ВЭФ 12 (Astrad VEF 12 Project) - including a transistor tutorial

[Mark Hennessy]

NM,

You might have missed my earlier question: We know the value of the emitter resistor, and we know the voltage across it. So what does that tell us?

This is the key to solving the riddle. We're very, very close

If it helps, we are using the most basic of principles here. No magic, no smoke and mirrors.

Please, don't be afraid to "state the obvious". The obvious is usually the answer I'm seeking - I generally don't believe in trick questions (though some teachers derive a perverse pleasure from that).

[Nowhere-Man]

Just let me print the whole thing out at the library and I'll give you the whole breakdown. Diode drop always has been an issue in diode isolators where battery banks are charged. It affects the 14 volt charge from alternators so you need compensation. When I print it out tomorrow I'll give you my whole take on it.

[Nowhere-Man]

There are times I get engrossed in media affairs. I've been very curious over the French elections. In fact I was up till 5.00 am. Trying to figure out who will win.

[Nowhere-Man]

O.K., I just printed an enlarged copy of Mark's diagram at the library. This is much better for me than trying to follow the thread on a 2 inch mobile screen. I will be back on this topic with a full breakdown of the circuit, to the best of my understanding so far.
Apart from that, I found today these P and MP type transistors even in Russia are only available as NOS. The Russian hobbyists themselves appear to be using substitutes.
Oh, on the diagram I can see the - and + bias at each junction. With PNP types I know the emitter base junction will have a + bias while the collector base junction will be negatively biased. So resistance at the base collector junction will be pretty big.
On Mark's diagram the 47 K and !0 K have obviously been chosen to set the correct bias.
That's it till later.

[Nowhere-Man]

Diagram of Soviet PNP transistor connections. All statistics on this page link. Emitter at top here and collector in middle:
http://radioelpribori.ru/p422-p423.html

[Nowhere-Man]

Now I can see with the large diagram another voltage divider which runs between + and - and the collector emitter terminals. Thus we have the divider of 8.8125 and 0.1875 (one resistor being just 1K. ) As said before the drop of 0.6 volt is significant. However, I am pretty hopeless on transistors and am sure it gets quite deep as you go into it.

[Mark Hennessy]

The 4k7 and 1k resistors are not a voltage divider.

If you go back over my recent messages, you will see that I have been asking careful questions to lead you down a very specific path which will lead to understanding. I don't expect you to stare at the diagram and figure it all out yourself. Each of my questions was designed to get you there in very small, very manageable steps. I'm afraid that you haven't been a good pupil because you keep bringing other subjects up rather than focusing completely on the question (but trust me, I know it's not always easy). So, now that you have the paper in front of you, there are no more barriers in the way. Let's have one more try.

1. Earlier, we decided that the 10k and 47k resistors are putting 1.6V on the base. That's a simplification, but an entirely valid one.

2. Next, we decided that there is 1V at the emitter.

3. I asked a really simple question. I will ask it again. If we know the voltage across a resistor, and we know the value of that resistor, what do we also know?

Please answer that question. Nothing else. It requires only the most basic of theory - it is not a trick question - and doesn't even need the schematic in front of you - I am asking about one resistor only. This is the penultimate step in our journey - we're nearly there




(Sorry to be firm, but sometimes a pupil needs that. Trust me, I've done this before )

[Nowhere-Man]

"If we know the voltage across a resistor, and we know the value of that resistor, what do we also know?"

Clearly, current values can be found.

I am going to be honest here and make some observations but first....:
I didn't spot the 4K7 and had to look it up. I was informed it indicates 4700. I have honestly never seen that before but, as stated, I use dated textbooks. So the RL is the load resistor. It drops 9 volts to 7.4 and the 1K reduces to 1.5. Both resistances are in series. The 47 K and the 10 K give 7.4 and 1.5.
So what's the problem? Well, any valve example would give current values with the load. Say, 50,000 R, a curve in milliamps and a voltage. For example -5 bias, 50,000 R load and, say, 0.002 amp. Plucking currents just from resistance and voltage without impedance values or a q bias quoted is unusual for voltage amplification.
Conclusion: This system may work fine with most students but not for me. I can say I had the same issues with biology classes and had to change the approach (because the first exam I flunked).
Another thing is I prefer old textbooks and find the graphs and tables easier.

[Murphyv310]

Basic electrical theory doesn't change and whether you are reading textbooks from the 20's 30's or the 70's these basic laws are the same.
I don't want to jump into Mark's work which is second to none but I feel NWM should concentrate on what Mark is setting out for you to do rather than bring in what you have read or are doing elsewhere, it is only confusing the matter both for you and your Tutor. I'm sure if you were in the classroom you'd have to follow the guidelines of the tutor as class progress would be hindered.
I understand you have a enquiring mind but getting confused by doing things the wrong way can lead to misconceptions about the correct way things are done and then it's harder to get the misconceptions eradicated from your mind. You'll not get a better tutor than Mark so take on board his help and leave everything else aside.

[pwdrive]

So what's the problem? Well, any valve example would give current values with the load. Say, 50,000 R, a curve in milliamps and a voltage. For example -5 bias, 50,000 R load and, say, 0.002 amp. Plucking currents just from resistance and voltage without impedance values or a q bias quoted is unusual for voltage amplification.

Hi there NM, Marks question wasn't about voltage amplification, that's a separate subject, the question was about determining the DC voltages and currents in the circuit that was provided from the information shown in the circuit.

I suspect the question was designed to see if you had a good basic understanding of DC current flow and voltage drop in a resistive circuit and the voltage drop across the base/emitter junction of a transistor, I suspect that you do but you need to put things across in a logical, reasoned and presentable manner otherwise it gives the impression that you don't understand.

Lawrence.