Isolation xfmr

Discussion in 'General Electronics Chat' started by d cook, Feb 8, 2005.

  1. d cook

    Thread Starter New Member

    Feb 8, 2005
    I have a lighting power supply circuit for a 747 aircraft and have a question
    about an inrush current caused by the incandscent lights load.
    From what i have read, an incandescent light has an inrush current(due to
    inductuion)of as much as 10x the normal resistive for 10-30 milliseconds.Does
    a switch on the primary side see that inrush current?
  2. dragan733

    Senior Member

    Dec 12, 2004
    Something I don't understand. This incandescent lamp is also from 747 aircraft?. Only one lamp does this inrush current or all lamps.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    Yep, the switch has to handle the inrush current. That's one of the more endearing features of tungsten loads. The halogen lamps are even worse. That's why the market in high output white led's is going to be big - assuming the output can be improved (emission and wafer yield both).
  4. David Bridgen

    Senior Member

    Feb 10, 2005
    No. It is due to the fact that the filament's resistance is non-linear, i.e. the value depends upon the current and is at its lowest when no current is flowing.

    When voltage is first applied across such a filament its initial resistance will be very low in value, hence the high current. Once current is flowing the resistance increases and the current therefore decreases.

    It is because of this non-linear resistive characteristic that small filament lamps have sometimes been used instead of a thermistor in the negative feedback loop of Wien oscillators to stabilise their amplitude.

    n.b. the next posting in this thread, by pebe, prompts me to add the following:

    A tungsten filament's resistance is dependent on its temperature, and the higher the current through it the higher its temperature will be.

    Thank you pebe for reminding me.
  5. pebe

    AAC Fanatic!

    Oct 11, 2004
    Strictly speaking, it depends on the lamp filament temperature.
  6. David Bridgen

    Senior Member

    Feb 10, 2005
    Quite correct.
  7. torpedopudding


    Feb 6, 2005

    To get back to the original question which concerned inrush current:

    There are ways to deal with it. Filament temperature is the big variable, so if you preheat the filament this will reduce the inrush. If you have a switch in series with the lamp, then an appropriate value resistor in parallel with the switch will keep the filament hot and reduce the inrush. This was commonly done when incandescent lamps were used in computer equipment eons ago.

    Another approach is to slow the rate of lamp turn-on. If you switch it with a transistor, this can be done easily but be aware that the transistor will dissipate power during the slow switching interval.

    Or you could first switch the lamp on in series with a resistor which would cause a dim glow, and after a short time (maybe half a second) apply full power.
  8. torpedopudding


    Feb 6, 2005
    Wait until high intensity plaid and camouflage LEDs hit the market!