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Voltage switch problem, most optimum solution ?

Discussion in 'Hobby Electronics' started by Mike, Mar 24, 2006.

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  1. Mike

    Mike Guest

    Hi chaps,

    I made a previous post about driving a flashing led from
    240v ac with a 'or' from two sources, turned out ok...

    Here's a different problem I've been toying around with
    and I dont like the feel of what I've done so far, so looking
    to do a little brainstorming, here it is...

    Have a single source of power that goes from 9v steady DC to 500v pulsed
    (not AC), in that in the first few seconds its a fairly steady 9v +20%,
    then starts pulsing at 500v with a duty cycle of 5% but returns to
    30v for 20% of the time and 12v for the remaining 75% of the period.
    Period is approx 30ms overall but can reduce to 10ms, complication is
    the 500v pulse is same period, so the duty cycle for that relatively
    goes up as overal period goes down.

    With this single source of power I need to draw off about 0.5W and
    drive a 'module' at 9 to 11v with some cmos logic along with a few
    leds etc.

    My problemo is keep the cost down <guh>, no relays unfortunately but
    drive this module without having it blow up or be unreliable
    because its only power source line goes from 9v steady to
    500v pulsed.

    The circuit I have so far is a high voltage transistor configured
    as a constant current drive into a resistor and 12v zener to spread
    the dissipation and filter with a 100uF electro 25VW.
    This seems to be ok but uses a few components and there isnt much
    gain at the low volts of the constant current source to charge up
    the electro quickly enough before the 500v pulses start coming...

    So I'm opening this to the public forum to see if any lateral ideas
    come up <shrug> cause I have exhausted mine (so far, afaik) :(

    * GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
    * Twin Tyres to suit most sedans, trikes and motorcycle sidecars
  2. Mike

    Mike Guest

    Interesting, I didnt know that, do they have a small relay to make
    it cheap, cause any semi to handle that isnt reliable if its cheap,
    unless its low down on the failure/repair curve etc...
    Now thats a good idea, the last thing I thought of using there was an
    inductor but it does make a lot of sense. The 500v pulse is only for roughly
    10 microseconds so that makes it a pretty viable idea, thanks :)
    The last one is providing some visual aid to the hearing impaired where
    a single row of switches runs an outside light and a couple of fans, they
    dont hear the fans and they can't always notice the outside light, having
    an inside flashing led on the switch cluster derived from an 'OR' was ideal
    as a reminder that something on the cluster was left on...

    This one is for a vehicle ignition system, be able to supply some power
    to some logic but make it plug compatible so installer doesnt have to run
    an extra cable for 12v power, get it via the coil return etc
    The 500v pulse is back emf from the coil, so I would guess fairly low
    in that the primary impedance is low, I dont want to snub the pulse
    if at all possible as it would then reduce the striking voltage to
    overcome the spark gap on the sec. The LC seems pretty much the perfect
    solution, the output of the LC would be seriously damped when the pulse
    arrives as the rise time is fairly short on the 500v pulse and yet have
    low impedance to get enough power inbetween pulses, prob wouldnt need an
    SMPS as a resistor, clamping zener and cap would be plenty for the amount
    of power I would like to supply which is only about 50 or so mA.

    Hey CHris, you didnt go to WAIT in WA some many moons ago did you ?

    * GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
    * Twin Tyres to suit most sedans, trikes and motorcycle sidecars
  3. Mike

    Mike Guest

    Yeah me too, fascinating, have speculated how to do that, can run a cmos
    pwm timer off a few microamps, then just need a hivolt mosfet as regulator,
    but heck it would have a really wide range pwm, pfm or ppm drive...
    Prob not that much an issue re saturation, afterall the 500v is fairly short
    and even if the core does saturate there will be a series resistor, just need
    to go through a few iterations. ie Some of the current not snubbed by the
    coil will be handled by a series resistor and some dumped into a zener, the
    trick will be to get the tree balanced in terms of dissipation whilst allowing
    enough current when the pulses arent present, will get to this during week,
    should be an interesting exercise :)
    I was going to use an MJE13007 as series current regulator, but the LC
    instead of those active devices would be heaps more reliable and more
    interesting, thanks
    no problem, he had identical name and was heavily into electronics, was
    a member of Salvation army and knew an old friend Phil - be about 25years
    * GMC/VL Commodore, Calais VL Turbo FuseRail that wont warp or melt !
    * High grade milspec ignition driver electronics now in development
    * Twin Tyres to suit most sedans, trikes and motorcycle sidecars
  4. 1. Are you sure the 500V is a low-impedance source? - EG Put 50 Ohm across
    it, and see if it stays at 500V (or smoke for R <1W Diss.)

    2. My first thought was a > 50V zener, or other similar fast switch turning
    OFF a series switch to a regulator.
    There are lots of alternatives.. like having the ">50V ? Detector"
    additionally loading the 500V spike to ground, through, EG a low value
    resistor or a 100uF capacitor, (that includes a parallel resistor).
    This is essentially an active, non-linear filter to attempt prenveting
    spike reaching low-voltage part of supply.

    3. As previously suggested, a passive L-C at front would do at lot of good
    to start with.

    Dave Merrett
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