Why can't I saturate this transistor?

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Discussion Overview

The discussion revolves around a user's difficulty in achieving saturation in a transistor circuit designed using Proteus Isis. The user is attempting to operate a BC547 transistor in the saturated region to drive an LED requiring 2.3V and 20mA, but is encountering issues with the collector-emitter voltage not dropping below 0.2V, indicating that the transistor is not fully saturating.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • The user calculated that a base current of 0.2mA should be sufficient to saturate the BC547 transistor, but is observing a collector-emitter voltage that does not drop as expected.
  • One participant questions the use of AC mode for meters in a DC circuit, suggesting that this could lead to inaccurate readings.
  • Another participant suggests increasing the series resistance to about 1kΩ, but the user reports that this causes the LED to turn off due to insufficient current.
  • There is a suggestion to increase the collector supply voltage to 12V, but the user finds that the current remains too low.
  • A participant explains that the collector-emitter voltage drop is not a fixed value at saturation and depends on the base drive, indicating that higher base drive can lead to a higher collector-emitter voltage drop at saturation onset.
  • One participant recommends building the circuit with real components instead of relying solely on simulation, noting that the accuracy of the simulation may vary.
  • Another participant points out that the datasheet specifies certain limits for the BC547 transistor, suggesting that the user is operating within those limits but still not achieving the desired saturation.
  • There is a discussion about the assumptions made by the Proteus program regarding the transistor's characteristics, with a suggestion to investigate further.

Areas of Agreement / Disagreement

Participants express various viewpoints on the issue, with no clear consensus reached. Some suggest adjustments to the circuit, while others question the simulation's accuracy and the assumptions made by the software.

Contextual Notes

Limitations include potential inaccuracies in the simulation software, assumptions about the transistor's behavior, and the dependence on specific configurations and component values that may not be fully accounted for in the simulation.

amenhotep
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Hi,
I started using Proteus Isis and was playing around just to get used to it. I designed a simple transistor circuit that should work in the saturated region. The LED needs 2.3V. For saturation, the collector and emitter terminals should be shorted or have a tiny voltage drop across it. So, since the LED requires 20mA, I calculated that the series resistance should be around 139. The BC547 transistor has a dc gain around 110 at that current. This means that a base current of 0.2mA or more should should be enough to saturate the transistor. The circuit is shown below. Whatever base current I used, ##V_{CE}## didn't get below 0.2V which indicates a transistor is near saturation. I've cross-checked values from the datasheet. Surely, something is terribly wrong !

2zsa051.jpg
 
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hi you
welcome to PF

why have you got all your meters in AC mode when they are in a DC circuit
Im not familiar with the inner working of Proteus, but maybe that's why you are getting oddball readings ?

Dave
 
davenn said:
hi you
welcome to PF

why have you got all your meters in AC mode when they are in a DC circuit
Im not familiar with the inner working of Proteus, but maybe that's why you are getting oddball readings ?

Dave

Thanks for the welcoming. I actually didn't notice that I was using the meters in the AC mode. I changed them to DC and the values were the same. Is something wrong with the circuit ?

23jg5xe.png
 
Try increasing R1 to about 1kohm
 
I've done all that. The current becomes so small that the LED turns OFF.
 
Try increasing the collector supply to 12 volts
 
amenhotep said:
I've done all that. The current becomes so small that the LED turns OFF.

yup that sounds about right for a 5V supply and ~ 20mA through the LED with a ~ 2V drop
200 Ohms give or take a bit is ok

the BC547 is going to turn on at ~ 0.6 - 0.7V and the 0.54 Vdrop across the C-E is indicating that its currently turned on.


the definition of saturation as in your common emitter case is ...

As base current increases the collector current increases to a point where any more base current will not increases the collector current at that point the transistor is saturated whereby current may flow in both direction and the two diodes will essentially be in a parallel mode and the voltage drop from emitter to collector will reflect the status as a low voltage drop.

as far as I can see, that's what is happening in your circuit
 
Last edited:
You also need to understand that the C-E Vdrop is not a fixed value at saturation ie. is isn't 0.2V and only 0.2V
it is dependent on the base drive, the higher the drive the higher the C-E Vdrop value at onset of saturation

cheers
Dave
 
I'd suggest building one from real parts. Your simulation's accuracy is at the mercy of a programmer someplace.

I looked for information on Proteus' parts library, to no avail.
However the BC547 is allowed (per a Fairchild datasheet at http://www.elektronik-kompendium.de/public/schaerer/FILES/bc550.pdf )
Vce as much as 600 millivolts at 100ma and 250 millivolts at 10 ma .
You're operating it in an area between those limits.

BC547 is a high voltage version of that transistor, 50 volt, so probably has a stout collector.
Try swapping it for a 30 volt BC548 and see if the program calculates any differently.
Then try a 65 volt BC546.

It's really educational to poke at things and learn their nuances..

old jim
 
Last edited:
  • #10
jim hardy said:
I'd suggest building one from real parts. Your simulation's accuracy is at the mercy of a programmer someplace.

I looked for information on Proteus' parts library, to no avail.
However the BC547 is allowed (per a Fairchild datasheet at http://www.elektronik-kompendium.de/public/schaerer/FILES/bc550.pdf )
Vce as much as 600 millivolts at 100ma and 250 millivolts at 10 ma .
You're operating it in an area between those limits.

BC547 is a high voltage version of that transistor, 50 volt, so probably has a stout collector.
Try swapping it for a 30 volt BC548 and see if the program calculates any differently.
Then try a 65 volt BC546.

It's really educational to poke at things and learn their nuances..

old jim

Thanks Jim. Tried different NPNs but can't bring VCE down enough.
 
  • #11
amenhotep said:
Thanks Jim. Tried different NPNs but can't bring VCE down enough.

Hmmmm ... one wonders what the Proteus program assumes about that transistor. Any details provided about that ?

I do notice Fairchild specifies Vcesat with Ic = 20Ib.
Fairchild's graph, figure 3 specifies Ic = 10Ib .
You're at 15.
Oughta work just fine.
How's your Vbe look compared to the datasheet ?

Poke at that simulation program a bit more... find its limits.
 

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