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# Need help interpreting an old circuit diagram: Transistor and RF Transformer

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#### kdausman

##### New Member
I will start by apologizing for asking what may be very simple questions; I am a scientist who has built some simple circuits before, but I am not by any means an expert in circuitry.

I am trying to build a simple circuit based on a diagram in a modern laboratory textbook, but the textbook's first edition was from the 1960s, and I strongly suspect that this particular circuit diagram and instructions haven't been updated since then. There are two components listed that I cannot figure out what they are, specifically, so I don't know how to purchase them. The circuit diagram and the caption explaining it are shown below:

First the transistor question: from what I can tell, the 1N3904 isn't a transistor. But the 2N3904 seems to be. Will I be ok using a 2N3904?

Second, the transformer question: It is listed as a radio frequency transformer with an inductance of 0.6 mH. This seems to be insufficient information to specify a transformer. How do I determine what I need to purchase?

In case you need further information to advise me, the general idea is that to create an oscillator where the frequency depends on the capacitance of the capacitance cell. The cell consists of a variable capacitor that can be measured in air or when immersed in a solvent. We start with the cell's variable capacitor in the "open" position (close to zero capacitance) in air, and adjust the other variable capacitor so that the oscillator frequency is around 1.3 MHz. That variable capacitor will stay in the same place for the remainder of the experiment. We then move the cell capacitor to the "closed" position, which will have a capacitance of ca. 50 to 200 pF (depending on which variable capacitor we use) and measure the frequency. Then we add the solvent and measure the frequency in both the open and closed positions. From those four frequencies we are able to calculate the dielectric constant of the solvent. (And the cool bit is that if we then add a solute to the solvent, we can measure the dipole moment of the molecules!)

Any advice on how to proceed in identifying those components would be greatly appreciated. Thank you!

You are correct about the transistor part number. OK, that was the easy part.

That is an ... unconventional ... oscillator circuit. It almost looks like a blocking oscillator except that the emitter isn't connected to the tank in any way. Running an NPN on *minus* 15 V like that makes it look like something based on avalanche. Been a while since I've seen that trick. I'll dig around a little.

ak

I agree about the transistor, a 1N3904 (if it existed) would be a diode. A 2N3904 sounds reasonable.

The circuit configuration is certainly odd, but does stand up to scrutiny (almost).

One winding of the RF transformer and Ct form a tuned circuit in the collector of the transistor.
The other winding of the RF transformer takes feedback to the base.
A simple tuned collector oscillator.

The collector circuit is at ground potential as far as the DC bias is concerned, so putting one side of the capacitor Ct and the capacitance cell Cx at ground potential for DC and RF to minimise the effects stray/hand capacitance as the circuit is adjusted.

Because the collector is at ground potential, to the emitter circuit must be at a negative potential.
(Collector is positive with respect to the emitter in an NPN transistor).

The emitter circuit is very odd.
I would expect the -ve supply to be decoupled to ground, we don't want RF on the supply line.
The 300pf Capacitor across R2 looks odd, it looks like a decoupling capacitor, but has a reactance of about 500 Ohm (if my quick and dirty calculations are correct).
There is no coupling capacitor between the emitter and the frequency counter to isolate the DC from the counter input circuit.
Most frequency counters have DC isolation on the input anyway, but if I was designing this circuit, I would put in a coupling capacitor.

Second, the transformer question: It is listed as a radio frequency transformer with an inductance of 0.6 mH. This seems to be insufficient information to specify a transformer. How do I determine what I need to purchase?
The quick and dirty answer is that the oscillator coil from an old medium wave AM radio receiver should be OK.
The longer answer is that there will be lots of places to buy one, but it is late here and I am off to bed so don't have time to look.

I will start by apologizing for asking what may be very simple questions
Simple questions, but well explained.
If only all questions here were so concise and to the point.

JimB

... the oscillator coil from an old medium wave AM radio receiver should be OK.
The longer answer is that there will be lots of places to buy one, but it is late here and I am off to bed so don't have time to look.

Even a 455kHz IF transformer would likely work.

I suspect that the RF transformer is a 465 Khz IF transformer. 0.6 mH with 100 pF would resonate at about 650 Khz which seems reasonable. The thing against the IF transformer idea is that IF transformers normally had the tuning capacitors inside the can. At one time I would have had IF trnsformers in my junk box but now I don't think I even have any old radios with one in or I would try the circuit. I suspect the 300 pF capacitor is to partly decouple the emitter resistor so there is enough RF current through the collector winding to give enough drive to the base winding. The circuit should have a decoupling capacitor across the supple lines. The circuit is easier to understand when drawn in the conventional way with a positive power supply. If the oscillator works then I think adding a frequency counter built around a micro controller with the code added to do the calculations would make a nice unit.

Les.

Is this just an elaborate way of measuring capacitance? Capacitance meters are now free with many multimeters, while in the 1960s they were expensive items.

Can you simply try the cell with a reasonably good capacitance meter?

Is this just an elaborate way of measuring capacitance? Capacitance meters are now free with many multimeters, while in the 1960s they were expensive items.

Can you simply try the cell with a reasonably good capacitance meter?

Elaborate? I wouldn't call 6-components "elaborate". What do you call nearly any other circuitboard with dozens to hundreds of components?

If you are instead referring to the method used, I would say it is clever.

As the OP suggested, it's a poorly drawn antique circuit, and just a crude oscillator with transformer feedback from collector to base. As with most of these text book circuits, it's not really very practical, and not a very good design or example at all.

Elaborate? I wouldn't call 6-components "elaborate". What do you call nearly any other circuitboard with dozens to hundreds of components?

If you are instead referring to the method used, I would say it is clever.
"Elaborate" is maybe not the best word, but it's a lot of work to build a circuit like that if all that is needed is a capacitance meter.

"Elaborate" is maybe not the best word, but it's a lot of work to build a circuit like that if all that is needed is a capacitance meter.

If he is building something from a 1960s electronics book, I would assume he is "Learning" about circuits instead of "needing" a capacitance meter. Basic electronics in the 1960s really hasn't changed from electronics of today - the lowly 2N3904 is still one of the most common small signal Transistors. His only challenge will be to find the once-common AM Radio parts.

If he is building something from a 1960s electronics book, I would assume he is "Learning" about circuits

Well if that 'circuit' is representative then he's not going to learn much, because it appears to be one of those crappy books that don't show working circuits or component values, just a rough idea of how you 'might' hook parts together.

Thank you all for the help and for the fascinating back-and-forth!

So no, this is not an exercise in learning circuitry. I am teaching a physical chemistry laboratory course, and the goal is to be able to measure dielectric constants of liquids with enough precision to suss out the effects on dielectric constant caused by relatively low concentrations of solutes. So the precision level required is quite high (I haven’t done a full analysis, but I suspect I might need somewhere between four and six significant figures on the capacitance measurement). That makes me skeptical that a multimeter capacitance measurement would be sufficient (though I could be wrong, and would absolutely welcome correction on that point).

As a further note, I was planning on using an oscilloscope instead of a frequency counter, because I happen to have a very high quality oscilloscope on-hand, and do not have a frequency counter on hand.

The consensus I am hearing, if I am interpreting the thread correctly, is that this is a poorly designed circuit. Can any of you recommend a better design? I am not limited to this particular approach; it simply is the method that came as part of the dielectric constant measurement lab writeup I was able to find.

As a further, minor point, and just to provide clarification, the book this is in is a physical chemistry experimental lab textbook whose latest edition is copyright post 2000 (I don’t have it with me at the moment, so I don’t have the copyright page), but whose first edition is copyright 1960s. It is speculation on my part that this particular circuit diagram hasn’t been updated since the first edition. This is not part of any kind of electronics textbook. It is written by physical chemists for physical chemists, with the assumption that physical chemists understand enough electronics to be able to follow instructions to build a circuit given a circuit diagram, but not that they know enough to be able to design circuits on their own.

Again, thank you all for the wonderful feedback! And I look forward to further suggestions!

As a further, minor point, and just to provide clarification, the book this is in is a physical chemistry experimental lab textbook whose latest edition is copyright post 2000 (I don’t have it with me at the moment, so I don’t have the copyright page), but whose first edition is copyright 1960s. It is speculation on my part that this particular circuit diagram hasn’t been updated since the first edition. This is not part of any kind of electronics textbook. It is written by physical chemists for physical chemists, with the assumption that physical chemists understand enough electronics to be able to follow instructions to build a circuit given a circuit diagram, but not that they know enough to be able to design circuits on their own.

Again, thank you all for the wonderful feedback! And I look forward to further suggestions!

Aah, sorry to hear that. There were very few books on electronics for the chemistry lab and the most famous one was from a professor at Michigan State. He kept making minor updates each year and I guess he made a fortune selling new a
Editions.

Anyhow, many frequency counters count over 5 digits of accuracy so you may see a good effect with a simple 555 timer. Two resistor and a capacitor wired to an LM555 chip.

I am a chemist and I could help you out. Where in the country are you located?

Thank you for the offer, gophert! I am in Boise, ID.

The textbook is the latest edition of Shoemaker, Garland, and Nibler (the author order has, I think, changed in the last few years). This is by far the most commonly used P-Chem lab textbook in use in the US.

Thank you for the offer, gophert! I am in Boise, ID.

The textbook is the latest edition of Shoemaker, Garland, and Nibler (the author order has, I think, changed in the last few years). This is by far the most commonly used P-Chem lab textbook in use in the US.
Ok,

I'm in Pittsburgh. I'll run a test this evening - I am determined to enjoy one of our few sunny days each year.

Unless you have a really high-end oscilloscope, with a dedicated frequency counter circuit, which is really rare, don't use it to measure frequency if you are looking for 4+ significant figures. Just about all oscilloscopes measure frequency by post-processing the waveform that is captured anyhow. I have seen oscilloscopes measure period to 0.1% and then give the inverse to more digits than is justified.

If you are trying to measure capacitance accurately, you are correct that multimeters will only give a few digits. However, a circuit like the one you have shown will be sensitive to temperature, supply voltage variation, and anything close to the coil will affect the inductance and therefore the frequency. It's also quite easy to accidentally pick up radio signals that can affect the frequency slightly.

This is not part of any kind of electronics textbook. It is written by physical chemists for physical chemists, with the assumption that physical chemists understand enough electronics to be able to follow instructions to build a circuit given a circuit diagram, but not that they know enough to be able to design circuits on their own.

That's a pretty poor assumption, there's far too little information in that 'circuit' for even electronic engineers to build it without a fair amount of work.

Incidentally, the frequency of the oscillator is about 650KHz with the preset capacitor at 100pF, ignoring the capacitance cell, which will make it lower, the more the capacitance of the cell, the lower it will go.

Interestingly my daughter is a chemist, currently doing Post Grad research in the Netherlands - however, way back when she was still doing her Masters she had to build an opamp based circuit, using a photo or solar cell to measure light levels through a liquid. Luckily, I had taught her to solder a number of years before, and she was the best solderer in the year, including the professor running the course

The reason for mentioning this, is that she emailed me the circuit she was given to build, and I was absolutely horrified at how badly it was designed - I couldn't believe that even Chemistry students would be building something so poor in the 21st century at a top University.

From what you've said, I now suspect the 'design' came from a similar book to the one you're using, where the 'circuits' are only partial, and intended simply to demonstrate a principle, leaving out many of the important components, and never intended to be used as a practical circuit (many electronics text books do the same, I've always thought it highly bizarre?).

I presume you also are at a University?, and hopefully has an Electronics department?, could you ask them for assistance?.

I realise there's often little or no contact between departments, I've mentioned here before a story my daughter told me - her boyfriend (now husband) is a technician at the same University, and at that time was working with a Physics group, and Melissa often called in and took them home made cakes. They were trying to make some kind of sensor, and the metal they were using wasn't working well enough, and they were discussing ordering a big 'lump' of a different metal to try, based on it's atomic number etc. Luckily Melissa overheard this, and pointed out that this would kill everyone on campus, their only concern was that it was a solid at room temperature. Their next choice was no better, as it reacts with air and gives off poison fumes - I'd really like to think that academics would consult with experts in over disciplines where required, but I fear this isn't the case?.

Anyway, back to the plot - you mentioned wanting high accuracy, but then propose using an oscilloscope to read the frequency - this is a VERY low accuracy way of doing it. A frequency counter would be immensely better, and reasonable ones are available quite cheaply.

Wonderful feedback, everyone! I am thrilled that I have found this forum!

The comments about frequency counters vs. oscilloscopes are well-taken. I am completely inexperienced in purchasing frequency counters, however. I see, for example, **broken link removed** that covers the frequency range I need while reporting what seems to be down to the 100 Hz position. Is that the kind of thing I am looking for? I see other instruments that go up to several hundred dollars. I have the money to make a purchase of a good instrument if it is necessary, but the general applicability of this approach will increase the cheaper the components I can get away with are.

With respect to other factors that can affect stability, I am taking careful pains to shield everything and control temperature. Aluminum can around the capacitance cell, shielded box for the electronics, BNC connectors to shielded coaxial cables everywhere. The aluminum can will have holes drilled to allow constant-temperature water in to surround the beaker holding the capacitance cell. Supply voltage I will keep an eye on, and I may go to a battery setup to provide extra stability (since this is a relative measurement, stability of output is more important than precise values of the voltage). I have handled high sensitivity measurements that require careful attention to shielding and temperature like this before in designing and building transient absorption spectrometers, so those aspects I feel relatively comfortable with.

I find the observation that the frequency of the depicted circuit is around 650 kHz fascinating, which reinforces my thought that this is an experiment that has been included in this textbook without anyone ever actually trying it since the 60s. Ideally, I would eventually like to build a version of this instrument that allows me to vary the frequency the measurement is being made at over a *very* wide range, because the frequency-dependence of the dielectric constant would tell me information about different aspects of the polarizability. At a high frequency range, the electronic polarizability of the molecules will dominate. At a lower frequency range, the dipole moment rearrangement of the molecules will dominate. And there are other effects even lower in frequency. Regardless, I think the first step is to get the apparatus working in one regime (the supposedly-designed-for frequency range), and only look at how to extend it once the kinks have been worked out.

Yes, I am at a university, and yes, there is an electrical engineering department on campus. I can certainly reach out to them, although there are not established collaborative relationships between our department and theirs yet. In my experience, it is often a slow process when dealing with academics (I am guilty of that issue myself), which is why I asked my questions here. I am thrilled to have gotten so much useful information so quickly!

So, as Nigel said, back to the plot! It is really sounding to me that the original circuit diagram is highly questionable. If the goal is to make an oscillator whose frequency will depend on the capacitance of "capacitance cell," where the open-frequency is around 1.3 MHz, and where the closed (in air) frequency when the capacitance of the "capacitance cell" is 200 pF is significantly different from 1.3 MHz but is still within the measurement range of a frequency counter, what would be a better way to design the circuit?

There's a simple formula to work out the resonant frequency of a coil and capacitor - and there are even simpler websites which include calculators to do it for you!

This is the one I used, as it was near the top on a google search:

https://www.1728.org/resfreq.htm

Back when I was at college we had to do it all by hand, as it was before calculators and home computers.

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