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Help with winding a toroid pulse transformer to drive back-to-back SCRs

Discussion in 'Electronic Design' started by MR COFFEE, Jul 22, 2007.

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    MR COFFEE Guest

    Hi folks.
    I am trying to put together Bill Bowden's thermostat circuit to
    trigger back-to-back SCRs to turn on a water heater element run on
    120v (to make a 500 watt heater).

    I don't have a 3 winding trigger transformer laying around, but I do
    have a nice toroid core I picked up surplus that is supposedly for
    pulse transformer use. Can I just wind a trifilar winding around it
    with some 28AWG magnet wire and have half a chance it will work? The
    core measures about 2 uh with 8 turns on it, so I figure it is a
    pretty low mu ferrite core.


    mr coffee
  2. My biggest worry would be the breakdown voltage of the
    magnet wire, especially after it has been dragged through
    the hole in the toroid many times. What is the peak voltage
    that will appear between any pair of the three wires? You
    may need better insulated wire, or an insulated core and
    three separated windings to achieve reliable resistance to

    What inductance do you need?

    Is the core black ceramic or painted? It might be a
    powdered iron core, that includes distributed gaps, that
    make it useful for designs that include DC in the windings
    or emphasize energy storage in the inductor, rather than
    simple transformer action of pure AC.

    MR COFFEE Guest

    about 170 volts
    I guess I assumed pulse transformers needed to be wound that way
    Separate windings are actually easier to wind from my experience.
    Actually, I haven't got a clue. I hoped someone more familiar with
    SCRs would fill me in on that. I've done searches but all I've found
    is on
    commercial sites that want to sell pulse transformers.
    Actually it is painted blue on one side and the other side and
    circumference is
    painted very light green. The core is from a Pulse Engineering
    transformer but
    I couldn't find any information on it anywhere on the web.
    I'm guessing it is ferrite from what I assume was it's intended use. I
    have two
    cores so I can bust one open if that would give helpful information. I
    think the low inductance I measured would mean it is a low mu core
    for high frequency use, but I stand to be corrected and educated on

    Thanks for reply.
  4. DaveM

    DaveM Guest

    These transformers are usually called "Trigger Transformers". Mouser sells the
    Vishay-Dale line, but looking at their stock, they only have one that looks like
    it would meet your needs
    Newark looks the same, and Digikey doesn't stock any. Pitiful.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    "In theory, there isn't any difference between theory and practice. In
    practice, there is." - Yogi Berra
  5. ehsjr

    ehsjr Guest

    Yes, with a mod to the circuit. The transformer is driven by
    6 volts, and just needs to turn the SCRs on. As drawn, with a
    home wound unknown transformer, you risk burning out the PNP's,
    so put a 100 ohm resistor in series with the winding that is
    connected to the PNP collectors. If you use sensitive gate SCR's,
    you can do that (turn them on) with very little current. Build
    the circuit, and try it out with a 6 volt supply to a 6V load
    on the SCR's rather than the heater, just to make sure it turns
    the SCR's on when it should.

    Once you're satisfied with the circuit, you can connect it to
    the heater.


  6. A much simpler solution, and likely cheaper, more robust, and higher
    longevity would be to use solid state relays to energize the AC fed
    heater elements, and use a simple Low Voltage DC circuit to perform the
    sensing, and relay activation signal.

    Probably more efficient as well, as the DC sense/control circuit could
    be built to us very little energy.;keywords=solid state relay;FV= relay&N=1323038 254134&Ntk=Mouser_Wildcards
  7. Terry Given

    Terry Given Guest


    oops, sorry about shouting.

    what you need to do is a "splat test"

    charge up a known, large cap to some voltage V, then "splat" it across
    onw winding (other open-cct) and measure current (eg 0.1R resistor off
    to scope). you will the current slope (dI/dt) gives you V/L, and if the
    cap is large (c.f. energy in choke) then V is pretty much constant so
    you can work out L. you can also see the core saturate (slopes upward, L
    decreases), which will tell you how many Volt-Seconds the core can
    handle (with that many turns), which then tells you what pulse width it
    can cope with - you know the volts the driver cct will apply.

    trifilar wound produces lowest leakeage, so best pulse shapes. if
    leakage is too high, primary saturates before 2ndary goes very far.

  8. I wouldn't trust ordinary single layer varnish to stand
    that. I would probably use kynar wire wrap wire or Teflon
    insulated wire if I was going to do a trifilar.
    It improves the energy transfer at the edges to do trifilar,
    but the compromise with separate windings may be acceptable.
    A first cut would be to calculate the inductive current over
    the pulse duration and the transformed load current. As
    long as the inductive current is less than the transformed
    current, you are getting close.
    I think it is a powdered iron core, not ferrite, so it is
    probably non conductive enough to not worry about insulating
    the core. But the colors probably rule it out as a
    Micrometals core:

    But, regardless, based on your test of 2 uH with 8 turns,
    you can predict pretty well the inductance of other windings
    based on the inductance being proportional to the square of
    the number of turns.
    The inductance is probably too low for the core to be
    ferrite. But powdered iron is fine for a pulse transformer,
    since you don't have to worry about forcing exactly zero DC
    bias in the drive circuit. The distributed gaps in the
    powdered iron material make these cores much more resistant
    to saturation than ungapped ferrite is.

  9. Double strength mag would/should be fine. Even single strength has
    over 350 Volts breakdown resistance, on a bad day. Also, if the turns
    count is low enough, he could pipe the mag wire through a Teflon sleeve.

    A good thing to do is wrap the toroid with transformer tape the way
    they used to wrap tires a few decades back. It usually has about 1000V
    per mil strength, if not more.

    One could also dip the toroid in q-dope paint.

    MR COFFEE Guest

    Thanks for all the replies.
    Do I assume correctly that the resistor goes across one of the
    secondaries in the "splat test"?
    I will use some kynar ww wire for stronger insulation as suggested. I
    will also put a resistor in series with the primary - good suggestion.

    BTW - I tried searching both digikey and mouser for a SS relay that
    would handle the amps - no dice. I know I've seen them surplus from
    time to time but iirc they were pretty pricey compared to a couple of
    SCRs (3 bucks).

    I'm trying to use stuff I've got laying around. I've got all the parts
    except the pulse transformer and SCRs.
  11. Terry Given

    Terry Given Guest

    yes. unless you happen to have a current probe (most people dont).

    MR COFFEE Guest

    Thanks Terry.
  13. Werty

    Werty Guest

    MOC3xxx will trigger parallel SCR's .

    trigger pulse may need to be extended

    because of inductive loads on the SCR's ,

    so i dont use transformers , they of course

    would saturate , stop triggering the SCR's

    just before they could accept the trigger

    pulse !


    Measure inductances with a scope .

    known voltage dumped into the inducter ,

    the slope is so consistant at start , it

    has no change , so put a strate edge on scope

    and use lots more scope boxes , to get a mo

    accurate "division" of the hor and vert boxes .

    volts/henries = dir(amps)/dir(time in seconds).
    -------- or ----------
    henries = volts/ ( dir-amps/ dir-seconds ).

    Put a low value R in the ground leg of your

    ckt to send signal to scope .


    figure the caps inductance to be far lower than

    induct under test , figure the inductors R to be far

    higher than all other R's ...

    I got SolidState Relays from

    BG Micro that use SCR's , rather than TRIACs .

    They also have ALTERNISTORS , about $0.40 .

    BTW I live in Guadalajara , San Juan de Dios ,
    by the Tube ...
    I saw the nut and bolt store south of me on
    Revolution street

    , tossing
    the old 8 foot flour tubes and replacing with 8 twist
    C.F. tubes ! Dark as a cave ! Fools .

    Compact Flour' lites are BOGUS , they have starters
    and heaters that limit life .

    I had a 220 vac , 8 foot tube , i got used ,
    it lasted the 23 years , til i sold the house
    in 2000 ! No heaters , no starter .
  14. joseph2k

    joseph2k Guest

    You do not seem to have mentioned doing the dip before the windings are
    wound. Though it was implied.
  15. Jasen Betts

    Jasen Betts Guest

    If you can't get your hands on a scrap "Mr Coffee" to salvage parts from
    (sorry, couldn't resist) consider using a MOC3010 (or similar)
    optocoupler to switch a triac. (datasheet available from

    I used it, a thermistor, and a LM304 to make a thermostat a few years
    ago. no PCB - I just epoxied the ICs down and soldered the Rs and Cs
    point-to-point. it worked great. for the adjustment I used a multi-turn
    pot from an old VCR.

    I mounted the whole lot in an earthed metal case.

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