Transistor Discussion

[magpie]

Would anyone happen to know of a compatible replacement for the BC547B transistor?
I am building some signal generators (the Velleman type that nievasoliveras posted) and don't have BC547B transistors.

Thanks.

[qbjorn]

Since this is a transistor discussion I'll take a shot:
Is there no one here with some more enlightened thoughts about William Fogal's transistor, http://www.eskimo.com/~ghawk/fogal_device/ ?
According to Bearden it should contain some spectacular scalar & potential properties (like infinite bandwidth and gain), but of course it was silenced - well, isn't that a surprising development ...
There not enough information in his patents (actually 2 different patents on the same device) to duplicate the device without some further knowledge, but Fogal was supposed to have come up
with a prototype device at his kitchen table which implies that anyone here - almost at least - should be able to do the same.
We just need to know the secret ingredient ...

[electricme]

@ Gyula,
Thanks for the handy link and the explinations to my questions, you are very knowledgeable with transisters.

This is a good thread here, and I can see it will have a lot of hits as soon as people know about it.

One last question, would this thread consider putting up a number of www addresses in the beginning here before it gets so big that one has to read every post to find info on a certain transister? It might save a lot of regular posts for simple questions, leaving you to answer only those which are considered the more unusual and need clarification.

eg Geramimium  go to this http
     Silicon          go to this http
     MOSFETs    go to this http
     IGBTs          go to this http

20 odd years ago, I had access to the "Towers" data book which was considered at that time the best which had equilivents to each transister, is there a http address for this item?

Thanks again Gyula

General Jim

[gyulasun]

I've read this thread back and forth a few times now:  Is it as a result of a path of lesser resistance that the LED in the joule thief lights up?

Ok, let's discuss it once more.

Please study this link, http://www.ctglabs.com/soliton.htm  it deals with testing Naudin earlier work on a (Caduceus) coil, my point is he switches a 12V battery across a coil (now it does not matter it is a Caduceus, it is still a coil of certain self inductance value and this is all that now counts in the explanation from your question point of view).

The scope shot shows the 5V input pulse from a generator  (upper waveform, Naudin used a 10V pulse, Ctglabs used a 5V pulse) to operate Q1 as a switch and also shows the so called flyback pulse (lower waveform, Naudin calls it back EMF pulse) and you can see this pulse has about 56V peak amplitude, ok?   It is measured with respect to the negative battery polarity. Vertical voltage scale is 10V/Divison for both waveforms.  Flyback pulse is created whenever you interrupt the current flowing in a coil.

I edited the picture to show the pulse time durations when Q1 (IFR840 power MOSFET)  switches ON and OFF.  You can see that the 56V pulse appears across the drain and source points just after Q1 is switched OFF.  Because the LED in a joule thief is also connected betwen these two points (i.e. between collector and emitter) it is obvious that this voltage is able to forward bias a LED (or several LEDs in series). 

So whenever the transistor is just switched OFF in a joule thief circuit, the voltage level is just the flyback pulse between the collector and emitter where the LED is connected and current can flow through the LED because it "sees" a forward voltage polarity, the pulse's positive peak amplitude appears on the coil wire end that is connected to the collector, with respect to the emitter.  The battery is a short circuit from AC point of view and I mention this because the pulse originally appears across the coil,  and the upper wire connection of the coil that is connected to the positive battery pole can 'see' the emitter wire via the battery.

After the energy of this pulse is consumed by the LED,  the LED becomes dark but human eye cannot detect it because it will be ON again very soon (a question of repetition frequency like we can see an incandescent lamp to light continuously when it has no current flow in it at the zero crossings of the 50 or 60Hz AC mains voltage).  Notice the collector-emitter path has a high resistance value when the transistor is in a switched OFF state, hence no or very very little current can flow through it.

And whenever the transistor is switched ON, it means the collector point is pulled down to near zero voltage with respect to the emitter (just the saturation voltage could be measured by a scope)  hence the LED cannot see any forward voltage to draw a forward current i.e to light up, ok?  The transistor simply shunts the LED well below its forward voltage, current can flow through the transistor only. 

Now the answer to your question: why would the current rather flow through the Light Emitting Diodes rather than through the Collector Emitter path of the transistor?  is that in OFF state the transistor does NOT conduct current and the flyback pulse from the coil is able to forward bias the LED so that current can flow through the LED. 

Hope you can see it all now?

rgds,  Gyula

[gyulasun]

Would anyone happen to know of a compatible replacement for the BC547B transistor?
I am building some signal generators (the Velleman type that nievasoliveras posted) and don't have BC547B transistors.

Thanks.

Hi,  it is a pnp type,  the 2N3906 is a possible substitute I think.