Transistor conduction

WBahn

Joined Mar 31, 2012
25,267
Math was never my strong point lol
Then you need to start coming up to speed on at least basic math, such as algebra, as math is the language of both science and engineering. There's really no way around it.

You also should start digging into some introductory circuits material that covers things like Ohm's Law and Kirchhoff's Laws to begin with.
 

ronv

Joined Nov 12, 2008
3,770
WBahn I disagree with your theory that Vcc should be made from Vbe + Vrb since Rc comes into play somewhere I would think? although I could be wrong....I didn't measure Vrc on my second test but maybe that voltage added to the Vbe and Vrb voltage = the Vcc supply?

I am using a craftsman digital multimeter 82082....love this little meter!

Yes the negative is what I am referring to as ground, I am using a bench power supply, not a battery.
So Rb is measured from collector to base?
 

WBahn

Joined Mar 31, 2012
25,267
WBahn I disagree with your theory that Vcc should be made from Vbe + Vrb since Rc comes into play somewhere I would think? although I could be wrong....I didn't measure Vrc on my second test but maybe that voltage added to the Vbe and Vrb voltage = the Vcc supply?

I am using a craftsman digital multimeter 82082....love this little meter!

Yes the negative is what I am referring to as ground, I am using a bench power supply, not a battery.
Just as the height difference between two floors of a building can be measured either along the inside of a stairwell, up an elevator shaft, or up the outside of a building, so to can a voltage difference be measured by summing up all the voltage differences along any path between the two points of interest.

You still haven't told us exactly what measurement you are performing to measure what YOU are calling Vrb. Where are you putting the negative lead of the meter. Where are you putting the positive lead of the meter?

Thank you for telling use what meter are you using. You still haven't what range you are setting it on. You still haven't said if you are using a single meter or multiple meters to take the different readings. I've tried tracking down meaningful specs or, better, a manual for it online, but no luck so far. My guess is that it has an impedance of 10 MΩ on the voltage ranges, but I don't know that for sure. Do you still have the manual?
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
I used a single meter to take my readings and my base resistor voltage was measured from base of the transistor to the collector.

I am happy with what I achieved however and as far as I am concerned I have some decent numbers to put into a graph and study.

time to move on to transistor characteristic curves me thinks.

Neil.
 

WBahn

Joined Mar 31, 2012
25,267
I used a single meter to take my readings and my base resistor voltage was measured from base of the transistor to the collector.
Then you measured a voltage that has NOTHING to do with the base resistor. You measured Vbc, the base-collector voltage.

You need to decide WHAT you want to measure and then you need to set out to actually measure it.
 

ronv

Joined Nov 12, 2008
3,770
I used a single meter to take my readings and my base resistor voltage was measured from base of the transistor to the collector.

I am happy with what I achieved however and as far as I am concerned I have some decent numbers to put into a graph and study.

time to move on to transistor characteristic curves me thinks.

Neil.
Double check the value of your resistors. The gain of the transistor is off by about a factor of 10.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
Then you measured a voltage that has NOTHING to do with the base resistor. You measured Vbc, the base-collector voltage.

You need to decide WHAT you want to measure and then you need to set out to actually measure it.
The base resistor connects the base to the collector.....I measured across the base resistor directly.....from the positive supply to the base of the transistor....look at my circuit.
 

hp1729

Joined Nov 23, 2015
2,304
The base resistor connects the base to the collector.....I measured across the base resistor directly.....from the positive supply to the base of the transistor....look at my circuit.
Measuring across the base resistor gets the voltage between the base and VCC, which is proper. Base to collector doesn't get you much of anything of value.
 

BobTPH

Joined Jun 5, 2013
2,260
You should get in the habit of measuring everything with respect to ground, or you will confuse yourself and us. The exception would be to measure the voltage across a resistor in order to determine the current in that resistor, and then immediately convert it to current. The resistor better be smaller than 100K though, or the resistance of your meter will effect things significantly.

Your description of you reading of Vb confused us all. When you say you measured from the base to the collector that means one probe on the base and the other on the collector, yet it turns out that you actaully measured it from the base to Vcc, which is not the same as between the base and the collector.

Bob
 

WBahn

Joined Mar 31, 2012
25,267
The base resistor connects the base to the collector.....I measured across the base resistor directly.....from the positive supply to the base of the transistor....look at my circuit.
The base resistor does NOT connect the base to the collector. Look at your circuit. The base resistor connects the base of the transistor to the Vcc supply rail. The collector is NOT connected to the Vcc supply rail, it is connected to the collector resistor and the other end of the collector resistor is connected to the Vcc supply rail.
 

Thread Starter

Neil Groves 1

Joined May 3, 2016
21
The base resistor does NOT connect the base to the collector. Look at your circuit. The base resistor connects the base of the transistor to the Vcc supply rail. The collector is NOT connected to the Vcc supply rail, it is connected to the collector resistor and the other end of the collector resistor is connected to the Vcc supply rail.
I stand corrected sir.....my apology.
 

Plamen

Joined Mar 29, 2015
98
Not really homework although I guess it is as i'm doing this at home lol

I am playing with transistors building really simple circuits and taking measurements so as to get some idea of how tthey function in a circuit, I have a transistor which is fully turned on and I have the expected Base emitter at 0.67v, how can I turn off the transistor slowely whilst taking the voltage reading at this point so I can watch it fall as the transistor turns off please?

Neil.
Petkan:
If you have dual channel scope and a function generator apply a sawtooth voltage to the base (via resistor).
Synchronise the scope from the function generator and probe the B-E voltage
 

Plamen

Joined Mar 29, 2015
98
ok I just been staring blankley at this circuit for a while and I can't get my head around why when there is no current flow as in when the transistor is switched off why there would be a voltage across the transistor since no current flow means there cannot be a voltage present?

can someone give me a blow by blow account of what is going on here please?

sorry for being so dumb but this is very interesting to me and I want to understand.
Petkan:
All events in the circuit occur within the power frame. As the collector starts coming out of saturation the Vce starts rising from say 0.3V to 5V, while its complement, measured in respect of the other rail (right across the resistor) declines from 9V to 4V.
Vce = Vcc-I.Rc i.e. it is the complement of the resistor voltage to the rail Vcc.
The behavior of your transistor is normal, you just did not continue to completely shutting the transistor. You say voltage across collector resistor drop from 9V to 4V and had you continued raising base resistor value - you would have see nothing across the collector resistor i.e. full 9V across the shut transistor. (second Kirkhoff law)
 
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