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Thread: Simmer supply for LASER flash lamp

  1. #1
    Join Date
    Apr 2017
    Location
    India
    Posts
    3

    Question Simmer supply for LASER flash lamp

    Hi guys, I am currently working on developing Holmium-YAG LASER for medical purposes. I am using a xenon flash lamp for pumping,
    The specifications for the same are-

    Outside diameter- 5mm



    Inside Diameter--4mm


    Arc length--- 115mm


    Type-- xenon


    Envelope material-- cerium doped qurtz


    Pressure--- 700 Torr

    Now in the prototype model, I am using simmer circuit made up with huge resistors and transformers and what not (I haven't designed the circuit)!! This simmer works perfectly fine but the resistors and transformers are heating up and overall it makes my machine very bulky.

    Now I understand that the simmer circuit provides the high voltage pulse to flash lamp initially for arc to establish (600V in our case) and after that it maintains the constant current (365mA in our case) through the flash lamp.

    What I understand is that the household tube light choke also does the same work. So can we use tube light choke as the simmer circuit for LASER? IF we can't then why? And if we can then what are the precautions that should be taken (I don't want to lose my flash lamp)?

  2. #2
    Join Date
    Feb 2011
    Location
    New Hampshire
    Posts
    3,513

    Default

    I will say this respectfully. If you are working on something as basic as a simmer supply for the flash lamp then do not use this laser for medical purposes (even on yourself).

    Now, the simmer supply needs to be sized to match the lamp tube dimensions in order to get the most benefit from its use. The lamp you describe is pretty small. The cross section determines the required current to enable the discharge to fill most of the tube's diameter and reduce the impedance of the lamp for the main discharge. 30mA would be fairly typical for that size lamp. The only negative consequence of using higher current would be the additional heating of the lamp and the increased capability required of the simmer power supply.

    The required voltage to drive this 30mA depends weakly on the pressure in the tube, but that is likely to be about 450 torr. And so, a voltage of a little less than 1KV will probably work. However, immediately after the main discharge there is a period of increased lamp impedance and the steady state voltage may not be able to re-strike the discharge. A simple method to establish the best circuit for this simmer is to use a transformer capable of achieving several KV, charging a storage capacitor (10-20 uF is a reasonable starting point), pass this voltage through a high voltage resistor, pass this through a high voltage diode to prevent the main discharge from reverse flow into the simmer supply and then pass this through the lamp. Now, measure the voltage drop across the tube while it is operating (main discharge is disabled). You can then easily calculate the actual current through the tube. Adjust the high voltage resistor's, resistance to meet the 30mA requirement.

    The storage capacitor will continue to build voltage well above the steady state during the firing and non-firing interval,enabling the simmer to re-strike the tube if the discharge shuts down immediately after the main discharge. The simmer transformer should be ballasted to minimize heating and this is best done with an inductor located in series with the line voltage. The easiest method is to use additional, identical transformers with their high voltage secondary lines shorted. This turns each of these transformers into inductors that have identical ratings as the operating transformer. Continue to add additional "inductos" in series until the simmer supply performs well, but the transformer no longer over heats. In my experience, 2-3 of these "inductors is just about right.

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