Cheaper ESR meter

[gs3]

Re: Cheaper ESR meter

I forgot one more diode 1n4148 and the PCB.

I guess it is difficult to assess value on the parts because just the galvanometer and box would make a big difference depending on what you are using. Not counting those, the rest comes out at maybe 2 ~4 dollars if you have to buy it.

The PCB I built is tiny. If you do not mind a bigger size you can use regular strip PCB.

You can see I did not use jacks for the test leads and the probes are just rigid copper wire. Everything except the PCB is recycled.

[gs3]

Re: Cheaper ESR meter

I agree that it can be built for very little. As I said, I already have what I need as I have built several. The only thing is that I am involved in this for fun and I would like to see this mass produced just for the heck of it. I believe it could be sold for just a few dollars.

By the way, here is an approximate parts list. As I say, I keep making changes and no two are alike.

16 resistors
4 resistors 1% tolerance for the bridge
Trimmer resistor to adjust ammeter end of scale

4 x 1 uF tantalum caps
1 x 1 nF
1 x 100 nF

DS1,DS2=1N4007
DS3=1N4148
1 LED
1x BC547
1x BC557
IC = TL084

Box, galvanometer, switch, 9V battery, Test leads and probes....

I think that's it

You can use a multimeter and save the cost of the galvanometer.

I am attaching photos of one of my models. The box was shoddily made from PVC. I had the mini galvanometer in my junk box. You can see the PCB is tiny when you compare it with the size of the IC and of the 9V battery.

[gonzo0815]

Re: Cheaper ESR meter

If this thing is so darn cheap, there is no ense in "massbying" it fpr international people. Considering the enormouse shippment cost.

[whiz]

Re: Cheaper ESR meter

I was thinking of approaching some Chinese company and selling them the design and prototype in exchange for a few samples of the finished product...

if it is possible...why not?


P.S i think it would be nice if you start a group buy for all the others that might be interested building an ESR meter...that is what i wanted to say before...

[gs3]

Re: Cheaper ESR meter

Well, the switch, like the box, ammeter, etc. is something you can use anyone you like or have handy and it makes no difference. I happen to have in my junk box a push switch whch pops a green button when it is on so the situation is immediately visible. I may use that if I ever build a "final" sample.

As long as you understand the circuit, you can build it with the schematic as it is. But if you can only follow exact instructions without being able to troubleshoot the circuit then I would advise that you wait.

As I said, I have built several already so I am not really interested in participating in any "group purchases". Also, depending on the ammeter and other factors you will need different components.

It seems to me that if you decide to make a bunch of them all the same then the best course is for someone to build them and sell them.

I was thinking of approaching some Chinese company and selling them the design and prototype in exchange for a few samples of the finished product... Of course, I have also thought of learning Chinese but never got around to it.

[whiz]

Re: Cheaper ESR meter

I think the best solution may be a power button which changes color (mechanically) when it is on. That uses no energy.

I have several other thoughts. Maybe rechargeable batteries. Maybe two or three cells and a switching circuit to raise the voltage....

try to keep the circuit as simple as it can be with easy to find components even in a local store! and ofcource effective!
keep in mind that most of us we don't live in the same country, and from my latest expirience with a project...i can tell you must go for safari to get some components!

how about after you fisinsh it to make a group buy for the board?!

[gs3]

Re: Cheaper ESR meter

Good idea whiz. The thing though is that I was thinking of using the same LED that is already in the circuit. Also, you have to weigh the soncumption of the "reminder" LED with the consumption of forgetting the thing on. Maybe it is not worth it. I think the best solution may be a power button which changes color (mechanically) when it is on. That uses no energy.

I have several other thoughts. Maybe rechargeable batteries. Maybe two or three cells and a switching circuit to raise the voltage....

[whiz]

Re: Cheaper ESR meter

I am still considering adding a circuit which will make the LED flash briefly at intervals as long as the instrument is turned on. This will remind us to switch it off and therefore save batteries.

don't use cirquit...just put a led flasher costs almost as a simple led and it doesn't waste any space...the power consuption i think it's the same (or less) as if you were making a cirquit with transistors....

[gs3]

Re: Cheaper ESR meter

I made a small PCB but this design does not exactly match the schematic because I have introduced changes. The resistors in series with the ammeter depend on the particular instrument. As I say, I have made several but no two exactly alike. I am trying to make some final improvements and then give it final form. The you have to assemble several to make sure they all work well and within tolerance in spite of component variation etc.

I thnk this could be cheaply manufactured in China and sold at a low price. Similar to those analogue (or even digital!) multimeters that cost $2.

One of the guys at the other forum built the PCB to fit inside one of those meters and transformed it from multimeter to ESR meter.

[gonzo0815]

Re: Cheaper ESR meter

Well nice. Have you did an PCB or just soldered on a standard board?

[gs3]

Re: Cheaper ESR meter

Hmmm, it seems that page www.qsl.net/iz7ath/web/02_brew/15_lab/06_esr/ is down now. I found the google cached page but it does not show the graphics which are the most important part. Maybe it will come online again soon.

At any rate, here is my version as it stands now. Still a work in progress though. Notice some values are not final yet and some labels are not consecutive etc. But I will answer any questions.

The ESR meter has two test probes to connect it to the capacitor under test without removing it from the circuit. This injects a signal of about 100 Khz and under 100 mv(pp). This low level signal will not be enough to trigger semiconductors in parallel with the capacitor and most components one can expect would have higher resistance and would not interfere with the measurement.

The circuit is a bridge of four resistors which is normally balanced and the capacitor is placed in parallel with one of the legs which introduces an imbalance which is amplified by an OpAmp. The lower the ESR, the greater the imbalance and the higher the reading.


OpAmp A, first from the left, divides the 9V power supply into two halves so we have +4.5 and -4.5 with respect to the center point which becomes our virtual ground. This symmetrical power supply is required by the opamps which follow.

Second opamp, B, is the basis for the 100 Khz oscillator. In practice I have noticed that the frequency tends to be not close to 100Khz, probably due to component tolerance values, so it is best to check it and trim values to bring the frequency close to 100 Khz. While this is not essential, it permits better comparisons between units built. At the output of this opamp we should have a square wave between -4 and +4 V pp.

The transistor which follows serves several purposes. It shifts the level of the signal so it is always positive, between 0 and +4 and it isolates the load that follows from the output of the opamp which does not have a low enough output impedance to drive the bridge directly.

At the collector of the transistor we find the bridge which is the central part of the unit. The two upper resistors are much larger in value than the two lower ones so that the voltage which is put to the capacitor under test is a small fraction of the output of the transistor. Any imbalance in this bridge is amplified by the next opamp, C. Let us analyze the different cases carefully.

1- When the leads are open, not connected to anything, the bridge is balanced and the output of the opamp will be a constant zero volts.
2- When we connect a good capacitor we are shorting one branch of the bridge to ground in AC only, not in DC. Therefore the output will be an AC waveform with no DC component, i.e. it will be centered at 0 V and swing up and down from 0V.
3- When we further short the capacitor, then the branch of the bridge is shorted to ground in DC also and the output of the opamp will be shifted upwards so that it has a positive DC component. Now the entire waveform will be above 0 V.

At the output of the opamp we separate the AC and the DC components. First we see a low pass filter, composed by a resistor and capacitor, which allows the DC component to drive the base of the transistor which controls an LED. This LED will light if the capacitor under test is shorted or has high leakage current.

The AC is allowed through a high pass filter, formed by a capacitor (which blocks the DC component) and a resistor, into the input of the last opamp which is a rectifier. The lesser the ESR of the capacitor under test, the greater the rectified voltage will be. I adjust the circuit so that the maximum voltage out of the rectifier is about 1 volt. Then I have inserted a diode in series with the instrument. The purpose of this is to expand the range of the instrument in the low ESR values and compress it in the higher ones. As the voltage rises, at first the needle hardly moves, but as the voltage passes 0.6 volts, the needle moves faster. This means the lower 0.6 volts hardly move the needle and the upper 0.4 volts are expanded to almost the entire range of the instrument.

By adjusting the output to less than 1 V we could make the instrument even more sensitive.

I have built a couple of prototypes using cheap galvanometers I had in my junk box. I have considered modifying the design to use a bar of LEDs but I decided not to do this for two main reasons. One is that a galvanometer can usually be found cheaper and gives more resolution than a bar of, say, 10 LEDs. Another is that a galvanometer will use much less battery power which is an important consideration when using 9 volt batteries.

I am still considering adding a circuit which will make the LED flash briefly at intervals as long as the instrument is turned on. This will remind us to switch it off and therefore save batteries.

[whiz]

Re: Cheaper ESR meter

Very nice!!! keep going!

[gonzo0815]

Re: Cheaper ESR meter

Didn't found a schematic, can you provide one? A cheap cheker is always a good thing for me. Your improvements are interesting too.

[gs3]

This is a very cheap ESR meter. Initially published in the Italian magazine Nuova Elettronica as an accesory to use with an analog multimeter. Shown in this page and discussed in Spanish here.

It is extremely simple to build. It only has an TL084 quad opamp IC, a couple transistors and a few other pasive components. Not much else. I have made some modifications of my own to improve it. I removed one transistor which was not needed and I modified the output part so it would be more sensitive in the lower ESR values. I have built several prototypes and they all work well but I am still tinkering with some improvements and do not consider the design finished yet. My PCB design is tiny (33 x 38 mm) with full size components and you only need to add a 9V battery and an analog ammeter or external multimeter. I made the box with a bit of PVC conduit and the whole thing cost me close to nothing.

One thing it has over others and which I consider indispensable is that it will light an LED if the capacitor is shorted or has high leakage. While it may not be as precise as digital ones costing many times more, it is more than enough for quick capacitor checks when doing repairs. You do not need exact values because if a capacitor is bad its ESR has gone way out of range, not just increased by a bit.

You can build it as shown in the page I linked to above or wait and I will post my modifications as soon as I get around to it. Mine is definitely more sensitive but, as I say, I build them as prototypes and tinker with them to the point where no two are alike. i will try to come up with a final design and then I will probably make a page with all sorts of explanations on how it works and how to build it and use it.