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How to Step down from 12v - 9v?

Discussion in 'General Electronics Chat' started by vinvin, Sep 20, 2006.

  1. rob001

    rob001 New Member

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    This didn't work as expected. In the first test run, the fan burned up and the 15w resistor got very hot. In the next test, I achieved the ideal fan speed by connecting 3 of those resistors in series. I have no idea why it could be off by this much, but, the supply that I am tapping in to is 19V, 3.42A
    I used 15 ohm 15 watt resistors, three of them to get the fan speed I wanted. I think this means that the fan is operating on .42A, not far off from the manufacturer's specification. But using 3 resistors to achieve this result seems excessive, and they take up too much space.

    So, to specify some details I didn't mention before, I need to step 19V, 3.42A down to 8V, .38A (at or around)
     
    Last edited: Apr 22, 2010
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You used 45 ohms in series with the fan that you wanted to work with 8V.
    Your supply is 19V and the fan is 8V so the resistors have 11V across them then the current is 11V/45 ohms= 0.244A, not anywhere near what the fan is spec'd at.

    Why did you say the fan is using 0.42A? Then the 45 ohm resistors will have a voltage across them of (0.42A x 45 ohms)= 18.9V. Then the fan is almost shorted.
     
  3. rob001

    rob001 New Member

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    That's what the label says on the fans. Most of them say .38 to .42A. So what would be a better way to step 19V 3.42A down to 8 or 9V and about half an amp?
     
    Last edited: Apr 22, 2010
  4. dave

    Dave New Member

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  5. Thunderchild

    Thunderchild New Member

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    use PWM control or a SMPS
     
  6. Birdman Adam

    Birdman Adam New Member

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    And please don't just tap into power at a random area! I'm unaware of any power connectors that output 19V! Why don't you just use the 5V directly from a Molex connector? This is inside a computer right?
     
    Last edited: Apr 22, 2010
  7. rob001

    rob001 New Member

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    If you haven't realized that I'm not talking about a desktop, please don't give me any advice. Audio guru's advice has been factual and he is clearly competent enough to be giving this kind of advice. No molex connectors here. I promise.

    19V 3.4A needs to be stepped down to 8 V, .5A. How about a voltage regulator?

    I found this one:

    http://www.westfloridacomponents.co...+1/2A+.5A+500mA+5V+Voltage+Regulator.html
     
    Last edited: Apr 22, 2010
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The voltage regulator will need a heatsink.
     
  9. rob001

    rob001 New Member

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    I can do that. It looks like that particular voltage regulator can support up to 19V input. Does that look like a good one for my application? I can pick some up in person tomorrow morning and try this!
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 78M05 limits the current to around 500mA but it might not be available. An ordinary common 7805 limits the current at around 1A and is available everywhere.
     
  11. fatalhud

    fatalhud New Member

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    Hi all
    Another Numpty Here
    I have been trying to drop the voltage from a 12 volt battery to 9 volts
    I have built this circuit I found on a link from this site

    http://www.electro-tech-online.com/attachments/12v-to-9v-dc-gif.9585/

    when connected to the 12 volt battery I am getting 9 volts out OK
    The problem is when I connect the 9 volt side to the device I want to power it does not work
    The device is a camera trap which uses an open source microprocessor "Arduino Duemilanove" which also has a 4 x 20 line LCD display
    I have measured the current drawn by the box at 0.2Amp
    I used all the components, listed in the circuit except the NTE184 was replaced with a STMICROELECTRONICS - 2N5192

    STMICROELECTRONICS|2N5192|TRANSISTOR, NPN, TO-126 | Farnell United Kingdom

    and R1 I bought a 560ohm 5watt resistor

    The LED indicator on the microprocessor does light up, but it seams to flicker and does not initialise
    The camera box does work of a normal 9 volt supply
    Does anyone have any ideas what I may have done wrong

    Here is a link to the camera controller
    DIY Camera Controller – Part 1 Louises Place

    Alan H
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The value of R1 is marginal to supply only the zener diode but does not supply enough current for the transistor to work.
    For an output of 200mA the transistor needs a base current of at least 4mA. If the value of R1 is reduced to 330 ohms or less then the output transistor will work but the zener diode might get too hot when there is no load.

    EDIT:
    R1 is 560 ohms and has 3V across it. Then it dissipates only (3 squared)/560= 0.016W! Why did you buy a huge 5W resistor?
     
    Last edited: May 12, 2010
  13. fatalhud

    fatalhud New Member

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    Thanks for the reply
    Because I'm not that clever at Electronics and thought I was playing safe :(
    Would you recommend I try a 330 ohm in this circuit or is there an easier solution to drop the voltage

    Alan H
     
  14. Hero999

    Hero999 Banned

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    Why not use the LM317, LM7809 or 7808 (8V but more common than the '09 and should still power your circuit)?
     
  15. belpha

    belpha New Member

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    Hi there!

    I have a power supply taken from an old compact pc. It's 12 V - 2.5 A output.
    I read the previous posts, so I tried to calculate the value of the "simple-resistor" version.

    Could you confirm that I'm right?

    from 12 V - 2.5A to 9 V - 2.5 A
    R = (12 - 9) / 2.5 = 1.2 ohm
    W = (12 - 9)^2 / 1.2 = 7.5 W

    So for easly convert the the voltage I need a 1.2 Ω 10W resistor?

    Thanks and sorry for my bad English.
     
  16. Hero999

    Hero999 Banned

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    No it won't work, 2.5A is the maximum current capacity of the power supply.

    The value of the resistor and indeed whether you can just use a resistor or not will depend on what you have connected to the output of the power supply and is nothing to do with the current rating of the power supply.
     
  17. belpha

    belpha New Member

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    I understand.
    I want to use it for power guitar effects, but they could be 2 or 10, depends time by time.

    So if I want a "variable-current-load" 9 V power supply I need to use an IC.
    Maybe two 7809 (max output current 1.5 A) in parallel for use all the initial 2.5 A?
     
    Last edited: May 21, 2010
  18. Hero999

    Hero999 Banned

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    It depends on how much current the guitar effects pedal needs?

    I think a single LM7809 will be fine.
     
  19. belpha

    belpha New Member

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    About 300 mA each pedal...
     
  20. Hero999

    Hero999 Banned

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    Then your adaptor can't supply enough power for 10 pedals.

    The current capacity of the LM7809 can be boosted by adding another transistor but the maximum current will still be 2.5A because it's the maximum current your PSU can supply.
    [​IMG]
    Increasing Regulator Current

    In reality you'll have to measure the current because I doubt each pedal will consume 300mA continuously.
     
  21. belpha

    belpha New Member

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    "10" was just an example for say that the number is not always the same.
    I have pedals that require 200/250/300/350 mA.

    I know that it can't ask more of 2.5 A, but I want the possibility to use all of them.
    This is why I thought about two 7809 - 1.5A, but with your suggestion of boost the power it should work as well.

    I don't know if a pedal need 300 mA continuously, how can I test it?
     

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