I am trying to figure out what is wrong with this.

Hextejas

Joined Sep 29, 2017
187
I have posted about this schematic before but for different reasons. Now I am trying to figure out why 1 reading is not correct.

The readings are VC and IC.

In trying to solve this myself, I read this in the education section here.
"electrons can easily flow from the p-type base to the n-type collector as long as the base-emitter junction is forward biased (meaning the base is at a higher voltage than the emitter)."
So I started taking readings to see if I could figure it out. The readings that I got were:
IB = 8.3 uamps. Yay, 1 right !
IC = 00 ???? or so small that I can't measure it.
VC = 4.2 mv ?????
Supply = 9.1v
C-E = 4.2mv
B-G = .501v
C-G = 4.2mv
E-G = 0
G is ground.
BEG is base emitter collector and I think that I have them connected properly.

I still don't have a 1meg pot so I am using a 1meg resistor.
So, where and how do I start measuring so as to get VC and IC closer to what the thread predicts.

Thanks

AlbertHall

Joined Jun 4, 2014
11,598
If the circuit is connected as shown then the collector of the transistor is at 0V (near enough) so the full supply voltage appears across the milliammeter and the 510Ω resistor in series and the milliammeter should be very low resistance so you should have almost 9V across 510Ω or approximately 18mA flowing.

Check the voltage at the junction of the resistor and the meter.
If that reads near zero then the problem is with the meter or its connections.
If it reads near supply voltage the problem is with the resistor or its connections.

BobTPH

Joined Jun 5, 2013
4,034

Bob

hobbyist

Joined Aug 10, 2008
890
Anytime you come across a situation like this, always the first thing to do is remove all ammeters, then hook up the circuit and take only voltage readings, that will ensure the components are hooked up properly, and are in working condition.

Unless you have very top quality ammeters, ammeters settings can affect the amount of current they allow to pass through, when working with very small currents in the uA region it is probably better to take voltage readings across bias resistors.

Hextejas

Joined Sep 29, 2017
187
That was it and now I can forge ahead.
Thanks folks.
Now to plug in a 1meg pot and see what happens.
Hmmm, are there different kinds of pots and what should I look for ?

George KG5TKY

WBahn

Joined Mar 31, 2012
26,398
That was it and now I can forge ahead.
Thanks folks.
Now to plug in a 1meg pot and see what happens.
Hmmm, are there different kinds of pots and what should I look for ?

George KG5TKY
Doesn't really matter given what you are trying to do. For most basic work what is called a "linear taper" will probably have more utility for you than an "audio taper".

Hextejas

Joined Sep 29, 2017
187
I have one more question, I hope. And it's about the potentiometer.
I wired in the pot and started measuring and calculating b? Is that beta ?
I took hobbyist advice and took out the ammeters.
Well, I think that the pot was doing weird stuff as the readings were all over the place.
So, I went back to 3 resistors only.
1meg, 760k, and 550k and the readings for b were 229, 228, and 231. I call that a success.
I found it interesting in that if I were to graph the miliamps and microamps against the resistance, they tracked it, though in the opposite direction.
As one went up, the other went down. I guess the transistor was doing his job.
Also based upon the resistor value, they seemed to be proportional.
IE, twice the resistance = 1/2 the microamps and 2x the miliamps.
I call this a success, oh yeah, I guess the pot was messing me up.
Onward !

Last edited:

WBahn

Joined Mar 31, 2012
26,398
I have one more question, I hope. And it's about the potentiometer.
I wired in the pot and started measuring and calculating b? Is that beta ?
Depends on what you mean by 'b'. How did you calculate it?

I took hobbyist advice and took out the ammeters.
Well, I think that the pot was doing weird stuff as the readings were all over the place.
If it's a pot that's worn, they can be flaky. How are you wiring it up? Be sure to connect the wiper to one of the terminals in order to make a two terminal device (commonly known as a "rheostat").

So, I went back to 3 resistors only.
1meg, 760k, and 550k and the readings for b were 229, 228, and 231. I call that a success.
Sounds reasonable and that's a good indication that you are calculating the beta.

I found it interesting in that if I were to graph the miliamps and microamps against the resistance, they tracked it, though one in the inverse.
As one went up, the other went down. I guess the transistor was doing his job.
Also based upon the resistor value, they seemed to be proportional.
IE, twice the resistance = 1/2 the microamps and 2x the miliamps.
This I don't follow. If the microamps (by which I assume you mean the base current) goes down, then the milliamps (by which I assume you mean the collector current) should go down by the same amount.

If they don't, then how can you explain getting beta values that are nearly constant over your range of base resistor values.

Hextejas

Joined Sep 29, 2017
187
WBahn, I said that wrong. The currents went the opposite way of the resistance.

WBahn

Joined Mar 31, 2012
26,398
Ah, I see what you meant to say, now. Thanks for the clarification.