Electronics > Beginners

transistors

<< < (9/10) > >>

tooki:

--- Quote from: Zero999 on August 16, 2019, 08:14:35 pm ---
--- Quote from: tooki on August 16, 2019, 06:53:52 pm ---
--- Quote from: Zero999 on August 15, 2019, 09:41:16 pm ---Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth. (Attachment Link)

--- End quote ---
Your example image is very hard to read, whereas the OP's scanned (not photographed!) original is easy to read. So no, please don't do this!

--- End quote ---
I admit it wasn't my best attempt, but I find the lack of contrast in the original difficult to read. More importantly I forgot to add the base resistor to Q2.
(Attachment Link)

--- End quote ---
That’s awful, too. The OP’s scan isn’t perfect, but there has to be a happy medium between it and the mock faxes you create. Crushing the image to 1-bit black and white produces maximum contrast but low detail with tons of jaggies that make reading much harder.

Zero999:
I disagree. I find high contrast much easier much easier to read, than fuzzy greyscale.

--- Quote from: queennikki1972 on August 14, 2019, 11:53:02 pm ---I will try to the circuits again in Getting Started in Electronics.

--- End quote ---
Beware, those books contain numerous errors in the schematics. The author, Forrest Mims, doesn't have a formal training in electronics. Don't get me wrong, I found his books informative and he has some great ideas, but his lack of education does show. It's a shame they didn't get someone with more experience to proofread the books before they were printed.

Jwillis:

--- Quote from: fourfathom on August 17, 2019, 02:11:14 pm ---
--- Quote from: Jwillis on August 17, 2019, 08:12:50 am ---When the LED is off only 0.6 volts is passing from emitter to base .During the off state the capacitor is charging. When the capacitor discharges a, negative current flows from the base of the PNP via the NPN .That current is passing through 100K resistor so at the base of the NPN the current is only 0.09mA. thats like 18ma at the base of the PNP.When the base of the PNP goes negative it switches on and most of the current through the LED is also passing through the emitter and collector which lights the LED .Very little current ever passes through the base of the PNP transistor.

--- End quote ---

I do understand how this relaxation oscillator works.  That 100k resistor is a variable one so I figured that 10K would be a reasonable minimum value.  If we limit the range to 50K and up then I agree that the with the likely range of NPN gain the PNP base current (and LED current) won't be too bad.  I still don't like relying on NPN gain specs to limit this current, but perhaps that's just excess paranoia on my part.

--- End quote ---

4 modes to any BJT  .Cut off ,saturation,active and reverse .

                                      NPN                                                                                                               PNP
VE < VB < VC          Active Forward (current flows from C to E proportional to B current)             Active Reverse (current flows C to E proportional to B current)
VE < VB > VC          Saturation (Current flows freely from C to E)                                                 Cutoff (No current flows from E to C )
VE > VB < VC          Cutoff (No current flows from C to E)                                                             Saturation (Current flows freely from E to C)
VE > VB > VC          Active Reverse(Current flows from E to C proportional to B current)             Active Forward(current flows from E to C proportional to B current)

The 10meg resistor provides bias to the NPN .The current and voltage is so low that the PNP never goes into saturation. The Diode limits feedback to the NPN Base .While the capacitor is charging The PNP is in cutoff mode because VE < VB > VC .
The PNP must go Active Forward  to light the LED( VE > VB > VC).   
I've never  been very good at writing things to make them understandable.This explains all those low marks in English composition.
All I can say is try the circuit and put t to the test.

fourfathom:

--- Quote from: Jwillis on August 17, 2019, 09:19:41 pm ---The 10meg resistor provides bias to the NPN .The current and voltage is so low that the PNP never goes into saturation. The Diode limits feedback to the NPN Base .

--- End quote ---
That "Flash Rate" resistor also provides bias (or current) to the NPN when the PNP begins to conduct.  This is part of the positive feedback loop.  The diode clamps the DC voltage at the NPN base (clamps at about -0.6V), protecting the base from reverse breakdown and ensuring that the base drive (from the "Flash Rate" resistor) will be adequately positive.  Without that clamp diode the NPN base-emitter junction would clamp and the capacitor C1 would charge negative, limiting the NPN base current.  The diode actually enhances the feedback to the NPN base.  By the way, I see the PNP going heavily into saturation -- about 0.02V VCE at the start of each flash.

I suggest that you simulate it to check the transistor and LED currents.  I had mentioned that with my sim I needed to increase the "Off Delay" resistor to 20M in order for the circuit to oscillate.  Not today though -- even 5M works.  I double-checked that PNP base current again -- still quite high with the 10K "Flash Rate" resistor.

mikerj:

--- Quote from: Jwillis on August 17, 2019, 09:19:41 pm ---The 10meg resistor provides bias to the NPN .The current and voltage is so low that the PNP never goes into saturation.

--- End quote ---

This is an oscillator, it has positive feedback.  Think about what happens to the NPN base current when the PNP transistor switches on.  Then consider the resulting current flow through the LED and the BE junction of the PNP.

I'm sure the current averaged over the entire on time of the LED is reasonable, but for lower values of the 10k pot the peak current will be much higher than you are trying to set with the 470R resistor.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod