This is because the variation in thickness and doping profile of the base region. When the voltage is high enough the depletion region in the base extend far enough to make electron tunneling possible.
Back in the day in 'Everyday Electronics' they published a project called "The Dream Machine" which was a transistor in this configuration and a simple amplifier to drive a small speaker and it would generate 'Pink Noise' this can aid sleep in some folks by mimicking the noise of light rain or the gentle babble of a stream. It worked exactly as described and in the theory bit it was stated that 'actual Zener breakdown' ended around the 4v area and the avalanche kicked in after that. There was also talk of experimenting with different transistors as some 'Won't work' as you clearly demonstrated !!......cheers.
Transistors are designed asymmetrically on purpose, but collector and emitter can always be interchanged. The asymmetry affects the gain: lots of gain in the normal orientation, very little gain (or even gain less than 1) when reversed. But we get something back for this gain degradation: the saturation voltage is lower. So, back in the days when bipolar transistors were the power devices in power supplies, they could be used as switches in low-voltage switching supplies that way.
Another good one is the Unijunction Transistor. Makes good relaxation oscillators. I don't think UJTs are made anymore but you can still find them around. Old school for sure!
This is the design fault made in many power supplies that start discharging your laptop battery if plugged into the latop but pulled out of the wall outlet during charging.
There are special avalanche transistors that use the same principle for generating pulses with very fast edges, in the picosecond range, useful for testing equipment and transmission lines.
Nice demo! Those who are unaware of the result of the following circuit should also try this: Turn off the +12 V source. Put a voltmeter in place of the LED. Ground the base of the transistor. You can remove or leave the capacitor at your option. Predict the voltage to be read by the voltmeter. Now turn on the +12 V source and note the collector voltage. The result can be surprising. (This is an example suggested by the late great Bob Pease to show that the spice circuit analysis program does not always give the right answer.)
Thanks for the time to put this relaxation oscillator circuit on your bench. It's been years since I've touched this circuit. Talking about knocking the rust off. Already got my breadboard out...
Interesting, I did a lot of bipolar transistor characterization in the 1980 and 1990's but, never witnessed the base-emitter breakdown unto a negative resistance region. Most small signal transistors would breakdown at about 6 volts and appear as a 6 volt zener.
Back in my "Starving Student" days - I used the reverse-biased E-B junction of a 2N3904 as a ~5V Zener reference in a lab project. Worked perfectly, with no apparent damage the part !
As Absurdengineering pointed out, you can get very low saturation voltage, when you swap the collector and emitter on certain transistors. That was used (before the MOSFET produced nearly zero saturation voltages) for turning a low DC measurement voltage to AC for purpose of greater amplification in AC stage and then again rectifying it to DC. I recall benefit of more than a decade, say from 200 mV to 2 or 5 mV saturation. I also recall there were some special transistors sold exactly for this application. Finally, I think the “best” special transistor may have been PNP, rather than NPN types. Maybe you like to try and demonstrate this feature?
Heat up the transistor and the LED stops flashing )) Just will light steady. I use old Ge transistors as Heat and light sensors, attaching them into the scheme this way.
You could make this much more reliable by using 2 transistors connected to be the equivalent of an SCR. Just make sure the charging current isn’t enough to keep it latched!
You can use a similar circuit to generate pulses with really fast rise time. I remeber using it as a hamonic generator for a frequency multiplier. Like a step recovery diode for poor people!
This is because the variation in thickness and doping profile of the base region. When the voltage is high enough the depletion region in the base extend far enough to make electron tunneling possible.
Back in the day in 'Everyday Electronics' they published a project called "The Dream Machine" which was a transistor in this configuration and a simple amplifier to drive a small speaker and it would generate 'Pink Noise' this can aid sleep in some folks by mimicking the noise of light rain or the gentle babble of a stream. It worked exactly as described and in the theory bit it was stated that 'actual Zener breakdown' ended around the 4v area and the avalanche kicked in after that. There was also talk of experimenting with different transistors as some 'Won't work' as you clearly demonstrated !!......cheers.
Transistors are designed asymmetrically on purpose, but collector and emitter can always be interchanged. The asymmetry affects the gain: lots of gain in the normal orientation, very little gain (or even gain less than 1) when reversed. But we get something back for this gain degradation: the saturation voltage is lower. So, back in the days when bipolar transistors were the power devices in power supplies, they could be used as switches in low-voltage switching supplies that way.
Another good one is the Unijunction Transistor. Makes good relaxation oscillators. I don't think UJTs are made anymore but you can still find them around. Old school for sure!
This is the design fault made in many power supplies that start discharging your laptop battery if plugged into the latop but pulled out of the wall outlet during charging.
There are special avalanche transistors that use the same principle for generating pulses with very fast edges, in the picosecond range, useful for testing equipment and transmission lines.
Nice demo! Those who are unaware of the result of the following circuit should also try this: Turn off the +12 V source. Put a voltmeter in place of the LED. Ground the base of the transistor. You can remove or leave the capacitor at your option. Predict the voltage to be read by the voltmeter. Now turn on the +12 V source and note the collector voltage. The result can be surprising. (This is an example suggested by the late great Bob Pease to show that the spice circuit analysis program does not always give the right answer.)
Thanks for the time to put this relaxation oscillator circuit on your bench. It's been years since I've touched this circuit. Talking about knocking the rust off. Already got my breadboard out...
Interesting, I did a lot of bipolar transistor characterization in the 1980 and 1990's but, never witnessed the base-emitter breakdown unto a negative resistance region. Most small signal transistors would breakdown at about 6 volts and appear as a 6 volt zener.
Back in my "Starving Student" days - I used the reverse-biased E-B junction of a 2N3904 as a ~5V Zener reference in a lab project. Worked perfectly, with no apparent damage the part !
Yes. The negative resistance is necessary for this oscillator to function. 8 V Zener diodes are more reliable in the base of a chopper transistor.
I just tried this one a week ago Messing up a transistor is so much fun. I used a bc547 but needed above 12 volts to let the led flash.
Didn’t Look Mum No Computer make 1000 audio oscillators like this for his museum?
As Absurdengineering pointed out, you can get very low saturation voltage, when you swap the collector and emitter on certain transistors. That was used (before the MOSFET produced nearly zero saturation voltages) for turning a low DC measurement voltage to AC for purpose of greater amplification in AC stage and then again rectifying it to DC. I recall benefit of more than a decade, say from 200 mV to 2 or 5 mV saturation. I also recall there were some special transistors sold exactly for this application. Finally, I think the “best” special transistor may have been PNP, rather than NPN types. Maybe you like to try and demonstrate this feature?
Instead of using a diode, I guess we could also use a resistor?
Heat up the transistor and the LED stops flashing )) Just will light steady. I use old Ge transistors as Heat and light sensors, attaching them into the scheme this way.
That is a really cool effect. I like your breadboard power rail PCB!!
All against transistors abuse.
You could make this much more reliable by using 2 transistors connected to be the equivalent of an SCR. Just make sure the charging current isn’t enough to keep it latched!
You can use a similar circuit to generate pulses with really fast rise time. I remeber using it as a hamonic generator for a frequency multiplier. Like a step recovery diode for poor people!