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Design problem for scientific lamp current source.

Discussion in 'Electronic Design' started by Geoff, Nov 13, 2012.

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  1. Geoff

    Geoff Guest

    This circuit is to drive a scientific lamp which is low pressure
    neon of strike volts 600V and typ run volts of about 200V. The
    design intent is DC current from 0 to 60mA. This circuit
    traditionally runs to about 30mA with no problem and is well
    proven at these currents. Normally it is not darlington pair but
    it is something I have tried..

    The trouble I have with this circuit is that at above currents
    of about 35-40mA the lamp starts to flicker indicating a higher
    current than is dialed in by the pot. An oscilloscope at the
    opamp output confirms that at the point where it starts to
    flicker, the loop starts to try to lower the current since the
    opamp output volts drops. The onset can be made to be a periodic
    dip (tens of hz dip about a volt) before the lamp begins to
    flicker. When the lamp flickers on and off more severely, the
    opamp tries to go negative to compensate. This to me shows that
    the output transistor is conducting, not the failure of the loop
    control.. I have played around with the drive impedance, ie
    with/without the 1k base resistor. The point of flicker can be
    less severe with the resistor, or latches hard without. Maybe
    the VCEO spec is being stretched here. I have tried a variac and
    reduced the volts to about 580V, still does it but onset is
    higher current (50mA). I have tried a BUL 416 transistor also. I
    might just be applying too much voltage, but I can't see why,
    might have to look at the op curves a bit harder. Maybe I could
    use a MOSFET or IGBT.
  2. Bill Sloman

    Bill Sloman Guest

    This is a slightly Russian circuit.

    A more subtle approach is to use a transformer to apply the 600V
    striking voltage to the lamp for long enough to get the discharge
    going - less than a microsecond for a glow discharge, some
    microseconds for an arc - which lets you get away with a supply
    voltage a lot closer to the 200V running voltage of the lamp.
    What you are saying is the circuit is unstable. This means that you
    need to look at the frequency compensation. That 10nF capacitor is
    decoupling the op amp from the output stage at higher frequencies, but
    it may not be the right value to do what you want, and it could be
    contributing extra phase shift where you don't actually want it.

    Get hold of a text-book that talks about stabilising negative feedback
    loops and read it carefully. "The Art of Electronics" by Horowitz and
    Hill might be one such textbook - though it covers a lot more than
    just negative feedback.

    Some of our regular posters have published more specialised texts, and
    may be prepared to stick their necks out further.
  3. Tim Williams

    Tim Williams Guest

    SOA says you can only run 600V up to a hair over 10mA, maybe 12mA. BJTs
    are a bad idea for dropping high voltages due to second breakdown.

    Why not choose a tube like 6L6GC? Current production, doesn't take much
    heater power, and takes the voltage easily. Okay, a MOSFET would be
    smaller and cheaper (though probably about even if you include the
    heatsink, come to think of it).

    Note you're trying to carry a maximum of (800V - 200V) * 0.06A = 36W,
    which is a moderate amount for a small transistor. Just in case, you'd be
    better off with a TO-247 package, or two TO-220s in parallel.

    Switching methods are much more efficient, even if you can't run the thing
    directly on AC (I assume that's part of the design requirement).

  4. Why the Darlington?

    If you have voltage to throw away you can always try putting in more

    And sure why not a high voltage FET.

    George h.
  5. Geoff

    Geoff Guest

    I am looking at MOSFETS now.
  6. Geoff

    Geoff Guest
    Your point about separating the run and strike ccts is well
    taken. I might go that way if I can't find another solution
    using the present transformer.

    I don';t think it is a stability problem because at the point
    of breaking down, the volts to the base are only dipping,
    indicating the loop is trying to prevent the output from
  7. Geoff

    Geoff Guest

    I agree I'm pushing the limits of the SOA. Now that this is
    becoming clear that it is the problem, I can tackle it in the
    best way I can. I had thought about a tube too. For the moment I
    might try a MOSFET in a TO247 pack. The SOA is a bit more
    friendly at the high voltage end.

    Switch mode is out of the question at present, I know the
    customer has a huge dislike, so I am doing it the way he wants.
  8. Guest

    an upside down buck similar to this should be doable:

    could make one with current sense on the low side

  9. Geoff

    Geoff Guest

    I had adapted the circuit from a similar one. I am not using the
    darlington now. The problem persists, probably second breakdown.
    That is worth doing if dissipation becomes a problem.

    Trying that next.
  10. Jamie

    Jamie Guest

    Trying to control the break over point on neon gas is going to be
    tricky with so many slow components in line.

    Using darlington adds to the problem with the storage time in the
    trannies and the use of that diode in the base with nothing but a 1k
    to pull it up isn't helping much.

    That circuit needs the integrator/miller cap greatly reduced, diode
    removed and possibly a Cap lead network from the feed back to give it
    a little derivative in the - feed back. At least that will help with the
    phase error and reduce os-kill-La-Trons! :)

    MeSelf, I think a PWM would be nice but then again, we don't know what
    the adverse effect of flicker would be..

  11. Bill Sloman

    Bill Sloman Guest

    I wasn't that specific. I just said that it wasn't doing what it had
    presumably been put in there to do.
  12. Where does the 800V come from?

    One thing I've done in the past is to make a weak voltage doubler that
    drops down to about half when you start drawing serious current. The
    effective series impedance would help stabilize the circuit, and it
    would dissipate a lot less power.

    With your circuit, you could try a 10K 100W resistor instead of the
    330R. They even come in TO247 if that's what you really want.

    Best regards,
    Spehro Pefhany
  13. tm

    tm Guest

    That's exactly how many HeNe gas laser power supplies work. The first stage
    of the CW multiplier have large capacitors. Then it has enough stages with
    smaller capacitors to make the start voltage. A proper current limiting
    ballast resistor establishes the run current.
  14. Owen Roberts

    Owen Roberts Guest

    Take a look at the I-V curve for the lamp. You may be entering a
    region of oscillation in the plasma.

  15. Geoff

    Geoff Guest

    800V is from volatge double from a 270 volt secondary winding at
    50VA. The caps are a pair of 450V 47uF. I think that is a good
    idea too, effectively reducing the supply regulation.
  16. Geoff

    Geoff Guest
    I might just do that too. There is a chance it acts like a
    relaxation oscillator. The lamp does not misbehave with a simple
    R in series with a HV supply at 50mA or so.
  17. Robert Baer

    Robert Baer Guest

    Try a simple resistor in series with the lamp and adjust the resistor
    and/or supply for that current range.
    Very possible the lamp is in a negative resistance region.
    BTW, the secondary ionization voltage for neon is 40.9V and the third
    ionization voltage for neon is 63.2V - meaning you better see about 63V
    across it when operating properly.
  18. Bill Sloman

    Bill Sloman Guest

    Not if it's operating as a glow discharge. Getting electrons out of
    the cathode by positive ion bombardment needs quite a large voltage
    drop across the anode glow region.
  19. Guest

    You seem to be missing the obvious for the circuit and that is you need an isolation resistor between the OA output and transistor base such that the rolloff pole formed by that resistor and the worst case transistor Miller capacitance is at least a decade higher in frequency than the RC feedback ofthe 10n and 1K (really). There's nothing exotic about a crummy VCS for a neon lamp, scientific or not, it is hack simple. A lot of your circuit values are screwy.
  20. Martin Brown

    Martin Brown Guest

    That's about the rating of a few elemental reference low pressure
    emission lamps. A low pressure sodium lamp is pretty bright even at 12W.
    Seems reasonable to me. Replacing the 330R with ~3k9 30W ought to do it
    by dropping about half the HT voltage across the external resistor after
    the lamp strikes and the extra series resistance should then discourage
    it from becoming a negative resistance relaxation oscillator.
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