Help understand this circuit

[Gaber Mohamed Boraey]

Hello everyone , hope you all fine and in a good health

I am willing to do this circuit for capacitors test, as I’m into electronics repair and measuring capacitor with meter, “ measuring only capacitance “ not enough for me, that’s why I’m thinking build this circuit

I’ve read about and know that it’s colpitts oscillator, what I Don’t understand is the q2 purpose, blog author say it’s for rectifying, which i don’t understand , can you clarify for me?

Here is the blog

Simple ESR meter circuit diagram

How to build a simple equivalent series resistance meter, its circuit diagram is described in this article

zpostbox.ru

Attached the circuit
Thanks

 

[D.A.(Tony)Stewart]

Yes it oscillates with a maximum less than 2 diode drops, so that the current rises with low ESR and Q2 only sees the peak of the envelope to produce an impulse of current for each peak. This is not a great amplitude detector but with a large cap across the output it averages the rectified current.

The range of ESR is quite limited to about 1% or 2 decade adjustable by the pot R1 for full scale. The diodes are irrelevant as the Q is limited by the impedance of Q2, R2 and hFE8R4

When in doubt how to explain how something works, learn how to make a quick simulation.

--- Updated Dec 2, 2023 ---

For the given bias of R2, what range of ESR did you find useful?

Do you know how Vbe acts as a negative diode clamp to AC coupled signals on the base?
What do you expect for ESR*C for a good cap?

 

[Gaber Mohamed Boraey]

Yes it oscillates with a maximum less than 2 diode drops, so that the current rises with low ESR and Q2 only sees the peak of the envelope to produce an impulse of current for each peak. This is not a great amplitude detector but with a large cap across the output it averages the rectified current.

The range of ESR is quite limited to about 1% or 2 decade adjustable by the pot R1 for full scale. The diodes are irrelevant as the Q is limited by the impedance of Q2, R2 and hFE8R4

When in doubt how to explain how something works, learn how to make a quick simulation.

--- Updated Dec 2, 2023 ---

For the given bias of R2, what range of ESR did you find useful?

Do you know how Vbe acts as a negative diode clamp to AC coupled signals on the base?
What do you expect for ESR*C for a good cap?

The esr for good capacitor I get from table, this table

Equivalent series resistance or ESR of Capacitor.

Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However, they can be treated, to a very good degree of approximation, a…

ehpforyou.wordpress.com

 

[D.A.(Tony)Stewart]

Yes ESR improves with voltage rating and size of C and size of the cap as well as the quality of design and supplier.

"Low ESR" types are typically 1% of the "std" electrolytics in datasheets.
For any given family ESR*C = T is pretty constant.