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convert 150 - 300mV into 5Vdc

Discussion in 'Electronic Design' started by Winfield Hill, May 28, 2004.

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  1. Watson A.Name - \"Watt Sun, the Dark Remover\" wrote...
    Perhaps someone else can answer the furnace-thermopile question.

    John Damaschke's 100mW 300mV-to-5V dc-dc converter is interesting.

    In the article he touts the importance of MOSFETs in performing the
    work of converting as little as 150mV of source voltage into a nice
    5V supply, but in truth this is a simple task given say 8 to 10V to
    run the MOSFETs. Damaschke created a 8.5V source for the job, and
    that circuit is the real innovation. The tough task of working with
    as little as 300mV at startup is provided by a nifty JFET blocking
    oscillator, which creates sufficient voltage to start running the 5V
    converter. Once the +5V output voltage is well on its way, a MAX630
    (RC4193) step-up converter takes over, providing 8.5V for the FETs
    If high load conditions on the 5V output drag the input thermopile
    down to as low as 150mV, the full converter continues to work.

    Damaschke says his blocking oscillator is similar to those used in
    pacemakers, which are credited to Wilson Greatbatch's 1958 invention, In Greatbatch's own words,

    Let's examine Damaschke's JFET circuit. At startup the depletion-mode
    JFET is on and current starts flowing in the transformer, charging the
    1nF cap with a negative voltage. When the current rises sufficiently
    to saturate the transformer core, the secondary voltages collapse, and
    the JFET gate is forced negative, ending the cycle. A 1M resistor
    discharges the 1nF cap, setting the cycle period to about 1200Hz.

    | 1:45:65
    | +in ,------,
    | O--|----, | ,-----|>|----+---- +8.5V
    | | # # # |
    | | # # # |+
    | | # # # ---
    | | | | | 1.0nF --- 470uF
    | | |-' '-|-+--||--, |
    | '->|-, | | | |
    | | | '-/\/\-+ |
    | J105 | | 1.0M | |
    | O-------+---+--------+---+---- gnd

    Sadly, Damaschke doesn't have much to say about the critical aspects
    of this circuit, e.g. transformer properties or JFET requirements.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  2. Fred Bartoli

    Fred Bartoli Guest

    This can probably be simplified by eliminating the MAX630 stage and getting
    the "high voltage" supply from the T3 gate connection.
    Plus the choice of an 1N4001 for D2 can be improved (small schottky), or
    simply deleted.

    For the transformer, probably nothing critical. Just make sure that the
    transformer saturates before the lowest IDSS value of T1.

  3. Fred Bartoli wrote...
    [ snip Win's post ]
    Good call.
    Delete. Damaschke appears to ignore the MOSFET's intrinsic diode.
    Lots of turns.

    How low can the starting voltage be? Damaschke implies 300mV.

    - Win

    (email: use hill_at_rowland-dot-org for now)

  4. Did you see Greatbatch's AIDS patent 5,324,643 ( or better
    yet, use Amazing. It actually has the genetic code
    for the virus. I treasure your posts, Win.
  5. I used that very same blocking oscillator circuit in my old timer
    germanium transistor V boost circuit. It actually works down to .18VDC,
    but with the white LED barely visible. At .6V it's going pretty good,
    where the usual silicon transistor hasn't even started.

    So I got a bunch of LR44 / 357 button cells from Ebay, 50 for 8 bucks.
    I used a regular spring clothespin, drilled holes in the clip end and
    put two #3 brass screws in them for contacts. I use this as my button
    cell holder, and connected my V boost circuit to a LR44 button cell.

    It will run for a minute or so, then dim down to nothing. The voltage
    sag is very great, from 1.5V open circuit down to only .6V with a few
    tens of mA load. The light dims down to nothing after a few tens of
    seconds, so I opened the circuit. After a few minutes, the cell
    recovers, and I could do another load test, but the cell was much
    quicker to dim down to nothing. I can repeat this after an hour of
    recovery, and I've done it many times, and the cell seems to come back
    from the dead every time. It's getting to the point where the cell is
    taking longer to recover and putting out for a much shorter time, so I
    guess it's getting 'used up'.

    It's quite apparent that the load is too much for this little cell. Yet
    many of the laser pens and flashing LED keyfob lights use these button
    cells, probably because they're so small and cheap. Just don't expect
    them to give light for long enough to do anything serious. Putting a
    key into the lock is no problem, tho.

    I previously mentioned that I went to Radio Shaft to buy an LR44 button
    cell, and found that they want $3.19 apiece for them. Yikes! For four
    cells, that's a couple dollars more that what I paid for the LED keyfoib
    light, which was only ten bucks _with_ the four LR44 button cells!

    Meanwhile, the other day I was walking down the mall, and in the walkway
    was a watch and bracelet kiosk. I asked the salesman if he had an LR44
    button cell and he said yes, so I asked how much? $9.99, was his
    answer, so I asked was that with installation, and he said yes. So I
    asked, how much for the cell without installation, an he said $9.99! So
    his price is three times as much as Radio Shaft, and 62 times the price
    I paid for the LR44s on Ebay, including shipping! Now that's amazing!
  6. It says 1:45:65 but I would assume that there's more than one turn on
    the primary winding. So if it had ten turns, then the others would have
    450 and 650 turns respectively. But then if it had 20 turns... >:-O
  7. Terry Given

    Terry Given Guest

    ..... your transformer manufacturer will hunt you down and beat you with a
    stick :)

  8. mike

    mike Guest

    Why do you need three windings at all? Move the RC to the Gate side????
    What am I missing?

    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Yaesu FTV901R Transverter, 30pS pulser
    Tektronix Concept Books, spot welding head...
  9. Besides, why does he use such a high ratio of turns on the feedback
    winding? I realize that the input is only a few hundred mV, but why
    should the JFET need more than a few volts to switch? They shouldn't be
    as demanding as power FETs are for gate voltage.
  10. 45 x 150mV = 6.75V, you want the FET to 100% cut off.
  11. Ensure that Idss > I saturation of the transformer. This will result in
    the transformer requiring quite a lot of turns when this is in the few
    ma range.

    Lots of turns so that it satuates at low I.
    Well, I was quite intrigued by this, so I have now updated SS with an
    example oscillator of this type (LVBlockingOsc.sss,
    , ignore the update date). It runs from about 150mv upwards.

    The "design" is pretty simple. I only used a transformer with 2 windings
    to try and minimise space. It uses my newly added hysteretic non-linear
    core model (thanks for the core idea Mike).

    Its quite subtle really, it just looks like a normal oscillator circuit.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  12. Kevin Aylward wrote...
    You used a J109, which has Idss = 40mA (min), whereas Damaschke used a
    J105, with Idss = 500mA (min, at 15V vds). That's quite a difference.
    At 300mV (Damaschke's specified minimum startup voltage) the J105 with
    Rdss = 3 ohms (max), the minimum peak current will be 100mA, assuming
    a minimal transformer winding resistance of course.

    The thing that worries me about Damaschke's proposal to use this circuit
    with a low-voltage furnace thermopile is the *very slow* risetime of the
    300mV input whenever the furnace starts. Won't this completely fail to
    start the blocking oscillator? Other low-voltage applications, like
    seawater rope batteries, may also have this problem.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  13. That's what I was worrying about since I saw your original Damaschke
    reference: Is the circuit going to start up reliably? If not, can
    anything be done about it?
  14. Noise, in the linear range it will be amplified?
  15. Stefan Heinzmann wrote...
    Yes, of course, toss the whole silly business. The task of converting
    100mV or whatever up to 5V is trivial, given the fantastic low R_on of
    modern power MOSFETs. Assuming, of course, a power supply is available
    to power the FET gate-drive circuits. Under normal operation this can
    be provided from the 5V output, as in Damaschke's circuit. But during
    startup or load-fault conditions this is provided from a small battery,
    eliminating clever and painful low-voltage converters. A simple low-
    power 150mV comparator circuit can be used to disable the power-hungry
    conversion circuitry from draining the battery whenever an inadequate
    input voltage is present.

    - Win

    (email: use hill_at_rowland-dot-org for now)
  16. Terry Given

    Terry Given Guest

    nice. There are plenty of ways of skinning cats....

    I bought a digital micrometer a while ago. No off button - the battery lasts
    for 3-4 years, so why bother.

  17. Joel Kolstad

    Joel Kolstad Guest

    Yes, but it still seems to me like there's a place for the
    'ultra-low-voltage/low-power input to super cap' converter chip out there,
    even if it is just to provide the start-up voltage for a much better (more
    efficient) power converter. That way I'd think you could realistically
    expect equipment (I'm thinking solar powered here) that would keep working
    for perhaps a decade without maintenance, whereas with a battery you'd have
    to replace it every 2-3 years if rechargeable.

    Although you might convince me that with a lithium battery you could still
    get ten years...
  18. I just used whatever was in my SS lib. It was only a proof of concept
    thing. One alternative would be to use a depletion mode power mosfet.
    There are some, but I couldn't be bothered setting up a model for the
    one that I found.

    This could be an issue. I did try a 10sec ramp, and this seemed to work
    ok though. The output stays at 0V through the ramp up, then collapses
    quite fast (to -15V) at the saturation point of the transformer, forcing
    the ocilator to start. I suppose one could use a realy attached to the
    300mv so that it switchs the voltage at the appropiate point.

    I'm also never happy with circuits that only work dynamically. i.e. have
    more then one stable state.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  19. Terry Given wrote...
    After 3-4 years goes by and it no longer works, you'll be thinking
    about that off button as you make the trek to Radio Shack for the
    hearing-aid battery. After 6-8 years has gone by you'll be sorely
    missing that off button when it fails again. After 9-12 years...

    - Win

    (email: use hill_at_rowland-dot-org for now)
  20. Kevin Aylward wrote...
    Yep, the J105 is better, you should add it to your library. $1.22 qty
    25 at Newark, #38C7317,
    I suspect your transformer model. Real transformers don't have the
    square loops I remember from your model, IIRC, and their saturation
    is gradual. A slow 30mV/sec ramp wouldn't create enough secondary
    voltage with a real transformer to negatively charge the 1.0nF cap
    in Damaschke's circuit above, providing a way to turn off the JFET.
    But the relay circuit would have to operate on 300mV. :>(
    Indeed, one wakes up at night worrying about the extra non-operating
    state. But if circuitry was added to detect this state and kick the
    oscillator, then the benefit of simplicity is lost.

    - Win

    (email: use hill_at_rowland-dot-org for now)
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