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Using diodes to drop voltage

Discussion in 'Electrical Engineering' started by Skenny, Jun 9, 2006.

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  1. Skenny

    Skenny Guest

    I have a small computer speaker amp that uses a 9 volt wall wart rated at
    400MA.
    Since the voltage drop across a diode is approx. 0.6 volts, is it feasible
    to series connect 5 diodes (1 amp each) between the amp and a 12 volt
    computer power supply?
    Shouldnt that drop the voltage to 9 VDC?
    5 * 0.6= 3.0
    12 VDC - 3 VDC = 9 VDC.
    Anyone see any problems with doing it this way?
    Thanx..
    --Skenny
     
  2. Roby

    Roby Guest

    The string of diodes will reduce the voltage just as you describe. There
    is one difference in powering the amplifier this way: the 12 volt return
    is connected to the computer power common. With a wall transformer, the
    amplifier supply voltage is isolated from the computer supplies.

    That's *probably* not a problem and seems worth trying. Perhaps another
    reader has done it and will jump in here.

    I am often tempted to pitch out the multitude of wall warts nested behind
    my computer (speakers, modem, router, etc) and tap the computer supply or
    build a one-supply-for-all box.

    Roby
     
  3. Skenny

    Skenny Guest

    Thanks Roby,
    This is for an aracde cabinet that Im building, since the speakers and amp
    wont be coming into contact with any other parts, except the power supply, I
    dont think I will have any trouble with isolation.
    The idea about a common power supply for everything on a PC sounds like I
    good idea. Please keep us posted if you pursue this.
    --Ken
     
  4. Beachcomber

    Beachcomber Guest

    A better solution is to buy a 9V. regulator from some place like
    Jameco Electronics. This will take 12VDC in and give a constant 9VDC
    out no matter what the load, up to the capacity of the regulator.

    The problem with series diodes is keeping the voltage drop across each
    equal and balanced. Some power supplies do this with resistors.

    Beachcomber
     
  5. repatch

    repatch Guest

    The voltage drop of a forward biased diode is NOT constant.

    It has a non linear relationship to the current going through the diode.
    0.6V is a "rough" number based on a typical amount of current (usually in
    the 10-30mA range in my experience). The ACTUAL drop may be much more,
    especially when nearing the upper end of the current spec. I've seen drops
    of over 1V when dealing with currents of > 1A.

    So, with all that said, 0.6V is certainly a "low" estimate (when dealing
    with power), and as such is generally safe. Chances are you speakers will
    see less then 9V, that's usually OK, you'll just get less volume.

    If sound is important to you then this is a bad idea since you will be
    introducing distortion (the larger input signals will cause a larger dip
    in the power supply), but since the word computer is in there twice I
    doubt 100% audio is a concern.

    TTYL
     
  6. Skenny

    Skenny Guest

    Thanks everyone for your input.
    I didnt know that the voltage drop across a diode is linear to the current.
    So I agree, this may not work, since the current wont be constant (due to
    sounds changing), the supply voltage will not be constant, so I could see
    where distortion could result.
    Actually, I didnt give all the data either. The wall wart output is 9VAC.
    The power leads to the amp board go directly to a full wave rectifier (4
    small diodes on the board), so I figured 9VDC should be OK. I hooked it up
    to a 9v transistor type battery and it seems to work good, but I didnt crank
    up the volume much.
    I dont really want to use a zener or a regulator chip.
    Do you think it would matter much if I used the full 12 volts on the amp?
    (Might let out the smoke?)
     
  7. repatch

    repatch Guest

    It's not linear, in any way, it's actually closer to exponential.
    Well, this is an interesting question. In reality, the wall wart you have
    WILL measure probably more then 12V when unloaded. The 9V rating is at the
    rated current. OTOH the 12V rail in a computer usually is close to 12V. So
    you've got a situation where the electronics in the speaker can
    likely handle 12V+ at low currents, but at high currents expect to be fed
    closer to 9V.

    Therefore, when the speakers aren't producing any sound everything is
    fine, when the volume gets turned up things will start getting
    interesting. Chances are if you don't blast it (i.e. never let the
    speakers clip) you'll be fine, but it's also possible the amp will
    dissipate more power then it's designed for and burn out.

    Only way to tell would be to try it. Generally computer speakers are
    designed in such a way that they are usually OK with being driven by a
    higher voltage, but that's no guarantee.

    TTYL
     
  8. Beachcomber

    Beachcomber Guest

    Is there any particular reason you don't want to use an external 9 VDC
    regulator? It's cost is probably pennies and it will do the job.
    At 400 ma, you probably don't even need to worry about a heat sink.

    DC output wall wart transformers come in both regulated and
    non-regulated varieties. When you need a fairly precise output
    voltage or you don't know if putting too many volts will harm your
    device, its best to go with the regulated version.

    Beachcomber
     
  9. Skenny

    Skenny Guest

    I think using the regulator is what I will do.
    Thanks to everyone for the valuable input.
    It's good to have forums like this where you can get different approaches to
    a problem.
    BTW, Im not an engineer. I am an electrician, have been for nearly 30 years.
    Most experience has been industrial, so I have done some "fly by night"
    engineering too. LOL
    Currently I work in an aluminum rolling mill. We maintain several AC & DC
    drives, PLC's, HMI's, etc.
    Ok, Im off the soap box now, once again, thanks for the help!
     
  10. JohnR66

    JohnR66 Guest

    Yes it will work fine. The 12v computer supply is pretty well regulated, so
    you get pretty close. Small audio amp ICs that are likely used in you
    speaker don't need tight regulation anyway.
    John
     
  11. Skenny

    Skenny Guest

    Thanks.
    Im not sure I care for the voltage difference between speaker ground and
    computer audio out ground.
    Do you think it would be enough to hurt the computer?
    The speakers are cheap, I think I paid around 7 bucks for them at Big Lots,
    but the computer wasnt quite that cheap.

    "Alfredo E. Torrejon"
     
  12. ehsjr

    ehsjr Guest

    It will work fine.
    The ~.6 volt drop is at very low current (10 mA) for a
    1N4001 diode. At 40 mA that diode will drop ~.7, at 100
    mA it will drop ~.8, at 200 mA it will drop ~.85 and at
    400 mA it will drop ~.9 So guessing that your amp will
    draw an average of 50 mA, 5 diodes would give you a 3.5
    drop.

    Those cheap computer amplified speakers will work fine
    at anything from 9 down to 7 volts (and maybe even less).

    You could also use a LM7809 regulator.
    See the datasheet, page 6, for a 3 component circuit:
    http://www.ortodoxism.ro/datasheets/unisonic/LM7824.pdf
    It will give you 9 volts, regardless of what current
    your amplifier draws.

    Ed
     
  13. PanHandler

    PanHandler Guest

    Every electronic component I've ever owned RUNS on smoke. When the smoke
    comes out, it quits. The only variable is the the more expensive the
    equipment, the thicker and smellier the smoke.
     
  14. Skenny

    Skenny Guest

    Ive had a few cars that ran on smoke too, but they never seemed to run out,
    just kept on smoking.
     
  15. Any reason you can't just use a 3 terminal 9V regulator?
    An LM7809 or LM7909 (depending on the load current) should probably work
    just fine.
     
  16. Beachcomber

    Beachcomber Guest

    I think some are not familiar with "regulators" (through no fault of
    their own of course). It's sounds difficult to use something you've
    never used before. But an LM7809 or LM7909 is as simple as can be.

    Back in my high school days, I had a shop/electronics teacher who only
    taught tube theory because he didn't understand transistors.

    Beachcomber
     
  17. This is true. When the OP presented his problem, some try to educate him
    on the particulars of the Ebers-Moll model, which is a noble pursuit.
    But some people just need a practical solution. The existence of a
    device such as a 3 terminal regulator may be of much more value in the
    short term.
     
  18. Skenny

    Skenny Guest

    Thanks guys, but I do know about the small regulator chips, I have built
    several circuits with them.
    I just wanted to know how everyone else feels about the diode idea.
    Again, thanks..
     
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