Hi, thanks for the video. I think that the discharge path is not via 10k resistor, but via BE junction of the upper transistor and conducting lower transistor. If the discharge path was via 10k, You would have signal shape close to the triangle (much more symmetrical).
Yeah. I was also gonna point out - hinted at with the pulse output - that when it discharged it also drops the normally 5v trigger voltage made by the potential divider to much lower, so the transistor doesn't turn off immediately as it starts to discharge - a bit of hysterisis
At 5:19, if instead of the added resistor one places the base of an NPN transistor one may use its collector to discharge the capacitor quicker. This circuit works well due to the hysteresis produced by the resistive pair on the right, All oscillators work because of the limitation in their power supplies unless they have a hysterics effect on their reference point, Oscillators with and without a hysteresis effect are very interesting to follow. Those without hysterics effect will never work with one capacitor or other " phase changing element,
The discharge is through the NPN's base-emitter diode and the PNP's emitter-collector. This current also turns on both transistors. Since the current pulls the 1k to ground, the capacitor also provides the emitter-base current for the PNP through the (now) parallel divider resistors.
At first I thought How can that oscillate. It was then I realized the 1K load resistor on the NPN allowed for voltage gain and kills the bias to the PNP as well. A clever relaxation oscillator, similar to what a P.U.T. does.
There is a circuit very similar to this from Charles Wenzels' site (techlib). That one uses slightly different connections, with many different uses for the circuit detailed. Makes a change from the usual 2 transistor astable multivibrator circuit.
Thanks Mr. IMSAI for doing the overview and explaining the operation using an oscilloscope. I am assuming you were born about the same time I was (1955), which was ideal for growing up on vacuum tubes, transistors, and integrated circuit design. Making a career of designing RF and Analog circuitry, I found great interest in your presentation. When I first saw your circuit (which was completely new to me), I immediately began analyzing it. At first, I couldn't understand how it could oscillate with the substantial negative feedback with the NPN transistor but, then I noticed the 1k resistor was in series with the entire circuit, which would alter the PNP bias, and then it made sense. So the 10k resistor at the NPN is only charging the cap and the discharge path is through NPN's base. Not a good circuit, the NPN will be damaged by surge current after a while, might I suggest a resistor in series with the NPN base to limit current? Sorry to be critical but, enjoyed whole analysis, so thanks! By the way, I expect this oscillator to be temperature compensated since you have the Base-Emitter of the NPN in series with the PNP.
The two transistors and the resistor divider on the right form basically a programmable unijunction transistor simulant. Maybe an idea would be to do an episode on the PUJT as chip of the day. AFAIK OnSemi still manufactures (or did until recently) 2N6027 parts. And you can make relaxation oscillators with PUJT that are very very similar in schematic to the one in this episode.
I've seen a similar circuit like that before, but both transistors were replaced with an SCR that had two gates. An old circuit where the dual gate SCR is unobtainium anymore.
There's two ways too look at the circuit. The way I look at it is that the PNP transistor works kinda like an emitter follower and the NPN can't turn on until the voltage of the capacitor reaches about VCC/2 + Vbe. That way it looks like the classic way to control transistors - via the base.
this is so neat, I bet you could use this if you ever wanted to make a super cheap and simple dc-dc converter. the short pulse could be used to determine the max pulse width and the ramp could be compared to a feedback signal in order to make an appropriate pwm signal for a switching transistor.
It'd be quite interesting to see if the ramp waveform could be made more linear by replacing the 10k resistor with a current source. Could find functionality as a sweep generator for a CRT oscilloscope. Discharge path looks more to be through the B-E junction of the top transistor and the E-C path of the bottom one.
Well, I'm just wondering why circuits like this oscillates in the first place... Why isn't the capacitor just charget up to the point where the first transistor starts to barely conduct, and then it opens and current flows trough the transistor(s), and at that point the capacitor is not charget anymore. So it reaches some sort of equilibrium where everyting is in balance.
It's the feedback due to the 1K resistor in the collector circuit. When the transistors begin to conduct the voltage on the collector drops which drops the voltage divider voltage. The divider voltage dropping turns on the transistor more which causes the drop to be more, and more and more! Thus the very fast switch due to the feedback. Without the 1k resistor the circuit would act as you first described.
A nice little sawtooth generator. Wanna impress your friends even more? Use a neon lamp, a resistor and a capacitor... Needs way higher voltage though, so it's not for the faint of heart, haha.
Similar to the LM3909, just that here you use a high supply voltage, but the 3909 uses some extra transistors to allow it to also use the charge on the capacitor to flash the LED. You can put the LED, with a resistor, in the collector of the PNP transistor, change the 10k bias resistors to 1M, and use a 470n ceramic capacitor, and the third 10k as 1M as well, and get a nice ultra low power blinker off a 9V supply. Using it at higher power you just have to make sure the transistor you use for the NPN is capable of handling the base current to discharge the capacitor, as that can be very high with large capacitor values.
It goes much simpler: 1 BJT, 1 capacitor, 1 resistor. Also a relaxation type oscillator. There is a video about it under "World's Simplest Single Transistor Oscillator".
I'm reverse engineering a fairly old Uniross rechargeable Ni-Cd torch - it has on, off, and flashing. The circuit has 4 transistors - 3 PNPs and 1 NPN - I will check to see if the flasher circuit uses one of these. It will certainly help me to draw the circuit schematic, if it is - LOL.
Not going to run well below around 4V, due to the base emitter voltages. Frequency is set by the RC time constant of the capacitor and the 10k resistor connected to it, and less so by the 1k resistor. At lower voltages the oscillator swing is going to be smaller and slower. Works best over around 6V.
Taking the output from the base might look ok with a Spice simulation. . Base is high impedance, emitter low impedance. Any load on the NPN base will change the frequency. There are much better oscillators that just use a single transistor, and even a multivibrator (2 bjt) is far better than this one.. it's output has an exponential rise from the capacitor charging, then a fast fall as the transistors turn on. The output will be like a triangle wave, so will have odd harmonics. A more interesting NPN/PNP oscillator circuit is the staircase generator.
An even simpler two transistor oscillator can be made from a super alpha pair ( similar to a darlington ) the collector of the PNP couples to ground via an 8 Ω loudspeaker ... the base of the NPN connects to the emitter of the PNP , via a 33 K Ω resistor , which also connects to the +ve of a 1v5 battery .... the base of the NPN couples to the loudspeaker via a 47 nF capacitor ... the other end of the loudspeaker connects to the emitter of the NPN and the negative of the battery ( ground ) .... the collector of the NPN just connects to the base of the PNP ..... the NPN is a high gain Silicon transistor and the PNP is best as a Germanium audio output type .... this circuit makes a SUPERB continuity tester , that will NOT damage IC's etc ... ( tried - n - tested ) ...... DAVE™🛑
Or, instead of a resistor in the second collector you put a speaker, and instead of permanent charging resistor you put a pot - first metronome of children playing electronics.
a broken igbt🤣 sorry, I couldn't resist! such an amplitude of comedy...I guess I could rectify this. but na.....so much fun....so little time....🤣😁🥔🌶🍖🍞🥗🥧🧋
Hi, thanks for the video. I think that the discharge path is not via 10k resistor, but via BE junction of the upper transistor and conducting lower transistor. If the discharge path was via 10k, You would have signal shape close to the triangle (much more symmetrical).
Yeah. I was also gonna point out - hinted at with the pulse output - that when it discharged it also drops the normally 5v trigger voltage made by the potential divider to much lower, so the transistor doesn't turn off immediately as it starts to discharge - a bit of hysterisis
Thanks for video sir! Classic line -> "Circuit of the day, everybody loves circuit of the day"
At 5:19, if instead of the added resistor one places the base of an NPN transistor one may use its collector to discharge the capacitor quicker. This circuit works well due to the hysteresis produced by the resistive pair on the right, All oscillators work because of the limitation in their power supplies unless they have a hysterics effect on their reference point, Oscillators with and without a hysteresis effect are very interesting to follow. Those without hysterics effect will never work with one capacitor or other " phase changing element,
The discharge is through the NPN's base-emitter diode and the PNP's emitter-collector. This current also turns on both transistors. Since the current pulls the 1k to ground, the capacitor also provides the emitter-base current for the PNP through the (now) parallel divider resistors.
I think the capacitor is being discharged through the base of the transistor which will have a fair amount of current under saturation conditions
I simulated it, it's goes crazy for a split second. probably best to put in a limiting resistor...
You're a good teacher
Great little circuit of the day
At first I thought How can that oscillate. It was then I realized the 1K load resistor on the NPN allowed for voltage gain and kills the bias to the PNP as well. A clever relaxation oscillator, similar to what a P.U.T. does.
Neat, and you were right, I was looking at the base of that other transistor. Thanks.
These videos are a treasure, Thank you
There is a circuit very similar to this from Charles Wenzels' site (techlib). That one uses slightly different connections, with many different uses for the circuit detailed. Makes a change from the usual 2 transistor astable multivibrator circuit.
I wonder changing 0.01uF cap with series LC circuit would create sinusoidal waves?
You should give it a try. Breadboard it or try it in Spice.
So the cap is being discharged through the base of the NPN. Neat.
Charged through the 10K hence the slope and discharged quickly through the Base Emitter junction ? I think Mr Guy had a slip of the tongue.
@andymouse Yes, I think so. @doktorpyta5904 saw it too.
Thanks Mr. IMSAI for doing the overview and explaining the operation using an oscilloscope. I am assuming you were born about the same time I was (1955), which was ideal for growing up on vacuum tubes, transistors, and integrated circuit design. Making a career of designing RF and Analog circuitry, I found great interest in your presentation.
When I first saw your circuit (which was completely new to me), I immediately began analyzing it. At first, I couldn't understand how it could oscillate with the substantial negative feedback with the NPN transistor but, then I noticed the 1k resistor was in series with the entire circuit, which would alter the PNP bias, and then it made sense. So the 10k resistor at the NPN is only charging the cap and the discharge path is through NPN's base. Not a good circuit, the NPN will be damaged by surge current after a while, might I suggest a resistor in series with the NPN base to limit current?
Sorry to be critical but, enjoyed whole analysis, so thanks! By the way, I expect this oscillator to be temperature compensated since you have the Base-Emitter of the NPN in series with the PNP.
I have no personal attachment to this circuit. Criticism of the circuit great.
@@IMSAIGuy Thanks for your comeback. Hopefully my criticism is constructive and no egos are harmed.
The two transistors and the resistor divider on the right form basically a programmable unijunction transistor simulant. Maybe an idea would be to do an episode on the PUJT as chip of the day. AFAIK OnSemi still manufactures (or did until recently) 2N6027 parts. And you can make relaxation oscillators with PUJT that are very very similar in schematic to the one in this episode.
I've seen a similar circuit like that before, but both transistors were replaced with an SCR that had two gates. An old circuit where the dual gate SCR is unobtainium anymore.
There's two ways too look at the circuit. The way I look at it is that the PNP transistor works kinda like an emitter follower and the NPN can't turn on until the voltage of the capacitor reaches about VCC/2 + Vbe. That way it looks like the classic way to control transistors - via the base.
Wow, you're coming up on 100k subscribers, congrats!
Thanks for video,is there a way make the ramp more linear?
Thanks, you are spoiling us with all this great videos.
this is so neat, I bet you could use this if you ever wanted to make a super cheap and simple dc-dc converter. the short pulse could be used to determine the max pulse width and the ramp could be compared to a feedback signal in order to make an appropriate pwm signal for a switching transistor.
It'd be quite interesting to see if the ramp waveform could be made more linear by replacing the 10k resistor with a current source. Could find functionality as a sweep generator for a CRT oscilloscope. Discharge path looks more to be through the B-E junction of the top transistor and the E-C path of the bottom one.
Fascinating! Thanks for sharing!
Why does the voltage not drop to 0 volts but rather about 2 volts before beginning to recharge?
Well, I'm just wondering why circuits like this oscillates in the first place...
Why isn't the capacitor just charget up to the point where the first transistor starts to barely conduct,
and then it opens and current flows trough the transistor(s), and at that point the capacitor is not charget anymore.
So it reaches some sort of equilibrium where everyting is in balance.
It's the feedback due to the 1K resistor in the collector circuit. When the transistors begin to conduct the voltage on the collector drops which drops the voltage divider voltage. The divider voltage dropping turns on the transistor more which causes the drop to be more, and more and more! Thus the very fast switch due to the feedback. Without the 1k resistor the circuit would act as you first described.
A nice little sawtooth generator.
Wanna impress your friends even more? Use a neon lamp, a resistor and a capacitor... Needs way higher voltage though, so it's not for the faint of heart, haha.
With only 2 resistors extra, using a unijunction transistor you can make a similar relaxation oscillator but for low voltages :-)
Neon: th-cam.com/video/BXD3DNALulQ/w-d-xo.htmlsi=OruyaJTpNoI9ix1s
Transistor weirdness at its best. These thumbnails always make me watch the video to find out how it works. Cheers :-)
Similar to the LM3909, just that here you use a high supply voltage, but the 3909 uses some extra transistors to allow it to also use the charge on the capacitor to flash the LED. You can put the LED, with a resistor, in the collector of the PNP transistor, change the 10k bias resistors to 1M, and use a 470n ceramic capacitor, and the third 10k as 1M as well, and get a nice ultra low power blinker off a 9V supply. Using it at higher power you just have to make sure the transistor you use for the NPN is capable of handling the base current to discharge the capacitor, as that can be very high with large capacitor values.
It goes much simpler: 1 BJT, 1 capacitor, 1 resistor. Also a relaxation type oscillator. There is a video about it under "World's Simplest Single Transistor Oscillator".
Merci for this video.
Yes in fact "I LIKE CIRCUITS",... lol
have not seen that prior,...
where did you find it?
I'm reverse engineering a fairly old Uniross rechargeable Ni-Cd torch - it has on, off, and flashing. The circuit has 4 transistors - 3 PNPs and 1 NPN - I will check to see if the flasher circuit uses one of these. It will certainly help me to draw the circuit schematic, if it is - LOL.
What transistors did you use, I'm thinking 3904 and 3906, sound good?
yes
Question can this run on low voltage say 3v? Also can the frequency be changed and if so how? what formulas to do so?
Not going to run well below around 4V, due to the base emitter voltages. Frequency is set by the RC time constant of the capacitor and the 10k resistor connected to it, and less so by the 1k resistor. At lower voltages the oscillator swing is going to be smaller and slower. Works best over around 6V.
@@SeanBZA Thanks very much.
You may also trigger it with a low-going pulse at the Base of the PNP Transistor.
Thanks 👍
Very interesting, i like using funny and strange circuits.
What's the Frequency?
similar (but not identical) circuit to 'Ultra Low Current Oscillator' by David Johnson - on Dave's Circuits.
My first guess was an oscillator, but I wasnt sure why.
I bet you could make these pretty fast with the right components.
This oscillator works in any device
If you put the pnp at the top, you don't need the 1k resistor.
Use a 741... Have lights that flash to the music..
save a 555 needs to be a tee shirt
Taking the output from the base might look ok with a Spice simulation. . Base is high impedance, emitter low impedance. Any load on the NPN base will change the frequency. There are much better oscillators that just use a single transistor, and even a multivibrator (2 bjt) is far better than this one.. it's output has an exponential rise from the capacitor charging, then a fast fall as the transistors turn on. The output will be like a triangle wave, so will have odd harmonics. A more interesting NPN/PNP oscillator circuit is the staircase generator.
Cool.
An even simpler two transistor oscillator can be made from a super alpha pair ( similar to a darlington ) the collector of the PNP couples to ground via an 8 Ω loudspeaker ... the base of the NPN connects to the emitter of the PNP , via a 33 K Ω resistor , which also connects to the +ve of a 1v5 battery .... the base of the NPN couples to the loudspeaker via a 47 nF capacitor ... the other end of the loudspeaker connects to the emitter of the NPN and the negative of the battery ( ground ) .... the collector of the NPN just connects to the base of the PNP ..... the NPN is a high gain Silicon transistor and the PNP is best as a Germanium audio output type .... this circuit makes a SUPERB continuity tester , that will NOT damage IC's etc ... ( tried - n - tested ) ...... DAVE™🛑
👍👍👍👍
Or, instead of a resistor in the second collector you put a speaker, and instead of permanent charging resistor you put a pot - first metronome of children playing electronics.
a broken igbt🤣 sorry, I couldn't resist! such an amplitude of comedy...I guess I could rectify this. but na.....so much fun....so little time....🤣😁🥔🌶🍖🍞🥗🥧🧋