Author Topic: One opamp esr meter  (Read 21076 times)

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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #25 on: November 23, 2014, 10:19:39 pm »
Lessons learned:

1) R11 / R12 are best to be resistors (vs. diodes): using diodes will make the meter very efficient, so efficient in fact that you have to use more attenuation and by-passing (lower value R7). Resistors in R11/R12 allow far easier time zeroing full scale reading - the adjustment is smooth.

2) R1 being a multi-turn pot: how much attenuation depends on supply voltage and opamps used: the higher supply rails, or the more R2R swing opamps, the more attenuation you will need. So don't put R1 during the build. After the circuit is built, put in a high value R1 (470k or so) and work your way down, before finalizing its value. I ended up needing more than 200x attenuation at 5v.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #26 on: November 24, 2014, 12:50:40 am »
I ran the resistor version of the design with 1n4148s: it works well.
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Offline free_electron

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Re: One opamp esr meter
« Reply #27 on: November 24, 2014, 01:27:43 am »
title says : one opamp... i count two ...
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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #28 on: November 25, 2014, 01:06:42 am »
couple of links you may be interested in...

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

That one is quite interesting. It uses a simiiar looking oscillator, at a much lower frequency. A transformer to attenuate the signal and lower its output impedance. I used an opamp to do that.

The scales are slightly different.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #29 on: November 25, 2014, 01:35:14 am »
If I were to put the DUT on the non-inverting end, I would get (near) linear scale for the read-out.

I will explore that more later.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #30 on: November 25, 2014, 04:50:58 pm »
A real precision rectifier, like yours, would greatly improve linearity of the readings. And you can do it via a quad now, :)

To achieve 1v/1ohm reading is trivial. To get to 10v/10ohm, you will need to increase the supply voltage as well.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #31 on: November 25, 2014, 10:59:17 pm »
Quote
in your case, all you need was to balance the output to a ua-meter scale (via R1 fine tune) right?

Yeah. The gain (50ua/20ohm) was determined in part by the use of the 20ohm resistor and attenuation (R1). 20ohm was selected due to the resistance scale of my meter.

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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #32 on: November 26, 2014, 10:51:34 pm »
The current going through the ammeter is dependent on the supply voltage.

As such, when I built the first copy, I decided to put a voltage regulator, gauged to run on my meter.

After a while, I thought there may be reasons to not use a voltage regulator and actually power the meter from different voltage sources as a way to use different ammeters - those with less sensitive scales can be used at higher supply voltage, and vice versa.

So, how does the meter behave under different supply voltage?

Here is the meter running on 5v (5.1v actually) measuring a 1ohm resistor (in parallel with other larger resistors).

Current when the dut is shorted: 90.9ua
Current when the dut is in: 86.2ua.

So the resistance is estimated to be (90.9ua / 86.2ua - 1) * (39//39ohm)=1.06ohm.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #33 on: November 26, 2014, 10:54:28 pm »
Here is the same meter, running at 12v:

Current with the dut shorted out: 406ua;
Current with the dut in: 385ua.

Resistance estimation: (406/385-1)*(39//39) = 1.06ohm, :)

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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #34 on: November 26, 2014, 10:56:24 pm »
The excitation voltage on the dut depends on the attenuation and in my case, the attenuation is about 200:1 and the output signal from the oscillator is just shy of 3vpp. So I am looking at 15mvpp on the dut.

That means you can safely utilized the meter for in-circuit measurement.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #35 on: November 26, 2014, 11:05:15 pm »
Quote
by repositioning the dut to the non-inverting end.

This approach has some severe limitations, as I found out.

As the attenuation goes up with a low ESR part, the gain on the amplifier has to go up significantly to overcome the non-linear region of the diodes. For a reasonably fast part like ne5532, 30x is about its practical top end. That means the input signal to the opamp needs to be 3mv or so if schottky diodes are used in the feedbck loop.

With 3vpp output of the oscillator, and 110ohm on the input of the attenuator, the minimum this approach can reasonably measure is 0.1ohm - I found out that anything shy of 0.5ohm will be over-estimated by the meter.

It can be solved via 1) the use of a fast opamp in the precision rectifier; or 2) additional gain stage(s).

I think a better approach is to go with a passive rectifier (as shown earlier), if you want voltage read-out.

I also experimented with using an instrumentation amplifier to read out the differential voltage from the rectifiers but that has the same issue with attenuation. More complexity but minimum gain.
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Offline 3roomlab

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Re: One opamp esr meter
« Reply #36 on: November 27, 2014, 09:34:19 am »
hmmm i guess its better off measuring AC instead of rectifying then.
 

Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #37 on: November 27, 2014, 01:28:27 pm »
Millivolt meters may not have the frequency response needed here -> at least through 50k. And many of the RF types that go above that simply uses diodes to rectify the input signal and then amplify it later -> it has non-linearity at the lower end as well.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #38 on: November 27, 2014, 06:46:34 pm »
Another option is to use a mcu to adc the voltage differentially, convert the voltage reading into ohm and then drive an analog meter, led display or lcd display.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #39 on: November 29, 2014, 07:18:52 pm »
One of the advantages of living in a poor country is the abundance of cheap equipment.

Here is a meter that I got for a song, with a center resistance reading of 10ohm.

Zeroing to full scale (voltage) reading now -> zero ohm.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #40 on: November 29, 2014, 07:22:37 pm »
Here is the meter reading a 1ohm resistor.

It looks spot on only because of the viewing angle -> in reality, it reads slightly bigger than 1ohm, more like 1.1-1.2ohm.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #41 on: November 29, 2014, 08:19:52 pm »
To read lower ESR values, it helps with a meter whose center resistance reading is lower.

Here is a toy multimeter, with a 5ohm+ center resistance.

The picture on the left is where the meter is being zero'd. The picture on the right is where the meter is measuring a 1ohm resistor.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #42 on: December 02, 2014, 10:49:34 pm »
I talked about earlier how this meter can be used to measure inductance.

Here it is, measuring a 47uh (norminal) inductor (JW Miller 2309).

The reading is just shy of 25ohm. As my oscillator runs at 64.5Khz, a 25ohm reading translates into an inductance of 62uh - if you consider the fact that the inductor's impedance is orthogonal to the 5ohm center resistance, the number works out to be 60uh but 62uh is good enough for us.

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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #43 on: December 02, 2014, 10:51:55 pm »
If you run the oscillator at 16Khz (15.9Khz to be exact), a 10uh inductor will read 1ohm. So you can simply multiply the resistance reading by x10 to obtain your inductance reading. Most opamps, even the lowly 741, can run at this kind of frequencies.

If you run the oscillator at 159Khz, a 1uh inductor will read 1ohm. Unfortunately, you will need a really fast opamp to do that. NE5532s that I used here are not sufficient here.
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Offline 3roomlab

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Re: One opamp esr meter
« Reply #44 on: December 04, 2014, 12:39:57 pm »
hmmm interesting. maybe i should try n grab a cheapo china meter
 

Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #45 on: December 04, 2014, 02:07:06 pm »
I'm working on a version that will provide a linear reading on dmm, either current or voltage.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #46 on: December 06, 2014, 03:49:05 pm »
Measuring 5ohm (10//10ohm resistors):

This is the calibration point.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #47 on: December 06, 2014, 03:50:42 pm »
Measuring a 10ohm resistor:

We should get around 1.00v reading, vs. 0.983 actual.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #48 on: December 06, 2014, 03:53:25 pm »
Measuring a 4.7uf cap:

I think it was measured to be about 1ohm ESR. The meter has a residual reading of 25mv (ESR from C1). So the actual reading of 134mv (=1.34ohm) should mean 1.34-0.25=1.1ohm, had I zero'd the reading.
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Offline dannyfTopic starter

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Re: One opamp esr meter
« Reply #49 on: December 06, 2014, 03:55:33 pm »
Measuring a 22uf cap:

A reading of 157mv means 1.57ohm - 0.25ohm = 1.3ohm ESR.
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