ripple voltage is based on the filter capacitor size and the load current.

    Assuming 300ma load, 50hz
    For full-wave bridge it is:

    Vripple = I/2*f*C
    Vripple = 0.3/2*50*.000470
    Vripple = 6.3Vpp

    For half wave it is:
    Vripple = I/f*C
    Vripple = 12.77Vpp

    If the load is 300ma then the regulator will dissipate approximately;
    10*0.3 = 3 watts. Will need a heat sink.

    With a 12 volt regulator, ripple would be removed using a full-wave bridge, but not completely with a half-wave.

    Also, you will probably get a volt or so higher ac from the transformer if it's not fully loaded.
  7. vk6zgo

    Active Member

    Jul 21, 2012
    Yeah,no regulator with the halfwave rectifier.

    The motor will only draw the required current from a 12V supply including a voltage regulated one,because it is designed for that voltage.
    If you increase the voltage you can draw more current.

    The transformer is already 16V rating at its nominal current rating,it can be more at lesser loads.
    Your motor may draw more than its normal rated current with a higher voltage applied.

    A series resistor will limit the current through the motor & prevent you cooking it.

    A series resistor isn't as sophisticated as a full regulated supply--but it isn't doing a sophisticated job!