One opamp esr meter

[dannyf]

We have had a few plans floating around trying to measure ESR: some are real ESR meters (I proposed a digitally-implemented ESR meter a while back), and others are just impedance measurements (I proposed one using a 555 timer).

I was thinking about recently how I would make it simpler.

Design goals:
1) the simpler the better;
2) the meter could function as a standalone esr meter or an external adapter to an analog meter.

[dannyf]

I implemented the circuit above on a ne5532, running off 5v rail (@ 60+khz). Most of the component values are the same, maybe the capacitors are 4.7uf for the most part.

My DUTs in this case are a 100ohm resistor: the dut is either in the circuit (forming a 200ohm leg, with the 100ohm current limit resistor), or it is shorted out. So we should see that the current readings are cut in half.

Here is the meter's reading for either scenario:

100ohm: 39.4ua;
100ohm x 2: 19.9ua

Not bad.

[dannyf]

My next question is to move it to an analog meter.

I have a radioshack analog meter, that looks like the meter below:

Mine is a 50ua / 250mv full scale, a resistive reading of 20ohm at the center.

For that, I will adjust the resistors to get a full scale current through my meter at 50ua, and use a 20ohm resistor as the current limiting resistor.

[dannyf]

Component choices: no critical component other than R1 and C4/R7;

1) opamp: I tried TL072 and NE5532. NE5532 worked well. TL072 has limited current capability and will need to dial in more attenuation. Other considerations are swing and slew rate.
2) power supply: will need to be regulated, or you will need to adjust R12 as the battery runs down. 5v is ok but with limited swings so you have to use a more sensitive meter. For meters with 250ua or higher full scale, you want to go to 9v or 12v. 9v battery is a good power source here;
3) D1/D4: I used two bat54s (actually one serially connected BAT54S). Germanium diodes should work, and good old silicon diodes (1n4148) should as well - just watch out for clipping; Higher cut-off voltage on the silicon diodes can be an issue for a slow opamp or low supply voltage;
4) R5/R6: for less sensitive meters, you want to use higher value R5/R6. Higher value R5/R6 will require more output swing from U1.

R1: this is determined by the resistive scale on your meter (for convenience). My meter has a center reading of 20ohm so I used a 20ohm resistor. R1 and the dc-blocking capacitor (C4)'s ESR (R7) form the center resistance. So it is desirable that R1 is slightly less than the 20ohm value, by the amount of C4's ESR. So if your C4 has a (known) ESR of 3ohm, R1 should take a value of 17ohm.

I do not advise the use of a pot here, for reliability concerns.

C4/R7: this is the most critical part of the meter. You want the device to have sufficient capacitance, but most importantly, you want it to have low ESR. You can achive that by paralleling multiple capacitors. I have a few TPSDs (200mohm ESR, or 1% off) and I will parallel two of them here; T520s are also good choices here - but they are substantially more expensive;

In the event that you don't wish to calibrate the meter with resistors; or you don't mind running the meter off dual rails, C4/R7 can be eliminated and you do not need to worry about it.

[dannyf]

Output frequencies: the meter runs at 60Khz+; I also ran it at 10khz and 20khz. No difference in readings so I think you don't need to worry too much about the operating frequencies.

Oscillator waveform: the oscillator actually exhibits fairly significant distortion, due to the limited swing to ground. You can improve it but not that much can be gained there - I drove the meter with a signal generator and not a zap of difference. I wouldn't waste my time there.

Power supply: if you have an isolated power source, dual rail is the way to go; otherwise, for a portable device, I would use a 9v battery and run the meter off a single rail.

Attenuation: if you run the meter at 9v or 12v, dial in more attenuation, by increasing R3 to 33k or 47k or more, so the input to U1 is limited to ~100mv - the meter runs fine to about 500mv input to U1.

[G4ZWI]

couple of links you may be interested in...

 http://ludens.cl/Electron/esr/esr.html

And

http://www.members.shaw.ca/swstuff/esrmeter.html

Some interesting aspects,...  (I never considered the test lead socket resistance, for one....)

Cheers,   Fred

[dannyf]

A few more pictures.

I calibrated my one-opamp esr meter to my radioshack multimeter (50ua, center resistance reading of 20ohm) by using two 10-ohm resistors in serial (they measure to 19.9ohm).

First, zeroing the meter.

[dannyf]

This particular capacitor has an esr reading of less than 1ohm, per the meter.

[dannyf]

A 15ohm resistor: a shade over 15ohm esr.

[dannyf]

A 10ohm resistor reads a shade over 10ohm esr.

[dannyf]

A cap plus the same 15ohm resistor: reads more like 17ohm ESR?

[at2marty]

Looks good!  I may give that one a try next.  I'm currently working on a variation of Jay_Diddy_B's design.

[Jay_Diddy_B]

Hi Dannyf,

I was looking at the oscillator that you are using:



I have not seen this configuration before. Without C6, the 22uF capacitor, the circuit will work as a square wave oscillator, with the slew rate being determined by the op-amp. The -ve input will oscillate between 1/3 and 2/3 the supply voltage. The frequency will be much lower than 60 kHz with the components shown (C5, R11).

With  C6 included, the ESR of C6, the 22 uF capacitor and the 10 K resistors (R8, R9 and R10) are going to determine the hysteresis, and play apart in determining the oscillator frequency.

Did I miss something ?

Regards,

Jay_Diddy_B

[dannyf]

U2 is the oscillator - runs about 50 - 60Khz now.

U1 is the detector / rectifier. R10 is your coil meter - set to 50ua for me (2kohm resistance, 20ohm center resistance reading).

C1/R3 is the DUT: R3 is its ESR.

R1/R2 form the attenuator, so the output swing of U1 is max 1v.

R4 is set to equal the center resistance of your meter. For me, it is 20ohm (actually 22ohm is used as I don't happen to have a 22ohm resistor).

C3 is optional - see later.

[dannyf]

Adjustment procedures:

1) once the circuit is done, put in your coil meter, with DUT being shorted.
2) Adjust R1 until the reading is full scale (0 ohm reading).
3) done! Your meter is operational.

Check: put in a resistor whose resistance is the same as R4. Your meter should have its needle in the middle, giving a resistance reading equal to that of R4.

In reality, due to C3's ESR, the resistance reading should be slightly less than that of R4's.

PS: R7 is optional - it is used to adjust for meter sensitivity, which can be taken care of by R1.

[dannyf]

As I used 22ohm R4 on a meter with 20ohm center resistance, my readings should be all off a little:

20ohm reading = 22ohm true resistance;
10ohm reading = 11ohm true resistance;
...

Here are pictures from a few tests:

1) zeroing.

Just shy of 0 ohm.

[dannyf]

2) 22ohm resistor:

We would expect a reading of 20ohm. The actual reading is just shy of 20ohm. Blame initial zeroing + C3's ESR (about 1ohm).

[dannyf]

3) 11ohm resistor (=2 22ohm resistor in parallel).

We expect a reading of 10ohm. Actual is slightly shy of that.

[dannyf]

4) a 22uf capacitor.

Reading = 1ohm -> 1.1ohm, if we had the right value for R4.

[dannyf]

5) a 9v duracell:

~3.5ohm reading -> 4ohm actual.

[dannyf]

Optionality of C3:

C3 is not needed, if you are measuring a passive device without voltage on it - resistors, capacitors, inductors (yes, this meter can be used as an inductance meter).

However, if you want to measure batteries' ESR, you need C3.

I would suggest that you build your meter with and without C3 (ie. with two terminal outs).

[dannyf]

The meter is essentially a current measurement instrument: it works by measuring the (rectified) current, knowing that the rectified current is inversely proportional to R3 + ESR, if the oscillator's frequency is sufficiently high.

Because of this, an analog multi-meter is ideally suited for this design.

If you were to use a digital multi-meter, it works, with the following math:

1) short the DUT and measure the current through the dmm (=Izero);
2) put in the DUT and measure the current through the dmm (=Idut < Izero);
3) ESR = (Izero / Idut - 1) * R4.

You would be going through the same math with an analog meter, except that the scale on the analog meter does the math for you.

[dannyf]

Component selection:

1) most parts are non-critical, other than the few listed below.

2) opamp - I used ne5532 and tried others. ne5532 was selected for its abundance and reasonably fast speed - the opamp needs to swing at least 1v/us.

3) R4: needs to be the center resistance of your meter, to minimize the amount of math needed.

4) C5: determines the frequency of the oscillator.

5) R1/R2: need to attenuate so that the detector's output swing is around 1v. Too high of a swing will require higher supply voltage or lead to clipping / inaccurate reading. I have found that the output from the oscillator can deviate considerably from simulation so you will need to play with R1/R2's values - it is not impossible that R1 goes to 330k or even 470k, vs. R2 of 1k.

6) D3/D4/R11/R12: D3/D4 are schottky diodes (I used bat54s. Germanium diodes work too). R11/R12 can be replaced with schottky diodes too. Silicone diodes can be used if the supply is 9v or more.

7) DUT protection: you can put a pair of diodes on DUT in case it is charged. Not drawn here.

lower voltage operation: I didn't try this but if you go with a r2r (output only) opamp, you can get the meter to work at 3.3v or more, making it possible to use phone batteries here.

9) current consumption is very low, at 4ma.

[dannyf]

Found two 39ohm resistors. Plus esr of the cap, I Will be reasonably close to the 20ohm center resistance of my meter.

[dannyf]

Built it with those 39ohm resistors.

Measuring the same 22ohm resistor used earlier, after the meter has been calibrated to full scale.

Spot on!