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Oscillator coil with common wires?
Oscillator coil with common wires?
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Old 11th July 2019, 04:45 PM   #21
Elvee is offline Elvee  Belgium
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Yes, it will: there might be small differences in the bias point of transistors, but for this kind of circuit, it is absolutely not critical.
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Old 11th July 2019, 05:30 PM   #22
PhantomBox is offline PhantomBox  Venezuela
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I don't understand the DUT section surrounded by a spotted line: Cap C2 "47µ" would be a 47µF electrolytic cap? And forgive my ignorance, but why would you put it there, since we are checking another cap for ESR?
I suppose that R11 ("R") is the point where the DUT is inserted?
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Old 11th July 2019, 06:22 PM   #23
Elvee is offline Elvee  Belgium
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DUT = Device Under Test, and that is C2, but to make the sim work its internal resistance (ESR) is shown explicitly, and the 47µ value is just a random example.

In practice, you will connect the test terminals instead of the capacitor+resistor (in parallel with the two diodes).

BTW the diodes are just for protection, you can use 1N400x, 1N540x, or similar (or nothing if you like to live dangerously)
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Last edited by Elvee; 11th July 2019 at 06:25 PM.
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Old 12th July 2019, 01:35 PM   #24
Elvee is offline Elvee  Belgium
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Default More streamlining:

The squarewave oscillator can be replaced by a sine one.

It spares one transistor, but more importantly, the cleaner waveform improves the performances (less sensitivity to the inductance of the test leads for example) and also allows some vector-sensitivity: the reactive part of the impedance will have less influence on the reading, improving the accuracy for low capacitance values.

It is not sufficient to make it a true ESR meter, but it is a step in the right direction.

Note that C1 and C4 need to be better than group II ceramic, like X7R and similar.

Plastic film, group I ceramics (not only COG), mica, etc. are suitable. Even mylar is OK
Attached Images
File Type: png EditedESR1.PNG (51.2 KB, 28 views)
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Old 12th July 2019, 03:15 PM   #25
PhantomBox is offline PhantomBox  Venezuela
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Thanks again for the info. I think I have some 22n mylar caps. I also found an old analog VU meter, that should do just fine. As for the diodes, can all of them be 1N400x/1N450x or does D5/D6 have to be 1N5711W/1N4148?
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Old 12th July 2019, 04:31 PM   #26
Elvee is offline Elvee  Belgium
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The 22nF are not critical: you can use ceramics or anything else. Mylar is perfect.

Only the 1.5nF and and 680p of the last version need to be a little better.

Only the two protection diodes have to be large ones.

D5/D6 should ideally be small schottky's: BAT42, 48, 81, 85 etc are suitable, but a small germanium type is also OK (even better): 1N34, OA90, AA119, etc.
1N4148's will work for test, and might even be satisfactory enough, but try to find the types indicated.

Note that the schematic I used is the simulation one. The real circuit certainly includes supply bypass caps, trimmers, etc: you should check the original project thread, but I will try to give some indications too, and I am in the process of simplifying the circuit again.
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Old 12th July 2019, 05:05 PM   #27
Elvee is offline Elvee  Belgium
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Default Further rationalization:

It is possible to eliminate one more transistor:

Oscillator coil with common wires?-editedesr2-png

I made a quick and dirty breadboard sanity check:

Oscillator coil with common wires?-editedesr3-jpg

Oscillator coil with common wires?-editedesr4-jpg

It works quite well. I didn't have a 50µA meter, so I used a 25µA one with series and parallel resistors to emulate a 50µA/20K instrument, but I had to add a supplementary series resistance in the form of a 47K trimmer.

The movement I used was pulled from an old megger, and by an extraordinary coincidence, the scale happened to be a close match for the new use, with the megohms becoming ohms.

With the mods, the instrument works more or less like the original, but most of the performances are improved, the number of parts is significantly reduced and the current consumption is ~halved.
The temperature stability is somewhat degraded compared to the initial circuit, but for this type of instrument, it is relatively unimportant.

The values are optimized from the reality; the sim optimum is slightly different.
Attached Images
File Type: png EditedESR2.PNG (49.8 KB, 31 views)
File Type: jpg EditedESR3.jpg (116.5 KB, 31 views)
File Type: jpg EditedESR4.jpg (120.8 KB, 31 views)
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Old 13th July 2019, 01:33 AM   #28
PhantomBox is offline PhantomBox  Venezuela
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I have trouble finding a suitable cap in my stockpile for the 680pF and 1500pF "better type".
And I have some 1N144 germanuim diodes. Would they be suitable for the circuit?
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Old 13th July 2019, 08:00 AM   #29
Elvee is offline Elvee  Belgium
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Unfortunately, the 1N144 is a relatively "big" Ge diode, having large leakage currents, which is not ideal.
For the preliminary tests, 1N4148 is probably preferable.
What type of cap have you found? Can you describe them, or better post a pic
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Old 13th July 2019, 12:35 PM   #30
Elvee is offline Elvee  Belgium
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Phantom, don't hurry too much building your meter: I am going to use my breadboard prototype to test various components (including ordinary 1N4148's as diodes), and see where the calibration trimmer(s) need to be located.

I will also provide a really buildable schematic, not just a replica of the sim.

Please confirm that you are going to use a 50µA meter, because it will have an impact on component values.
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