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Reduce voltage of DC adapter without reducing current

Discussion in 'Electronic Basics' started by suputnic, Jun 5, 2006.

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  1. suputnic

    suputnic Guest

    I have instant hot water heaters which take 2 X 1.5 batteries = 3V. I
    want to replace the battereies with DC adapters we have. The adapters
    have various voltages. I tried to drop the voltage by adding a resistor
    to the circuit, but then the current drops too low for the unit to fire
    up. Is there anything I can do? The measurements are:

    Batteries: 3.36V, 22.2mA

    DC Adapter, no resistor added: 5.9V, 22mA (Why does the current not
    increase with the increased voltage?)

    DC Adapter, resistor added: 5.9V, 15 mA and unit doesn't fire.

    One unit is running OK using the DC adapter without a resistor, but I
    think it would be risky to use the higher voltage adapters without
    reducing the output voltage to near 3V.
     
  2. John Fields

    John Fields Guest

    ---
    The heater may have an internal input voltage regulator.
    ---
    ---
    The resistor you added was too large in value.

    If you have a 5.9V supply and a load which wants to see 3.36V at
    22mA, then you need to select the [series] resistor to drop the
    difference between the supply voltage and the load voltage at the
    current the load needs to operate, like this:


    Vs - Vl 5.9V - 3.36V
    R = --------- = -------------- = 114.4 ohms
    Il 22.2mA

    110 ohms is a standard 5% value and since your heater seems able to
    stand an overvoltage, should work quite well. The resistor will
    dissipate:

    (Vs - Vl)²
    P = ------------ = 0.059 watts,
    R

    so a standard 110 ohm +/- 5%, 1/4 watt carbon film resistor would be
    fine.

    Another way to do it is to place a Zener diode in series between the
    supply and the load so that the Zener drops the voltage difference.

    What you do to select the Zener is to subtract the load voltage from
    the supply voltage and find a Zener closest to that voltage on the
    low side with a test current close to your load current
    requirements.

    For example, in the case you gave:

    Vz = Vs - Vl = 5.9V - 3.36V = 2.54V

    and Il = 0.02A

    So what you're looking for is a 2.5V Zener with a 20mA test current.

    Consulting:

    http://www.fairchildsemi.com/ds/1N/1N5222B.pdf

    the 1N5222B is a near-perfect fit, and you can also select the
    Zeners you need for the other supplies that way, the only caveat
    being that for a test current of 20mA the highest voltage Zener
    you'll be able to use in the 1N52XXB family is the 12V 1N5241B,
    which means that your highest supply voltage shouldn't exceed about
    15V.

    You could also use a Zener as a shunt regulator and, if you're
    interested, I'll explain how to do that.
    ---
    ---
    It depends on what the control input to the heater looks like. From
    your description I'd guess that there's a more-or-less constant
    current source in there driving an optoisolator which is driving a
    TRIAC or an SCR which is doing the _real_ work. But yes, there are
    limits, and if you don't know what they are you could easily hurt
    something in there.
     
  3. redbelly

    redbelly Guest

    There is something weird about these measurements. The battery
    measurement is giving 0.075 Watts, which is not much for an "instant
    hot water heater".

    Did you do the measurement with the heater immersed in water? If not,
    I wonder if the heater unit can sense this (perhaps with a temperature
    sensor that gets above the boiling point of water when the unit is not
    actually in water?), and goes into a "low current" mode when this
    happens?

    Try doing the measurements with the heater immersed in water.

    Mark
     
  4. Phil Allison

    Phil Allison Guest

    "redbelly"

    ** Yep - they are average values of a short pulse current waveform.


    ** ROTFL .....

    The OP is an idiot.

    His heater is powered by gas.

    The two batteries supply power to the ( electronic) spark igniter.






    ....... Phil
     
  5. redbelly

    redbelly Guest

    Well, shi-ite ...
    Never mind!
     
  6. suputnic

    suputnic Guest

    Thanks for all replies. I was only using a 47 ohm resistor, and still
    the current dropped too low. I tried to measure resistance between the
    positive and negative terminals when the burner was firing and when
    off, but it justs gives an open circuit reading. I just found out that
    the power cables feed into some sort of inverter? It (the white box)
    says

    Input: DC 3V
    Output >= DC 12 KV

    Not sure if this changes anything.

    I had read up about the first poster's zener solution, but won't this
    require a resistor to stop blowing up zener, and this circuit doesn't
    seem to like any resistor.....
     
  7. suputnic

    suputnic Guest

    Yes this inverter(?) does power the spark igniter.

    "The two batteries supply power to the ( electronic) spark igniter"
     
  8. suputnic

    suputnic Guest

    Actually it only seems to ever draw 22mA, so I suppose the appropriate
    combo of reverse biased zener diodes and forward biased silicone diodes
    in series would reduce the voltage to the right level, without using a
    protective resistor.
     
  9. ehsjr

    ehsjr Guest

    It's doubtful that precise voltage is needed.
    4 or 5 1N400x diodes in series between the adapter
    & igniter should work. If you want precision, you
    can use an LM317 voltage regulator:
    (View in fixed font)

    Adapter -----
    + ---+---Vin|LM317|Vout---+-----+---> To igniter
    | ----- | |
    | Adj [240R] |
    | | | | +
    [.1uF] +----------+ [1uF]
    | | |
    | [330R] |
    | | |
    - ---+---------+----------------+---> To igniter

    That will give you about 2.97 volts out to the igniter.

    Ed
     
  10. suputnic

    suputnic Guest

    No it doesn't need a precise voltage, batteries between 3.3V and 2.8V
    make it go. In fact I tried a 15.5V adapter (12V nominal) today, and it
    worked as well, but the igniter kept sparking after it was lit. Also
    the current was about half an amp, luckily it seems OK. I will try the
    chained diodes as a last resort, but I'd need over well over 10 in this
    case. Here are some of the options I have tried:

    15.5V adapter (12V nominal)
    0.5A ??
    -------------------------------------------------------
    Results
    worked but igniter kept sparking after ignition


    15.5V adapter (12V nominal)
    10V Zener diode in series
    3.3V Zener diode in series
    -------------------------------------------------------
    Results
    3.3V to the igniter as required
    46mA current, too high should be 22mA
    Did not work



    15.5V adapter (12V nominal)
    10V Zener diode in series
    -------------------------------------------------------
    Results
    4.7V to the igniter, acceptable
    68mA current, too high should be 22mA
    Did not work




    15.5V adapter (12V nominal)
    3.3V Zener diode in series
    386 ohm resistor in series
    -------------------------------------------------------
    Results
    5.3V to the igniter, acceptable
    18.8mA current, too low should be 22mA
    Did not work




    15.5V adapter (12V nominal)
    3.3V Zener diode in series
    330 ohm resistor in series
    -------------------------------------------------------
    Results
    5.6V to the igniter, acceptable
    20.2mA current, too low should be 22mA
    Did not work


    15.5V adapter (12V nominal)
    3.3V Zener diode in series
    267 ohm resistor in series
    -------------------------------------------------------
    Results
    6.2V to the igniter, acceptable
    22.9mA current, close to required 22mA
    Did not work



     
  11. John G

    John G Guest

    I have no idea where you live but sometimes it is better to have a
    battery ignitor because if the power is off (Storm etc) you may still
    want to use the heater.
    My daughter has a heater which has no provision for batteries and she
    was without hot water fo a week after a big tree brought down the power
    lines.
     
  12. Guest

    weather out there?

    So, I'm guessing you want to be able to power your hot water controller
    with a DC adapter, powered by the mains, instead of with batteries. Am
    I right?

    Why exactly would you want to do something like this?

    Could you get two AA NiMH batteries, and charge them when they go down?

    Another idea: get the correct size DC adapter, as close to 3.36V,
    22.2mA as you can find. Is there a thrift shop near you? Here where I
    live, at the local Goodwill, they charge US $1 each for assorted used
    DC adapters.

    Michael
     

  13. Diodes are made of silicon, bathtub caulk is made of silicone. ;-)


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  14. suputnic

    suputnic Guest

    So, I'm guessing you want to be able to power your hot water controller
    Beacuse we have many hot water heaters, and replacing batteries all the
    time is expensive. I suppose I could try the rechargeable option, if
    they last long enough. We have no power supply issues in New Zealand.
     
  15. Guest

    Ah. I'd recommend Energizer NiMHs, 2500 mAh, 1.2V. They come in packs
    of 4 or 8. We have about 20 of them scattered throughout our house...
     
  16. John Fields

    John Fields Guest

    ---
    The OP's problem is that he doesn't understand that in order to bump
    a few volts up to the 15kV or so required to strike the arc required
    by the ignitor he'll need to keep the impedance of the substitute
    supply down to what batteries look like, milliohms.

    Because of that, his assessment of the current required to do the
    job is flawed.
     
  17. I have a lot of trouble believing these ratings for an "instan
    water heater"....Maybe you should consult the manufacturer an
    possibly a lawyer about faulty advertising....The ratings you giv
    couldn't heat 10cc of water +10C in anything less than 4
    seconds.....
     
  18. Phil Allison

    Phil Allison Guest

    "Electromotive Guru"

    ** The heater is powered by gas - as the OP has now revealed.

    The two batteries supply power to the ( electronic) spark igniter.




    ........ Phil
     
  19. Jasen Betts

    Jasen Betts Guest

    first off 15.5V is kind of high to reduce to 3 using zeners,
    and also it's not a regulated supply so the output voltage will
    change with the supply voltage (which fluctuates somewhat)

    if you can't get a lower voltage plugpack you'll need to use the lm317
    circuit to reduce the voltage.
    yeah, that one.
    maybe a 10uF tantalum instead of the 1uf.
     
  20. suputnic

    suputnic Guest

    From your IP address, you appear to be from New Zealand. How's the
    Fine weather recently, but cold with the onset of winter
    Thanks for that information, I'll stop putting resistors in my circuit.
    I haven't assessed the required current, only measured it as 22mA.
    However I put on a 4.7V DC adapter today, it made the unit work. The
    measured current this time was 32mA, this is a different unit though.

    This attempt below, any ideas how to limit the current without
    inserting a resistor? Why is it drawing too much current anyway? Is
    that why it doesn't work?

    15.5V adapter (12V nominal)
    10V Zener diode in series
    3.3V Zener diode in series
     
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