PNP Germanium astable multivibrator- how low can you go - Page 1
Forum section: electronics / projects

author: starphot
author_rank: Contributor
timestamp: January 23, 2018, 09:20:10 am
content: I was cleaning out my bench and found my first personal project I put together while I was in the Navy in 1973. A prototype Sputnik type model rocket beacon made with a used fiberglass wafer board pulled out of a radar unit, cut and the through holes punched by a pick. This was before the 0.1 pitch breadboards were available. That perfboard and the LM-555 came only a little later. Most of the components were thrown out mil-spec parts that were in the storage bins too long and the others were mail order. The inset in the image shows one of the caps missing as this went somewhere else then thrown out. The remaining is too old to use. With new electrolytics, this PNP germanium multivibrator circuit still works, with a 1 Hz output to a PNP silicon LED driver. The LED is original.The thing with this circuit is that it was designed to run off of a 9v rectangular battery, but testing this circuit, I pulled the voltage down to see the failure point. When the voltage was around 1.8v, the led went dim, but the oscillation was still there on the scope, but at a higher frequency. I kept pulling the supply voltage down and finally it stopped oscillating around 0.2 volts! It returned around .2 volts supply when I returned it up from zero. This is due to the transistors are made with a germanium base with the .3v forward drop from base to emitter with some rolloff leakage at lower voltages and currents. The slight bucking from the timing caps helps keep it going. But the frequency output is about 10x the original setting at 5-9 volts.PNP Astable MultivibratorbyJoe Gafford, on FlickrJoe

author: danadak
author_rank: Super Contributor
timestamp: January 23, 2018, 11:30:52 am
content: Would have been interesting to observe current draw as you lowered V to .2 V.Must admit operating with no Vce, as a result of .2V Vcc I am surprised circuitstill worked. What was the freq at cutoff ?Regards, Dana.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 23, 2018, 01:12:01 pm
content: Quote from: starphot on January 23, 2018, 09:20:10 amLOL, another old fart. I still have my Graymark All American Five radio that I built in 1971...sitting on the shelf. Still works!

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2018, 05:38:02 pm
content: I have seen germanium transistors used in single cell inverters operating below the 1.2 volts provided by a NiCd or NiMH cell. They can self start and operate down to at least 0.6 volts so it might even be possible to use them with a single solar cell but they now have low voltage CMOS ASICs for energy harvesting which do the same thing.

author: starphot
author_rank: Contributor
timestamp: January 23, 2018, 05:53:53 pm
content: Quote from: danadak on January 23, 2018, 11:30:52 amDana,It was around 10 Hz near cutoff. Germanium devices has broader forward bias knee characteristics than a silicon device. The 1N34 Germanium diode has a knee that actually goes to a negative number before cutting off as a 1N4148 silicon diode has a full cutoff knee around .65 volts. The 1N34 has been replaced by the doped silicon 1N60 diode that emulates that in most of the detector circuits. I will try to get a video on that later. As with the new post by David as I write, I did made some coin cell circuits at the time.The video of the model rocket that I used the finished beacon in. It gave out a Sputnik type chirp and lit a LED in the nosecone. A newer version of that video later as more material has been found.https://youtu.be/gxfn2yNbCVwJoe

author: David Hess
author_rank: Super Contributor
timestamp: January 23, 2018, 07:39:31 pm
content: Quote from: starphot on January 23, 2018, 05:53:53 pmHuh? I have never heard of the 1N60 being other than a germanium diode.The closest modern diodes I have found for replacing germanium diodes are low barrier silicon schottky diodes but I still have a few thousand 1N270s that I use as needed.

author: starphot
author_rank: Contributor
timestamp: January 23, 2018, 11:51:04 pm
content: Your right David about the 1N60.I was thinking of something else, but not the Schottky diodes. I think that the 1N60 was preferred to the 1N34 was it is a more robust device.I am coming back from some long term health issues after retiring from the electronics repair industry and cleaning up the bench is one of the things getting me back. To me it is like Rip Van Winkle awaking up after a long sleep and seeing the world has changed. I awoke up to portable oscilloscopes that fits in your toolbox, brand names I never heard about, and a lot of smart kids here starting with an Aussie named Dave. Also working in repair, my own projects got on the back burner and got done literally years after inception. As I mentioned a few months ago on another thread, "The cobbler's children has no shoes".Joe

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2018, 03:10:00 am
content: JFETs and zero-threshold MOSFETs can also go very low. I've heard of 60mV from thermoelectric generators. Of course, you'll have a very tough time getting any meaningful power out of that, even if you can find FETs capable of sinking amperes!(Well, once you solve the startup problem -- you can bring in a conventional beefy MOSFET or BJT to do the heavy lifting. So there's always that. But a simple single stage, nah.)Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2018, 04:29:09 am
content: Out of curiosity, I've got a J105 on the breadboard. Source grounded. Drain load: 5 turns on a choke. Gate bias: 1M to GND, 47nF coupling cap to choke to GND.Choke is a 1mH "bobbin" style (SDR0905 family) with the primary winding added on top.Power output: 1N295 diode, cathode to gate, anode to filter cap (0.47uF). 100kohm load gives -9.5V (0.095mA) at 0.17V input. 26mA input. Oscillation frequency 433kHz.That's 4.4mW input, for 0.9mW output --> 20% efficiency. Surprised it's that high for such a poorly optimized circuit.Startup around 0.16V. Begins oscillating around 0.14, but doesn't overcome Rds(on) and load until later.More and fewer primary turns has mixed effect. I think because fewer turns gives more loop gain (lower startup voltage, more output voltage), but lower drain load impedance (and the J105 only has single digit ohms to work with). The primary impedance, at the resonant frequency, needs to be a few times larger than this, so that Rds(on) is the dominant load.More turns gives more primary impedance, but less loop gain.Much more inductance seems to be best. Maybe starting with a choke of, say, 10mH or more, and putting a few dozen turns on for primary, will do better.A smaller JFET like PN4391 might be usable as well, particularly with a higher impedance transformer (100mH or more, secondary?), and then use that to generate gate bias for a much beefier MOSFET oscillator, to get enough power to run an LED, say.Tim

author: starphot
author_rank: Contributor
timestamp: January 25, 2018, 10:25:10 am
content: Here's the specs of this circuit under different supply voltages. Temp in the shop, 16C. Below 1.5 v, frequency started ramping up. Below 0.2 volts the oscillation was unstable, cutting out.Volts_ma__Freq (Hz)9.0__67___25.0__38___1.82.0__15___1.81.5__9.5___20.2__0.7___20Joe