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pulsed DC and transformers

Discussion in 'Electronic Basics' started by [email protected], Jun 21, 2008.

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  1. Guest

    I'm trying to figure out if I could drive a transformer with pulsed
    DC, and have it work (not saturate, overheat, etc.)

    Ideally I'd like to pulse a self-wound toroidal transformer with
    12VDC, on the primary, and get 48VDC out of the secondary, center-
    tapped to get +/-24V on each leg. I plan to use a 555 in astable
    mode, 50% duty cycle, driving a mosfet to feed the transformer
    primary, at frequencies I haven't determined yet - anywhere from 100
    Hz to 100 kHz.

    Wikipedia has this:

    The time-derivative term in Faraday's Law shows that the flux in the
    core is the integral of the applied voltage.[11] Hypothetically an
    ideal transformer would work with direct-current excitation, with the
    core flux increasing linearly with time.[12] In practice, the flux
    would rise very rapidly to the point where magnetic saturation of the
    core occurred, causing a huge increase in the magnetising current and
    overheating the transformer. All practical transformers must therefore
    operate under alternating (or pulsed) current conditions.

    so it implies a transformer can be driven by pulses. In practice,
    what happens when driving a transformer with pulsed DC? Should I
    stick to low frequencies? Go for higher frequencies?

    Good idea / bad idea / horribly bad idea?


  2. Bob Eld

    Bob Eld Guest

    Keep the DC out of the transformer, especially a torroid as they have little
    tolerance for DC offset. Transformers or inductors that tolerate DC offsets
    have air gaps or the equivalent to limit saturation.

    The simple way is to drive the transformer through a capacitor, thus
    blocking all DC into it's primary. The secondary will convey the pulse, but
    the zero volt position will be somewhere in the middle of the pulse so that
    the area above zero equals the area below zero. DC can be restored with
    appropriate diodes.

    For example say the original pulse train was positive going 4 Volts for 1
    msec. then zero volts for 3 msec.

    If the transformer ratio is 1 : 1, then the secondary would show 3 Volts
    positive for 1 msec and 1 Volt negative for 3 msec. The positive area equals
    the negative area. Of course, if the pulse width changes, the zero line
    would shift to equalize areas. Faraday's law.
  3. Joerg

    Joerg Guest

    Yes, I'll second that. Else the time from core saturation to plume of
    smoke can be just microseconds. Last time I did that (experimental PWM
    stalled while I was doing other work in the office) it took two days to
    get the stench out of the area.

  4. Guest

    Gee, power electronics seems like so much fun. Maybe I chose the
    wrong major at the university... ;-)


  5. Joerg

    Joerg Guest

    Chemistry can be fun, too. One of my friends ended in the hospital. "My
    lab is now one hellacious black hole and the door is gone as well."
  6. Guest

    What did he try to do... make nitrogen tri-iodide or something silly
    like that?

    Yep, chemistry can be fun. So far I made 40 cc's of ethanol from the
    kids' leftover food scraps. Just have to scale it up by a factor of
    10^9 or so...

  7. Joerg

    Joerg Guest

    I don't remember. But it made the news.

    There was an ad for an ethanol machine that can make 30gal/week or so at
    home. In the fine print: You had to pour in lots of leftover booze.
    Yeah, right. Who'd have gallons of "leftover" Whiskey for that?
  8. Guest


    A co-worker told me he was going to buy this:

    Only $6,988 after tax credits.

    "To operate simply load EFuel100 feedstock (table sugar with ethanol
    yeast mix) into the fermentation tank and select the fermentation
    option on the control panel to begin the process. It will take between
    10lbs to 14lbs of sugar to produce 1 gallon of ethanol. The
    MicroFueler is capable of producing 5 gallons of ethanol per day once
    fermentation is complete."

    He asked me for my comments on the process flow diagram. I gave him
    about 5 areas where I would do it differently... plus I told him, with
    sugar about $2.50 for 5 lbs at Wal-Mart, it didn't seem economical.
    But he's already sent in the down payment... we'll see how it goes...

  9. Joerg

    Joerg Guest

    It's disturbing. I see many well-educated people fall prey to hype
    technologies. Predominantly the overly eco- bio- whatever leaning folks.
    The millisecond they get criticized they often react with "Ah,
    baloney" but without being able to furnish any data to support their own
    theory. Kind of like warmingists ...
  10. Guest

    On Jun 23, 11:51 am, Joerg <>


    Yes, truly.

    The site I posted above used to mention some sort of tax-free, cheap
    inedible sugar from Mexico. I tried seeking more info via the web,
    but no luck. (What kind of sucrose is inedible ??? And why?) The
    website's FAQ has since removed the reference to the cheap Mexican

    One of the ways I would have done it differently, by the way, would
    have involved just going to restaurants and collecting starchy food
    scraps (rice, bread, potatoes)... toss in a little bit of alpha-
    amylase enzyme (0.1% by mass) to convert starch to glucose, that the
    yeast can eat.

    Global warming... well, if it's true, I can start a coconut farm in

  11. Joerg

    Joerg Guest

    Very shortly thereafter you'll read in the papers that some crooks
    lifted a vat of starchy food scraps from the so-and-so diner. There
    ain't enough of that stuff. Look at the theft cases of "discarded"
    french fries oil. Some people are becoming desperate.

    A while ago I was passed by a Mercedes 300 TurboDiesel in the Bay Area.
    He pulled into the right lane in front of me but I had to back off. This
    car let off such a strong burger shop smell that my mouth began to water.

    Not likely. We used to be able to grow citrus around here. No more, that
    stuff dies in freezes every winter.
  12. Guest

    Ok, thanks.

    If I'm reading this right, I want RC to be <= the time period T.

    I'm not sure exactly what the primary resistance of my self-wound
    toroidal transformer will be, but let's just say it's one ohm.

    So let's say I'm operating my 555 driver at 7 kHz. So, C must be <=
    (0.000143 s) / (1 ohm); C <= 143 uF, correct?

    So a 100 uF capacitor should (just barely) satisfy the requirement.
    (Or a 10 uF, or a 1 uF... right?)

    Can it be an electrolytic? If so, the (+) side should be closest to
    the 12VDC input, right?


  13. Joerg

    Joerg Guest

    More like 22uF. Calculate it so it's less than 1/10th of the transformed
    load impedance, depending on your needs.

    Theoretically yes. But be careful, electrolytics can only take a certain
    amount of ripple. Found that out the hard way as a kid. Of course I had
    to build the biggest honking amp a 230V/16A circuit could stomach. A
    470uF/400V cap decided it had enough of this. Lift-off. Left a crater in
    the plaster ceiling and a hole in the carpet.

    IOW ceramic would be better.
  14. Guest

    On Jun 23, 2:11 pm, Joerg <>


    Ah, thanks! A good thing I asked the experts. "But be careful,
    electrolytics can only take a certain amount of ripple." didn't appear
    in MY circuits book.

    Just in time for Fourth-of-July fireworks...

    (I'd better test my circuits with small loads first.)

  15. Joerg

    Joerg Guest

    Small loads, see if anything gets hot.

    BTW the carpet turned out to be a pain. Needless to say my parents were
    not enthused and I vowed to fix that. Found a carpet remnant on sale at
    a store 5 miles away, 15ft by 7ft, barely large enough. Paid, rolled it
    up, strapped it to the frame of my bicycle and pedaled home. Got a cramp
    from that unhealthy position and had to push it the last 4.9 miles.
    Stopped at the library, picked up a book about floor covering
    techniques. At least I learned how to lay carpet and how to replace
    broken spokes on bicycle wheels. All because that one dreaded capacitor
    couldn't hang on.
  16. Guest

    On Jun 23, 2:52 pm, Joerg <>


    And the ceiling?
  17. Joerg

    Joerg Guest

    That was just a little crater, some of the plaster and concrete had
    fallen out. Pa gave me a gypsum bag, I mixed some up, went on a ladder
    and spackled it. Of course, then I had to re-paint the whole ceiling and
    that is not one of my favorite tasks.
  18. theamber


    Jun 24, 2008
    Pulsating DC to transformer.

    Use a "Pulse transformer" that is a transformer that is optimised for transmitting rectangular electrical pulses.
    The lower the freq. the better with transformers.
    A low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load.
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