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[noob] Lowering transistors fowarding voltage drop

Discussion in 'General Electronics Chat' started by laharl2k, Mar 14, 2011.

  1. laharl2k

    laharl2k Member

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    hi there, I'm biulding a fan controler (from the mobo pwm to control non-pwm fans)
    the thing is that the part that uses the 5v-0v input from the mobo's pwm to control the 12v-0V output with a pnp darlington (TIP105) but I've a problem with the voltage....
    i get only get about 10.5-11.3V from the 12V, and that translates to 3.54A instead of 4.15A (aka less speed, and less air flow)
    (12V, 5.4W X7)

    is there any way to lower the voltage drop ?

    [​IMG]
    (yes, it's a little ugly xD)

    PS: I'm learning slectronic all by mayself, using from google, to electronics for dummies xD, so i may have missed some really basic consepts that you get from school :p
     
  2. Diver300

    Diver300 Well-Known Member

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    One thing is that if you have the emitter of a PNP darlington connected to 12 V, you can't control it with a simple resistor from a 0 - 5 V signal. Either use an NPN transistor and switch the negative, or you need an additional transistor.

    Darlingtons always drop 0.7 V or more. That is because the base current for the larger transistor comes from its own collector, and the base-emitter voltage is always about 0.7 V. If the voltage were to drop below 0.7 V, there could be no base current, so the transistor turns off.

    If you need to reduce the voltage drop, either use an ordinary transistor and provide it with a larger base current, or use a MOSFET. MOSFETs need no gate current, but if you are using one for PWM, you need a quite a bit of current to charge and discharge the gate capacitance.
     
  3. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Being at work I can't see the linked to image (Websense prevails) but I can say that every time I have wanted to use the motherboard PWM to control a fan or dozen fans the solution was always with the use of a MOSFET. Additionally if I recall correctly the 12 volts was a constant with the low side of the fan(s) getting the PWM.

    Ron
     
    Last edited: Mar 15, 2011
  4. dave

    Dave New Member

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  5. laharl2k

    laharl2k Member

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    hi again, thanks for the reply. i've been doing some research with mosfets and yes they work like a charm.(will need a N-channel E-Type mosfet :p)
    talking about chargin and discharging, most of the mosfets i've seen that suppont 12A o more, need at least 20V at the gate, so i can either use a step up, or use a pnp darlintone with a 2222 as control and a very high current mosfet and hope that with 11V there will be enougth current for everything.....

    I'll see if i can find one with 30V 10A+ and 5 up to 10V for gate.
    Anyway I had it planed to use a step up to have 24V to be able to overvolt the fans, for some extra revolutions... (maybe some smoke too ^_^)


    EDIT: found this.....
    http://www.overclockersclub.com/reviews/sunbeamtech_rheosmart_6/2.htm

    but looks cheap and not optimally designed...
    that's something similar to what i want to build, add a step up to 24V (adjustable) and the pwm control for non-pwn fans.
    it looks like i'll have to use more mosfets in parallel than just a big one.
     
    Last edited: Mar 16, 2011
  6. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    I am assuming everyday home computer type fans here. How many did you plan to run? A typical 12 VDC home computer fan running at 12 volts has a current draw of about 200 mA or .2 amp. Even if we up that to 250 mA 1/4 amp with 12 volts and 10 amps you could run 40 of the things.

    Several years ago I was involved with a computer forum (actually a few of them) that did hardware reviews on everything imaginable. Nice source of free stuff till I goy burned out with it all. :)

    The gizmo you linked to looks like it uses PWM from the motherboard or just runs the fans on a pot. No real magic there.

    Attached is a circuit I played around with a few years ago that worked pretty well. Just a simple IFR 510 N channel MOSFET driven by a 2N2222. Using the IRF 510 MOSFET the thing could likely drive 20 fans.

    A few other MOSFETS you may want to look into are what is called Logic Level MOSFETs or MOSFETS requiring a low gate level for turn on. The IRL 3202 and the IRL 3302 may be of some help to you.

    As to the PWM thing. Just about any 12 volt brushless fan as those used in home computers are fine for PWM applications. Not sure whet you mean when you mention non-pwm?

    Ron
     

    Attached Files:

  7. laharl2k

    laharl2k Member

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    (****, 3rd time writing the reply x.x first one didnt appear after 3 days, and the 2nd time firefox froze)

    summarising 'cause it was a long post and now i'm lazy to write it up again.....
    non-pwm = 2wires, 12v and ground, and maybe the sensor, aka without the fourth wire (PWM signal)

    the standar fans are uselles to me, because i have my machine overclocked(X6 @3.5GHz, 4HHDs, HD 6950 that barely fit inside the case, etc) so it gets hot.
    the fans that i have now are some sunon maglev 3100RPM 0.45A, without pwm, nor sensor.... and i wanted to use some of those high current transistors to control them because runing always at maximum speed they get a little too much anoying ^^

    but there's also this fan that is a beast, 6400RPM, 3A. and i wanted to be able to use those too, so thats way i need it to be able to deliver that much current.
    http://db.sanyodenki.co.jp/product_db_e/coolingfan/dcfan/dc_fan_detail.php?master_id=1637

    I'm also thinking about making the step up go up to 30V cause i saw some fans taking 30V without burning....maybe not 30V for 24/7 but 20 could go without problem...
    making a cheap 7 bucks 1200 rpm at 12V go 2000 rpm at 20 looks like a good idea. and the 3100 go 5150 (+66%) even better.

    EDIT:
    thanks Ron, circuit helped me (who would have guesed that the load had to be in the drain side for it to work....) but the output signal is inverted...."NOT" function there :p
     
    Last edited: Mar 19, 2011
  8. ronv

    ronv Well-Known Member Most Helpful Member

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    Probably going to sound like my neighbors gas meter when he heats his pool in the winter. :)
     
  9. laharl2k

    laharl2k Member

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  10. ronv

    ronv Well-Known Member Most Helpful Member

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    Do you have what you need to build what you want or are you still thinking about a booster?
     
  11. laharl2k

    laharl2k Member

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    booster?...you mean the step up converter? (12v->30v?) sorry still lack a lot of technical languaje :(
    if it is that, then no, i still need to investigate about that and learn more about inductors and stuff to make that circuit....but it's not that necesary for now,

    i just finished investigating about the part to trasport the pwm signal to the 12V supply. and now it works.
    [​IMG]

    i still don't know way i get those pikes there after switching off, i mean how to filter them.....but they are beyong the 40┬Ás (25KHz signal) so thats more than a cycle so they won't appear (maybe a simulation error? :p)

    i used a pnp darlington to discharge the mosfet when the voltage is 0v using the mosfet remaining voltage, and the lamp is set to 96 watts just to be sure that the circuit works.
    There're some stuff still remaining to fix but i already have the concept.

    EDIT: i'll post my full circuit in just a moment....

    The PWM circuit and the power circuit
    [​IMG]

    still need to fix the 5V input current....will see it tomorrow.....nights....zzzz
     
    Last edited: Mar 19, 2011

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