A Simple ESR Meter

Simple ESR

I saw this ESR meter on zpostbox.ru while wander around the world.  ESR Meter is so handy tools I definitely need one.

zpostbox.ru is a good site collecting so many genius circuits. I like this one at once for it's simplicity. This circuits published on July 2008 issue of "Radio" magazine by Y. Kurakin. 

The heart of the ESR is a oscillator which is running at around 15 khz, use a FT-37-43 with 77 turns as L1. This oscillator is sensitivity of the series resistance, if the under testing capacitor Cx had enough capacitance, let's say 47uF,  the oscillator's output amplitude then only determined by the ESR of the Cx and not affected by the capacitance.

The Q2 is a DC amplifier which bias under conducting. The oscillating signal then been rectified and amplified, drive the PA1 to indicating the ESR.

Calibrate and Use

Before starting use the EST, we need short the test lead, advanced the POT R2, let the Meter reach to it's maximum scale. 

Then we could use 1 ohm or 0.5 ohm resistor to mark the meter's scale, this is one time marking work. Now we could use the Meter, release it's magic.

BTW, if use a good capacitor blocking the DC, this ESR could testing the battery's ESR also!

Making Suggestion

What the "Radio" magazine might not mentioned is how choose the oscillating critical components for your ESR meter.  As a free run oscillator, and even though the ESR meter don't criticize the frequency change, a qualified component is still much better than throw junk capacitors together.

The most problem with this is temperature coefficient, which leads to need calibrating every time we use it. I use V5R ceramic for C1, C2, C3, which make the temperature characteristic much worse. And then I replace them with Mylar and MKT capacitor, which is better. Suggesting a Mica or polystyrene.

Further Improvement

Use your finger to checking the most temperature sensitive component working very well on this little ESR meter. Very quickly I identified the most chief criminal:  2 transistor. while put the finger on them, the meter needle run away about half the scale away and even more!

Cure the transistor temperature characteristic need a little bit more sophisticated solution. I end up here this round.

1 uF Availability

To use the ESR for small capacitance is changeling, because the capacitance on our way.  We need alter the frequncy up to around 100 kHz. This is simple task, does that by changing C1, C2, C3 and the L1.

Ok, now the 1uF, even 0.47 uF capacitance won't affect the ESR reading too much, i just enjoy it.  The frequency i got is around 66 kHz.

Needle Damper

The 1 mA needle meter is driven by the Q2 quite violently. I damping the needle by series a 100 ohm to 500 ohm resistor.  which is working well, and the side effect is , the 'good' reading is compressed to very end the scale now. 1 ohm reading now located on the 80% to 90% scale on the meter, previously it's almost located on the middle. Choosing resistor as your wish.