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Switching/circuit breaker circuits

  1. Jul 16, 2011 #1
    Hi guys, Im Thava. Im working in optical communication. Currently I want to design a circuit to turn of couple of inputs to my laser.

    Say I have a supply of 5V to two pins of the laser, anode and cathode (pin 1 and pin 2). And then a bias current to another pin (pin 3).

    At some interval of time, I want to switch off the 5 V input to cathode and of course anode is grounded; also switch off the input bias current.

    The switching off time should be in about 100 kHz, and it should be controllable. It should be in a way that, if I supply a pulse with pulsewidth of 100 kHz, it should switch off exactly at the falling edge of the 100 kHz. Is it possible?

    And the switching time should vary, like I can always change the switching time.

    I just need some suggestions, ideas and discussion. I have so many ideas on doing it, but cant choose a proper way, so I just need some guidance. It will be much appreciated!! Thank you.
  2. jcsd
  3. Jul 16, 2011 #2
    Yes, 100 Khz should be very doable, but keep in mind there will be some finite switching time. Do you have any requirements regarding the "off" and "on" resistances of the switch? What about the amount of current going through?

    The simplest type of switch just uses a single MOSFET - when you apply a gate voltage above the threshold, it has a very small resistance between source and drain. When you apply zero volts, it has a higher resistance.

    One can then build more complex circuits uses multiple transistors to give a lower on-resistance and higher off-resistance, but at the cost of a slower response.

    Sort of - there will be an upper limit on the frequency of the control signal. If you increase the frequency too high, a practical switch won't be able to keep up - it will always be somewhere between "fully on" and "fully off", because of the internal capacitance.
  4. Jul 18, 2011 #3
    Thanks very much for your reply!

    May I ask what you meant by finite switching time? At the moment, there is no requirements on on and off resistance, may I also ask why I need this?

    Current rating would be from 40 mA to 150 mA.

    Do you have any reference circuits or model that will feed into this applications? I need to a circuit to start with. Thank you.
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