Author Topic: ESR Meter Adapter Design and Construction  (Read 102127 times)

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Offline Jay_Diddy_B

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ESR Meter Adapter Design and Construction
« on: December 02, 2013, 11:03:30 pm »
Hi,
I am going to share the design and construction of an ESR Meter adapter design and construction. The plan is to end up with a design that is similar in appearance to Dave's ucurrent that will allow a DMM to be used as an ESR meter. At this point in time I have not built the circuit.

The requirements are:


  • To have a simple design that can be constructed on a single-sided circuit board.
  • Operates at 100kHz.
  • Be a true ESR meter as oppose to an impedance meter.
  • Work in either the 2-Wire or 4-Wire mode.
  • Be well protected from charged capacitors and applied voltages.

I will be using LTspice to verify the design goals.

Simulation Basic Functionality



This the LTspice model configured for normal operation. U3 is configured as a 100 kHz square wave oscillator. 1/3 of a CD74HC4053 analog Mux / Demux  is used to provide a current to the capacitor under test through R3, R9 and C5. The voltage across the capacitor under test is sensed via R5 and C5. Another 1/3 of the CD74HC4053 is used as a synchronous detector. U4 provide gain, resulting in a scale of 1V=10 Ohms.
R16 and R17 are included in the model to explore the effect of lead resistance.

If I plot Vout versus ESR, I get:



Simulation of a Charged Capacitor

I wanted to make sure that the adapter is not damaged if it is connected to a charged capacitor. To demonstrate this a switch is used to connect the adapter to a charged capacitor, the charged capacitor is connected at t=2ms:



Results



After a very brief transient the meter actual reads the correct value of ESR.



There is a brief impulse of current in R9. A special 'pulse withstanding' resistor will be selected for the R9 location.


Application of Line Voltage

Although not recommended, it would be nice to know that the adapter will survive being connected across the line. In this test 240V ac at 60 Hz is applied to the input.



Results of applying line voltage


The upper trace shows the dissipation in R5. (930uW average power)
The middle trace shows the dissipation in R9. (18.6mW average power)
The lower trace is the line voltage across the terminals of the adapter.


Real Schematic



This is the real schematic. It is very similar to the simulation schematic. The 74HC4053 has been broken up into four parts to make the schematic easier to read.

J1 and J2 are used for 2-wire measurements. J1-J4 are used for 4-wire measurements. J5 and J6 are the voltage output to the DMM.

Board Layout

Here is a picture of the board layout:



I need to add a switch for the power.

I will add more details after I have built the prototype. The LTspice models are attached if you want to play with the circuit.

Parts are on order.... to be continued....

Jay_Diddy_B


« Last Edit: December 04, 2013, 04:22:25 am by Jay_Diddy_B »
 
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Offline Harvs

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Re: ESR Meter Adapter Design and Construction
« Reply #1 on: December 02, 2013, 11:36:31 pm »
Nice, looks like it should be easy to build as well.  :-+

Hope to see some pictures of the prototype.  Assuming it all works well, any chance of getting either a gerber of the artwork or scale PDF?  Looks like a nice project to etch and assemble, but it doesn't look like I use the same CAD package as you.
 

Offline dannyf

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Re: ESR Meter Adapter Design and Construction
« Reply #2 on: December 03, 2013, 12:36:51 am »
Nice work. Keep it up.
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Offline ElectroIrradiator

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Re: ESR Meter Adapter Design and Construction
« Reply #3 on: December 03, 2013, 08:47:08 am »
Nice design. Have you considered that the 4053 is quite slow to switch on or off? At +5V supply voltage the switching time is about 0.5uS, which might cause non-linear indication at 100 KHz sampling frequency / 5 uS half period.
 

Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #4 on: December 03, 2013, 10:39:10 am »
Have you considered that the 4053 is quite slow to switch on or off? At +5V supply voltage the switching time is about 0.5uS, which might cause non-linear indication at 100 KHz sampling frequency / 5 uS half period.

Thank you for your input, this is the kind of feedback that I was hoping to get by posting my design. :-+

I have considered the delay from the select pin, to the switch opening and closing.

The 74HC4053 is about an order of magnitude faster than the CD4053 with maximum switching times of around 50ns (I checked TI and NXP).

The 74HC4053 in the source will compensate for the delays in the sense circuitry.

I will be building the circuit soon.

I am looking for a suitable enclosure. There are some advantages to keeping the board dimensions less than 100mm x 50mm, so that the board can be made cheaply at places like itead. The other requirement is that the box is deep enough to hold a 9V battery.

I have found the UB5 Jiffy Box from altronics

http://www.altronics.com.au/download/Datasheets/H0151.pdf

This the same box that was used by Dave in the uCurrent. It was also suggested by Harvs in a PM. This box seems to be Australian design and manufacture.

Does anybody know of a box like this with worldwide availability?

Jay_Diddy_B
 
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Online BravoV

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Re: ESR Meter Adapter Design and Construction
« Reply #5 on: December 03, 2013, 11:21:18 am »
Finally, a DIY ESR measuring circuit which is hobbyist's grade friendly and capable of 100KHz, also looks like its accurate enough at ESR < 100 miliOhm, cmiiw.  :clap:

Thank you !  :-+


The requirements are:
  • To have a simple design that can be constructed on a single-sided circuit board.
  • Operates at 100kHz.
  • Be a true ESR meter as oppose to an impedance meter.
  • Work in either the 2-Wire or 4-Wire mode.
  • Be well protected from charged capacitors and applied voltages.

I think you also forget another feature point which in circuit testing since the test voltage is so low. I would say specifying that its capable of test voltage at < 100 miliVolt is not too overly ambitious.

To others, the various test voltages span across the above sweep, maxed out at 40 miliVolt P-P, again, CMIIW.  :-+




Although not recommended, it would be nice to know that the adapter will survive being connected across the line. In this test 240V ac at 60 Hz is applied to the input.

This too in the features list, but I guess you should word it more carefully.  ;)


J1 and J2 are used for 2-wire measurements. J1-J4 are used for 4-wire measurements. J5 and J6 are the voltage output to the DMM.

If I'm going to use or build two coaxial cables for 4-wires measurements, how are the connections setup for the coax outer shield and it's core for both cables ?


Parts are on order.... to be continued....

Damn, this reminds me of your cool LPKF ProtoMat machine there, an instant PCB in just a few minutes, feeling so jealous.  >:D

Offline matkar

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Re: ESR Meter Adapter Design and Construction
« Reply #6 on: December 03, 2013, 08:54:34 pm »
Very nice design! I have tried to add another opamp on the output to scale up the output by a factor of 10. The idea is to magnify the reading in the region I mostly use (measuring faulty low ESR caps). One benefit is also no 10x multiplication of the reading is needed. Unfortunately I'm unable to get good results...
@Jay_Diddy_B
Anyway it might be a nice addition to your device to add a range switch to switch between 1:1 and 1:10 scale.
 

Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #7 on: December 03, 2013, 11:59:59 pm »
Hi,
The construction has started.

Printed Circuit Board

I have milled a prototype board on my LPKF Protomat c60:



Outside View

Here is a picture of the outside of the box. The 4mm jacks are Keystone Electronics 575-4.



Let the soldering begin.

To be continued ....

Jay_Diddy_B
 
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Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #8 on: December 04, 2013, 04:16:17 am »
Hi,

I can now report that the ESR meter works.

Here is a picture of the assembled board:




I connected all 4 wires together to make a short and the adapter reads 7.3mV which is equivalent to 73m Ohms.



I can use the relative button on the DMM to zero the reading



I am now measuring a 470uF 16V capacitor. The adapter reads 132m Ohms. My HP 4274A LCR meter gives me a value of 125 m Ohms 5.6% difference.




I am now measuring a 47uF capacitor. The adapter reads 1.185 Ohms. My HP 4274A LCR meter gives me a value of 1.15 Ohms 3% error.



I have noticed that the frequency is around 77 kHz. This can be corrected by changing R3 to 4.7K, otherwise the circuit is the same as the LTspice model.


It is looking good. :-DMM

Jay_Diddy_B
« Last Edit: December 04, 2013, 04:21:13 am by Jay_Diddy_B »
 
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Offline w2aew

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Re: ESR Meter Adapter Design and Construction
« Reply #9 on: December 04, 2013, 04:44:46 am »
Ooooooh, LPKF, nice! 

Congrats on the working proto, looks real nice!
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Offline Spikee

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Re: ESR Meter Adapter Design and Construction
« Reply #10 on: December 04, 2013, 11:28:06 am »
Could you post the Gerber files ?
I want to etch and test this circuit @ college.
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Offline Fraser

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Re: ESR Meter Adapter Design and Construction
« Reply #11 on: December 04, 2013, 12:31:01 pm »
Impressive .... a working and pretty accurate prototype at the ALPHA stage. Excellent work.
 

Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #12 on: December 04, 2013, 01:15:07 pm »
Hi group,

Spikee asked:

Could you post the Gerber files ?
I want to etch and test this circuit @ college.


Well here are the files. There are two set of Gerber files, one for the Hammond box that was used in the prototype, the other is for the UB5 Jiffy box. The layouts are the same. I changed the outline and the mounting holes.

Consider this experimental.

Jay_Diddy_B


 
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Online BravoV

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Re: ESR Meter Adapter Design and Construction
« Reply #13 on: December 04, 2013, 01:27:22 pm »
JDB, on zeroing the offset, does it help if using an opamp that has the offset adjutment using external potentiometer ?

About the opamp, I can see you are using different one at the proto than the one in the schematic, what is the minimum specifications if using different opamp ? I guess this particular question will come sooner or later.  :P
« Last Edit: December 04, 2013, 01:36:22 pm by BravoV »
 

Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #14 on: December 04, 2013, 01:51:42 pm »
Hi,
The op-amp used on the prototype is the same as the one used in the LTspice models. I used the LTC6241. The op-amp on the 'real' schematic was a place keeper. It has the same pin out.

The LTC6241 op-amp has the right combination of GBW, Slew rate and Vos.

http://www.linear.com/product/LTC6241

The Vos for this op-amp is 125uV max, 40uV typical.

The Vos is multiplied by the high gain of the DC amplifier stage x55.

So we can up to 6mV from the Vos, typically 2mV.

There are lots of other sources for the non-zero reading.

The reading of 7mV is 0.35% of full scale.

I considered adding a zero pot. There were three considerations:

  • I needed a dual op-amp. Dual op-amps don't normally have offset pots. It would be tricky to design a stable offset adjustment circuit.

  • When used in the 2W mode for troubleshooting, It is necessary to zero the leads, which might be 0.2 Ohms or more. This would need a wide range on the zero pot.

  • Most of the better DMM have a Relative button. So I decided to take advantage of this feature and keep my circuit as simple as possible.


These were choices that I made during the design process. You can experiment with modifications. One modification would be to take the output and the meter 0V and measure these with an Arduino. This would make a nice self-contained unit. This would take care of the x10 scaling and deal with the zero adjustment.

Jay_Diddy_B

 

Offline quarks

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Re: ESR Meter Adapter Design and Construction
« Reply #15 on: December 04, 2013, 02:02:53 pm »
Looks very good, thanks for sharing
 

Offline Spikee

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Re: ESR Meter Adapter Design and Construction
« Reply #16 on: December 04, 2013, 02:16:35 pm »
Does anybody know if it is possible to get that UB5 box in europe ?
I have looked on the Altronics website but they don't list any dealers outside AUS.

I have also a uCurrent pcb laying around that needs a nice box =/

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Offline dannyf

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Re: ESR Meter Adapter Design and Construction
« Reply #17 on: December 05, 2013, 01:10:00 am »
Quick thoughts:

1) the switches on the front end (oscillator's output) aren't needed.
2) the unused switch can be utilized to introduce an automated zeroing function potentially.
3) you can build a stand-alone version with a mcu (that has a PGA): the mcu will generate pulse output, and the rectified output is then amplified by the mcu and digitalized via its onboard adc to drive a lcd, or to control a pwm pin to drive an analog meter. The autozeroing function can be implemented here.

The basic architecture of the meter is good and flexible.
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Offline Jay_Diddy_B

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Re: ESR Meter Adapter Design and Construction
« Reply #18 on: December 05, 2013, 01:53:50 am »
Hi,
The switches on the output of the oscillator perform three functions:

1) they have the same thresholds as the switches in the synchronous detector and the same propagation delays from the select input to the switches opening and closing.

2) The switches also increase the slew rate of the source. The op-amp has a minimum slew rate of 5V/us and a typical slew rate of 10V/us.

3) The switches also ensure rail to rail performance of the source.

I would be interested to see how the unused switch can be used to generate an auto zero function. Can you post a schematic?

Jay_Diddy_B

 

Offline dannyf

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Re: ESR Meter Adapter Design and Construction
« Reply #19 on: December 05, 2013, 02:17:51 am »
Sure. Cut open the wire between the dut and the resistor to its right. Insert a switch there. When zeroing is off, the switch reconnects the resistor to the dut (as it is now); When on, the switch connects the resistor to the virtual ground.

So when zeroing is on, the output on the opamp would be outputing a drift that needs to be netted out. When a mcu is utilized, it would periodically turn on zeroing, and measure the output of the opamp.

When the zeroing is off, the mcu can measure the opamp's output and then subtract from it the drift -> you have zero'd out the drift.
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Offline dannyf

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Re: ESR Meter Adapter Design and Construction
« Reply #20 on: December 05, 2013, 02:21:48 am »
This approach doesn't zero out the drift from the front-end (especially if you use an opamp as an oscillator or have asymmetrical up/down slopes).

A "better" approach is to use a low Rds mosfet as a switch to short out the dut periodically and measure the overall drift, in a similar fashion. It has the disadvantage of requiring a DC voltage / constant current source across the dut.

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Online BravoV

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Re: ESR Meter Adapter Design and Construction
« Reply #21 on: December 05, 2013, 03:46:24 am »
The Vos is multiplied by the high gain of the DC amplifier stage x55.

Ok, noted.

How about others specs, like minimum GBW ? Slew rate ? Noise ? Other else ? Will a cheap jelly bean fet amp like LF411 is "good enough/decent" as replacement ?

I have a limitation here, its not like I don't want to use those Linear op-amp as your recommendation, actually I can afford to purchase op-amp like LTC6241 say like $2 to even $5 a piece which is no big deal, but its just my location that most distributors and also like a direct purchase through Linear Tech, these guys will hammer me like $70 to $90  :o just for the shipping cost alone, which is not funny at all.  :(

So please understand me, finding for an alternative op-amp is the only way, and don't worry, even though I'm a electronics noob, but not that noob enough that I will blindly ask if I could use a 741 or LM324 as the replacement for this circuit.   :palm:

I wish I'm not alone here.


There are lots of other sources for the non-zero reading.

Such as ? Asking this cause I may ended up with low accuracy discrete components, just the critical parts that will affect the performance or accuracy, the plan is to use through hole parts.

Also about the gain, will a single voltage divider at the op-amp output will some how "temporarily" fix the gain error if I happened to use less accurate component ? I know, the best part is still use the exact components as your circuit, please bear with me here.  :'(


When used in the 2W mode for troubleshooting, It is necessary to zero the leads, which might be 0.2 Ohms or more. This would need a wide range on the zero pot.

Say I will use only 4W mode, the op-amp zeroing pot still feasible ?
 
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Online BravoV

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Re: ESR Meter Adapter Design and Construction
« Reply #22 on: December 05, 2013, 03:51:08 am »
Sure. Cut open the wire between the dut and the resistor to its right. Insert a switch there. When zeroing is off, the switch reconnects the resistor to the dut (as it is now); When on, the switch connects the resistor to the virtual ground.

So when zeroing is on, the output on the opamp would be outputing a drift that needs to be netted out. When a mcu is utilized, it would periodically turn on zeroing, and measure the output of the opamp.

When the zeroing is off, the mcu can measure the opamp's output and then subtract from it the drift -> you have zero'd out the drift.
This approach doesn't zero out the drift from the front-end (especially if you use an opamp as an oscillator or have asymmetrical up/down slopes).

A "better" approach is to use a low Rds mosfet as a switch to short out the dut periodically and measure the overall drift, in a similar fashion. It has the disadvantage of requiring a DC voltage / constant current source across the dut.

dannyf, please, really appreciate your efforts by helping this noob to visualize on what are you suggesting, if you don't mind.

It doesn't need to look pretty as long its readable, even a photo of a rough hand sketched drawing on paper should be more than enough.  :-+
« Last Edit: December 05, 2013, 03:58:14 am by BravoV »
 

Offline oldway

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Re: ESR Meter Adapter Design and Construction
« Reply #23 on: December 05, 2013, 10:05:31 am »
Thank you for sharing this very interesting project.
ESR meter is very useful for the repair and whe do'nt need a high precision ESR meter for this purpose.
Design is simple, low cost, reliable and well protected.

Some suggestions:
- The worst that could happen would be to measure a charged capacitor, the highest possible voltage is 400V (SMPS with PFC). Your design seems to be protected against such error.
- some people claims to use cheap and easy available components.
1) +/- 2.5V seems to be low edge power supply for most of the OP's.I would feed the OP's with half of the battery voltage. (+/- 4.5V).
2) For this reason, i would make a different power supply project, generating +/- 4.5V unregulated (half battery voltage) and +/- 2.5V regulated.
3) I would use 3 OP's of a LM324 (9V power supply) to do this.
- one OP with BC 847 + BC857 would provide a 0V center point.
- an output of another OP would feed the +2.5V power rail
- Output of the third OP would feed the -2.5V power rail.
reference votage of 2.5V would be provided by a TL431.
4) as oscillator, i would use a Cmos version of 555, (a TLC555 for example) with this very simple basic design running at 200Khz. (with 5V power supply)
http://wiki.xtronics.com/index.php/555_Delta-Sigma_Modulator
5) For 50% symmetry and inverted outputs, a would use halve of a CD4013B D type flip flop. (5V supply). (Square wave of halve the frequency of oscillator = 100Khz)
6) For switches, a CD4066B or better, a CD74HC4066 could be use in place of the CD74HC4053. (5V power supply)
7) as OP, i would use a TL081 with offset compensation and 9V power supply.

But you basic project is very good, nothing need to be changed if components are available and not too expensive.

 

Offline dannyf

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Re: ESR Meter Adapter Design and Construction
« Reply #24 on: December 05, 2013, 11:53:28 am »
Quote
1) +/- 2.5V seems to be low edge power supply for most of the OP's.

2.5v rails are good for usb powered devices.

I would actually go down to 3.7v/2 so it can be easily powered by li-ion batteries.
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