transistors

[ledtester]

On the subject of transistors, when it comes to learning how to analyze circuits involving semiconductors, for the beginner/hobbyist I highly recommend either of these books - both by Albert Malvino:

Transistor Circuit Approximations
Semiconductor Approximations: An Introduction To Transistors and Integrated Circuits

Used copies may be found on Amazon for cheap.

They are very practical books with good worked examples. The Art of Electronics is excellent but kinda skimpy on worked examples - largely, I believe, because they expect that to be provided for by teaching assistants in a recitation class.

[fourfathom]

I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.

This circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground.  Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base.

[floobydust]

OMG dusted off my 1980 Engineer's Notebook by Forrest Mims III. It was a real hoot building all that stuff, never had so much fun. The lamp flasher  was also our high school electronics class "strobe" light project, where we made our own pcb and cut sheet metal for the enclosure. I later turned evil and made it drive a car ignition coil...

It looks like it has a few weaknesses, like needing a resistor to limit current through the potentiometer so it does not smoke, the electrolytic capacitor polarity might be arguably flipped, and I would add a second resistor to limit base current for Q2.  This sch is the corrections I have in mind.

[tooki]

Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth. (Attachment Link)

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!

[Zero999]

Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth. (Attachment Link)

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!

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.

[floobydust]

The flasher circuit is also on Wikipedia https://en.wikipedia.org/wiki/Forrest_Mims#Author

[james_s]

I remember seeing a very similar flasher circuit in those warning lights they used to put on construction barricades. The old ones has an incandescent lamp and used a pair of 6V lantern batteries in parallel.

[Jwillis]

 

I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.

This circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground.  Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base.

The current to the LED is limited by the 330-470 ohm resistor.Because the LED has a forward voltage of 1.7V the current it draws is  I= (9-1.7)/330= 22ma ,Although that seems high the actual is lower because the battery is not at full potential (8.5V) . That puts the current at around 20mA . Since the LED is on a pulse ,short bursts of slightly higher current isn't hurting the LED .This has been measured and confirmed .The 470ohm resistor would be better limiting the LED to around 15.5mA .This is also the reason that no limiting resistor is require at the base of the PNP transistor.Because only 20mA is being drawn across the transistor the current at the base is very small.About 0.5mA has been measured at most.

[floobydust]

In the circuit https://www.eevblog.com/forum/beginners/transistors-204336/msg2618643/#msg2618643, I think the off-delay potentiometer needs a current-limiting resistor and I still see a high-current path through the LED from base current (E-B junction) for Q2. If Q1 ever stays on, the LED is roasted.

This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright.

[Jwillis]

In the circuit https://www.eevblog.com/forum/beginners/transistors-204336/msg2618643/#msg2618643, I think the off-delay potentiometer needs a current-limiting resistor and I still see a high-current path through the LED from base current (E-B junction) for Q2. If Q1 ever stays on, the LED is roasted.

This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright.

It was a quick thrown together circuit but your right if the resistance gets to low on that pot it would cause the LED to fail .But as I mentioned any value less than 3Meg theirs no visually noticeable delay. So it may be better to have a 3Meg resistor and a 5Meg pot .If a faster flash is required then lowering the value of the cap would do that.  I had it oscillating with a 470pF chiclet. Not noticeable to the eye but just wanted to see what was going happening on the scope.
Few extra resistors here and there probably won't hurt performance .

[fourfathom]

I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.

This circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground.  Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base.

The current to the LED is limited by the 330-470 ohm resistor.Because the LED has a forward voltage of 1.7V the current it draws is  I= (9-1.7)/330= 22ma ,Although that seems high the actual is lower because the battery is not at full potential (8.5V) . That puts the current at around 20mA . Since the LED is on a pulse ,short bursts of slightly higher current isn't hurting the LED .This has been measured and confirmed .The 470ohm resistor would be better limiting the LED to around 15.5mA .This is also the reason that no limiting resistor is require at the base of the PNP transistor.Because only 20mA is being drawn across the transistor the current at the base is very small.About 0.5mA has been measured at most.

It's not the current through the LED that I was commenting on, but the current through the PNP emitter-base junction and NPN collector-emitter path.  When the PNP is in saturation, that base current has little to do with the collector current, but is limited only by how much current that NPN transistor is pulling.  The NPN transistor also has no current limit other than the base current multiplied by the current gain since there are no resistors in the  PNP/NPN chain.  Hey, it probably works fine in practice, but since we're critiquing circuits here I thought I would mention this issue. 

I used LTSpice to test this (with 2n3904 NPN and 2n3906 PNP), just to see how much of an issue this actually is.  I had to increase the "off delay" resistor to 20M before it would oscillate, but it did oscillate.  As expected, the NPN collector current is influenced by the "flash rate" resistor value, but setting that at 10K gives me 100mA spikes.  That's a lot of base current for that PNP, but probably not damaging.  That current is also flowing through the LED, since that 330 Ohm resistor is not in that path.  Once the NPN comes out of saturation then the 330 resistor comes into play.

I changed the "flash rate" resistor to 50k and the PNP base current spike was reduced to around 25mA.  The LED current spiked up to 42 mA then stabilized at 16 mA during the flash. 

I added a 1K resistor between NPN collector and PNP base and then the currents were as you might expect: PNP base current 4 mA, and LED current 16-21 mA.

No big deal, I just prefer to not have these type of currents dependent on transistor parameters.

Since I can't figure out how to embed your schematic, here's a link to it: https://www.eevblog.com/forum/beginners/transistors-204336/?action=dlattach;attach=811830;image

[Jwillis]

I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.

This circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground.  Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base.

The current to the LED is limited by the 330-470 ohm resistor.Because the LED has a forward voltage of 1.7V the current it draws is  I= (9-1.7)/330= 22ma ,Although that seems high the actual is lower because the battery is not at full potential (8.5V) . That puts the current at around 20mA . Since the LED is on a pulse ,short bursts of slightly higher current isn't hurting the LED .This has been measured and confirmed .The 470ohm resistor would be better limiting the LED to around 15.5mA .This is also the reason that no limiting resistor is require at the base of the PNP transistor.Because only 20mA is being drawn across the transistor the current at the base is very small.About 0.5mA has been measured at most.

It's not the current through the LED that I was commenting on, but the current through the PNP emitter-base junction and NPN collector-emitter path.  When the PNP is in saturation, that base current has little to do with the collector current, but is limited only by how much current that NPN transistor is pulling.  The NPN transistor also has no current limit other than the base current multiplied by the current gain since there are no resistors in the  PNP/NPN chain.  Hey, it probably works fine in practice, but since we're critiquing circuits here I thought I would mention this issue. 

I used LTSpice to test this (with 2n3904 NPN and 2n3906 PNP), just to see how much of an issue this actually is.  I had to increase the "off delay" resistor to 20M before it would oscillate, but it did oscillate.  As expected, the NPN collector current is influenced by the "flash rate" resistor value, but setting that at 10K gives me 100mA spikes.  That's a lot of base current for that PNP, but probably not damaging.  That current is also flowing through the LED, since that 330 Ohm resistor is not in that path.  Once the NPN comes out of saturation then the 330 resistor comes into play.

I changed the "flash rate" resistor to 50k and the PNP base current spike was reduced to around 25mA.  The LED current spiked up to 42 mA then stabilized at 16 mA during the flash. 

I added a 1K resistor between NPN collector and PNP base and then the currents were as you might expect: PNP base current 4 mA, and LED current 16-21 mA.

No big deal, I just prefer to not have these type of currents dependent on transistor parameters.

Since I can't figure out how to embed your schematic, here's a link to it: https://www.eevblog.com/forum/beginners/transistors-204336/?action=dlattach;attach=811830;image

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.
 

[AVGresponding]

Unfortunately the only 100k variable resistor i had was used pull from old stuff and was a little flaky.
I do have one of the component testers as well. And the parts test ok before using.

There's nothing unfortunate about salvaged parts! It's a great way of building a nice collection of general purpose components, some of which can be horribly expensive if purchased new.

Parts recovered from decent quality dead equipment can often be more reliable than new parts from those cheap ebay Chinese suppliers.

If you're really lucky, you'll encounter the kind of dumpsters Dave Jones seems to find on a regular basis xD

[Psi]

When experimenting/learning to use transistors, make sure you have a good quantity of them.
They are cheap, so you can get like 100 for $1 on aliexpress, and even on digikey they are cheap.

You want to avoid the annoyance of giving up after 2 hours trying to get one transistor to work only to find you accidentally blew it up 1.9 hours ago.
So when in doubt, grab a new one

[fourfathom]

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.

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.

[tooki]

Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth. (Attachment Link)

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!

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)

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.

I will try to the circuits again in Getting Started in Electronics.

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]

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.

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.

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]

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 .

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]

The 10meg resistor provides bias to the NPN .The current and voltage is so low that the PNP never goes into saturation.

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.

[james_s]

This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright.

I've never seen one with a bimetallic self flashing bulb, maybe the really old ones worked that way but all the old ones I've seen used a little circuit with 2 transistors to do the flashing, they also had a CdS cell to switch them off during the day.

Here are two such circuits I found from someone who reverse engineered a few of these lights. These are tested working circuits from commercial products so if one wanted to build such a flasher one of these is probably a good starting point.

[floobydust]

It's mostly about a beginner's circuit that mysteriously stops working due to a turn of the potentiometer.
That kind of thing annoys me because when you are learning electronics, you already have enough things that can go wrong. It's not easy sometimes. The circuit operation is not explained anywhere, it's just handed to you and when it doesn't work, it can frustrate an experimenter.

In those two construction barrier circuits, the tantalum capacitor sees reverse-voltage pulses, and steady reverse voltage during daylight, around -500mV.
I was thinking the tantalum cap would fail. But it looks like that is possibly OK AVX tech pdf for up to 3-10% of rated voltage, maybe a volt, but failure rates are not known.

I found these flashing light bulbs realizing most people have never seen them before. Some so old they are made in USA and Holland.
Trying the KonMari method from Marie Kondo to clean up my electronics parts bins, I think they are only good for artistic value, like a toy robot nose or something.

[soldar]

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

I agree. Please show some consideration to your fellow forum users and use the least bandwidth possible. My recommendations:

Use .jpg only for photos and use .png for line drawings.

Post the smallest photo, in pixels and in file size, that shows what you want us to see. Do not post a 5 Mbyte file when a 50 KByte file will do just the same. Some of us have limited bandwidth. Learn to edit, process and compress.

Line drawings can be made very clear and very small in file size. I am attaching one just as an example. 6KB. No need to use 60 KB or 600 KB or 6 MB.

[tooki]

Yes, but it has to be done well. The lower the resolution, the more critical the preparation becomes, and frankly, for scanned things, it needs to be quite detailed, lest thin lines and letter detail go missing, exactly as in the first example.

[Zero999]

It's subjective to an extent. People differ in what they find clearest and easiest to read. Some like as higher contrast as possible, others prefer different coloured backgrounds.