simple ckt

Discussion in 'Homework Help' started by sharivan, Mar 19, 2013.

  1. sharivan

    Thread Starter New Member

    Mar 17, 2013
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    need some help.. im trying to do a circuit which utilizes a jfet, bjt and ldr. when no light strikes in the ldr, light turns on and when it senses light, it turns off.
    my problem is that, my ldr(R1. in the configuration i assumed ldr senses light) gets hot to the point that its legs turned yellow now. are the component values wrong? i know this maybe simple, but i dont get it. thanks alot!
     
    Last edited: Mar 19, 2013
  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    A circuit diagram would be very helpful.

    It sounds to me like you have the LDR connected directly between V+ and GND, and too much current is flowing through it, burning it up. We need to know how you connected it in order to know for sure, so please post the schematic you followed.

    Regards,
    Matt
     
  3. sharivan

    Thread Starter New Member

    Mar 17, 2013
    18
    0
    hello sir, here's the schematic
     
  4. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Hmm, odd--now it's showing up in the original post. I did not see it at first.

    The circuit looks okay to me, unless I am missing something. The problem must lie in how you set it up. Could you please take a photo of your current setup, the one that's giving you trouble? I think with that we should be able to figure out what is wrong.

    Matt
     
  5. sharivan

    Thread Starter New Member

    Mar 17, 2013
    18
    0
    sir, im sorry but i left my stuffs in our laboratory! probably, i can provide the picture later.
    i checked my setup many times, but the results were always the same. got worried about the ldr. anyways, thank you sir for helping! :))
     
  6. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    It occurred to me, if the FET is operating in saturation, or close to it, and there is a lot of light, current effectively flows directly through the LDR from V+ to GND, as I guessed before. Try putting a 220 ohm resistor in series with the LDR to limit the amount of current allowed to flow through it. This way, if the LDR has a very low resistance (lots of light hits it), there's still a resistance there to keep the amount of current at a safe level. Just a thought--I'm hoping someone else can verify this.

    I have never seen this circuit with a FET, I've always seen it with two BJTs. The BJTs drop about 0.7 volts between the base and the emitter. That 0.7v could make all the difference....

    I'm still thinking it has something to do with how you've connected it though. The suggestions I made at first go against many of the schematics I've seen for this circuit, so I'm not sure if they really carry any weight....
     
    Last edited: Mar 19, 2013
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