Power dissipation of transistor

Discussion in 'General Electronics Chat' started by HarrisonG, Aug 13, 2016.

  1. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    I need some help. I'm trying to calculate the power dissipation of an NPN transistor. I know that the formula is P=VCE x IC + VBE x IB. But in my case all the voltage falls across the collector resistor RC and the collector voltage is 0V. So what is VCE in that case? I know this is inperfect world and that there is some tiny amount of voltage drop across the collector-emitter junction, but what is it?
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You would need to check the datasheet for the transistor - they are not all the same and it varies with base and collector current.
     
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  3. WBahn

    Moderator

    Mar 31, 2012
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    What is the emitter voltage?

    Vce is, by definition, Vc - Ve. So just knowing that Vc = 0 V does not tell us what Vce is because if Ve happened to be, for instance, -10 V, then Vce would be 10 V.

    If Ve is 0 V, then you have a circuit that won't work since you can't get any collector current if Vce = 0 V and, without any collector current, then you don't have ANY voltage dropped across the collector resistor.

    It sounds like you are trying to run the transistor in saturation, in which case you will have Vcesat across the collector-emitter junctions. As a quick rule of thumb, Vcesat is often taken to be about 200 mV to 250 mV, but the best thing is to look at the transistor data sheet where they will usually provide a chart giving Vcesat under different conditions, so you just find the condition that is closes to yours and use that (or interpolate between the two conditions that straddle yours).
     
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  4. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Yes, Indeed! I'm trying to realy run a transistor in saturation mode and I need the collector to be at a voltage less than the needed to turn on another transistor aka im talking about transistor Not gate. It's nothing new to me, I have done such circuits before, but on this datasheet Screenshot_2016-08-13-23-42-47.png you see Collector-emitter saturation voltage is 1 volt and so if I am to connect another transistor to the output of a Not gate, since it turns on with viltages of 0,7v, and the Vce saturation of the first transistor is 1volt, then there is no way to turn off the seccond transistor since it will always receive atleqst 1 volt. Thank you, by the way, you answered my first question :D
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    But that 1V is specified with a collector current of 500mA (in the test condition column) and I presume you running it at a significantly lower collector current. This datasheet http://users.ece.utexas.edu/~valvano/Datasheets/PN2222-D.pdf includes this graph which lest you get a value for the collector current you are using.
    upload_2016-8-13_23-31-40.png
     
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  6. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Oh, I see now. So I must use it with current lesser than 500mA and everything will turn on and off fine. And if say according to your datasheet, my collector current is 150mA roughly, then the voltag drop across VCE will be 0,3-0,4v and I use this voltage to calculate the power dissipation of the transistor.Thanks!
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you can have a higher base current (between 3 and 10 mA), the dissipation will be much less, as the Vce will be about 0.1 volt.

    Bertus
     
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  8. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    That's because the transistor will be turned on more right?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you want to use a transistor as a switch, they even recommend a base current of 1/10 of the current to be switched.
    This is used to drive the transistor into saturation.

    Bertus
     
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  10. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Thanks!
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    What is it that you are really trying to do? What kind of load are you driving? It may well be possible to work with collector and base currents that are much smaller than the values you are talking about.
     
  12. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    2
    Well, It's about transistor NOT gate that I am talking about. Im just going back, fortifying my bjt knowledges, ie i've done countless of these circuits. And yes, the collector current can realy be much lesser than 500mA. I've looked a couple of different datasheets and I think I stumbled upon a power bjt the first time. Thats why it gave me Vcesat max 1V in the datasheet. Some other transistors like BC258C seems to be made for smaller currents and they have a Vcesat max of 0,2 A. And I guess it, the first transistor I mentioned was made to drive motors in H bridges or stuff like that and if I need a transistor to do logic gate, I better use the BC258C. I'm not building anything by the way, it's all just theoreticaly.
     
  13. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You could quite properly use the '2222 transistor at low collector currents to - it will wok well at 1mA or even 100uA.
     
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  14. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    It's a pretty good choice indeed
     
  15. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    2
    So with a risk to spamm my own thread, I got to say that I tested this on a circuit. IMG_20160814_155623.jpg The point of this is that T1 is a ZTX601Darlington transistor with Vcesat = 0,75 to.0,8 volts. T2 is turned on when 0,6 volts are applied to the base. The point is that when you turn T1 on, the voltage at the base of T2 will fall bellow the minimum turn on voltage and the Led will light off. But since T1 will never give a collector voltage less than 0,7 volts, there is no way to turn the led off. And it worked just as expected. So what I've learned from all of this is that if you want to build circuits like this one, you will need to take in mind that Vcesat max should be less than the minimum turn on voltage of a transistor. For example the T1 would have done it's job to turn off T2 if it was PN2222A with a maximum of 0,3 volts across the collector-emitter junction.
     
  16. dl324

    Distinguished Member

    Mar 30, 2015
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    You could put a diode on the emitter of T2 which would increase it's turn on voltage and adjust Rc2 to give the desired LED current.
     
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  17. dl324

    Distinguished Member

    Mar 30, 2015
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    Using a split darlington would give you current gain without the saturation voltage limitation:
    upload_2016-8-14_7-25-31.png
    But this requires using discrete transistors. SMBT3904 is a two discrete transistors in one package.
     
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