darlington transistor problem Please Help

Discussion in 'General Electronics Chat' started by samridh, Apr 30, 2012.

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  1. samridh

    Thread Starter Active Member

    Apr 25, 2010
    Hi everyone,

    I am supposed to make a replacement circuit for an original design.
    I have attached a picture displaying both. Please have a look at it.
    Opening the picture before reading further may help understanding what I am trying to explain

    In the original one it uses NOT gate to turn on a PNP transistor to glow LED which then turns the darlington pair MPSW13 to glow the bulb attached to the collector.

    The darlington pair that they have used is obsolete now so I went ahead and replaced it by MPSW45 darlington transistor.
    I am not allowed to use NOT gate so I am using a NOR gate which out;put is connected to a NPN transistor to glow the LEd and also to the darlington pair to glow the bulb at same time.
    The bulb being used by both is a 28V rated mianiature lamp 3A rated

    But there is problem in a particular spot at Q3. the darlington pair MPSW45 gets extremely hot. In one of the case it blew off as well and also once the 4 ohm resistor got so hot it almost melted.
    When I do calculation it doesnt make any sense to me.

    Am I using a bad replcament for the original transistor. WHen I compare the specs from datasheet it appears fine as the one I am using has higher ratings and bigger range than original.

    The only concern that I have is regarding the hFE(Beta) value as shown,
    For MPSW45(I am using)
    DC Current Gain hFE
    (IC = 200 mAdc, VCE = 5.0 Vdc) 25,000(min)150,000(max)
    (IC = 500 mAdc, VCE = 5.0 Vdc) 15,000(min)
    (IC = 1.0 Adc, VCE = 5.0 Vdc) 4,000(min)

    For MPSW13(original)
    DC Current Gain hFE
    (IC = 10 mAdc, VCE = 5.0 Vdc) 5,000
    (IC = 100 mAdc, VCE = 5.0 Vdc) 10,000

    I am not being able to figure out what this hFE is effecting in my circuit and also if this is the problem? If anyone can help regarding explaining what does this actually mean and if this is the corrrect problem.

    If you think something else is going on please reply back so I can at least have a direction to go. Cuz right now I am basically stuck.

    I am a newbie so plz any detailed explanation would be great. I have spent hours but it works for 30 secs and then the transistor gets so hot I have to shut the power off to save the transistor or let it blow after sometime.

    Thanks for any input and help.
  2. bertus


    Apr 5, 2008
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