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Split Power Supply for ESR Meter

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Musicmanager

Active Member
Hi Guys

Remember I'm a novice .. ...

I've built an ESR meter from a schematic I found on the web .. .. copy is attached.
Although intended to be powered by a 9v battery I set it up with my bench power supply because it has current limit protection and I thought ... if I've done my usual and put something in the wrong place .. .. .. you know what I mean !
Well, as anticipated it trips out the current limit immediately and I've thoroughly checked my work is exactly as the schematic ..

My question .. .. top left hand corner of the schematic shows 9v supply coming from the battery via a switch with two 10K Resistors ( R1 & R2) which I recognise as a voltage divider, feeding into pin 3 of Op Amp A, effectively pin 4 is 4.5v; pin 11 is -4.5v and pin 1 is Gnd or virtual ground as the author calls it.
I think the supply rail between the R1( marked V+) and Pin 4 of the Op amp (marked 4.5v) should be cut otherwise the supply will maintain 9v at Pin 4 ????

Any advice would be welcome ..

S
 

Les Jones

Well-Known Member
Most Helpful Member
The circuit looks correct to me. Your perception is all to do with which reference point you are referring to. If you are using the battery negative as your reference point (Zero volts.) then pin 4 will be at + 9 volts. Using this reference point the "virtual ground" will be at + 4.5 volts. If you now change your reference point to be virtual ground then that is now zero volts. The battery positive (Pin 4 on the op amp) now becomes +4.5 volts and the battery negative is - 4.5 volts. You can think about it in the same way as marking out a piece of material to have holes drilled equally spaced from the centre line. You would probably draw the centre line an then measure from that as opposed to measuring everything from one edge. It is possible that the negative of your power supply is connected to ground and you have something else connected to virtual ground (Such as an oscilloscope.) which will also be earthed thus connecting the +4.5 volt point (WITH REFERENCE TO YOUR POWER SUPPLY NEGATIVE TERMINAL.) This would only apply if the trip current setting was quite low as the op amp would not be able to draw very much current in trying to deliver 4.5 volts into a short circuit. I think it is more likely that you have a solder splash shorting out the power lines.

Les.
 

Musicmanager

Active Member
Hi Les

Thank you for that .. .. your explanation is exactly how I considered it when I started the build .. it is only when I found a problem that I gradually talked myself out of it !!
I've checked for solder splashes quite carefully .. .. however, I do have the current limit set very low ( so there would be only a little magic smoke :) ) I hadn't considered that possibility - may be it is working but I'm not allowing sufficient current for the op amp to function.

I've given up for this evening now, I'll have another play tomorrow

Thanks again

S
 

Musicmanager

Active Member
Morning Guys

I wonder if someone can further my understanding of this issue .. .. .

I've attached 3 'scope screenshots taken form pins 5,6 & 7 of a TL 084 Op Amp which is part of the circuit - schematic above .. ..

Pins 5 & 6 show a nice clean sine wave on the inverting and non-inverting inputs, but pin 7 is fluctuating all over the place, very messy (noisy) and I've got no input to pin 10 as a result, I presume. I've tried adjusting the resistors around the inputs with little effect, but when I tried to alter the resistance of the loop between 6 & 7, I either get no change or nothing at all .. .. .

Me no understand .. .. schematic speak with false tongue, Kemo Sabi !!

S
 

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large_ghostman

Well-Known Member
Most Helpful Member
Not advice to be recommended but maybe its your scope ground, at that voltage i would take the earth wire out the plug to the scope and try again. have you measured -4.5V with a volt meter? using virtual ground as the reference?
What is up with me today? I am tired and my brain is on strike!
 

alec_t

Well-Known Member
Most Helpful Member
Pins 5 & 6 show a nice clean sine wave on the inverting and non-inverting inputs
Not in the pics you've posted ;). Looks closer to half cycles only. Are you over-driving the inputs? Is your virtual ground sitting at half the rail-to-rail supply voltage?
 

tomizett

Active Member
Seems reasonable to me... With no cap installed the bridge is balanced (R9/R11 = R8/R10), so the signals at pins 5 and 6 are identical. The op-amp amplifies the difference between them, but there is no difference. The result? Very little at pin 7. You should see a signal appear here when you apply a cap (or a resistor) across the test terminals. With the terminals open, your meter should be reading off-the-scale maximum ESR.
 

Les Jones

Well-Known Member
Most Helpful Member
If you have nothing connected to the test terminals then the bridge will be balanced. (To within the tolerance of the resistors in the bridge.) If the bridge is balanced then the signals on the + an - inputs of the op amp will be identical. When the input signals are identical then there should be no output from the op amp. What you are seeing on the output (Pin 7) is noise plus a small amount of the DIFFERENCE of the input signal amplified. That stage should have a gain of about 27. (R15/R12 = 27K/1K = 27)

Les.
 

AnalogKid

Well-Known Member
Most Helpful Member
Back up a step. Please post scope shots of U1 pin 14 and of the collector of Q1. Without knowing what the exciting waveform looks like, you can't know what is noise and what is signal.

ak
 

Musicmanager

Active Member
Hi Guys

Sorry for the silence ... .. just after I made that post, my senior manager decided I was taking her shopping .. .. deep joy, as the saying goes !

Anyway, I've posted a screenshot of pin 14 and the collector of Q1 as requested ... .. also I've used a DMM between virtual ground and the emitter of Q1 and found 4.35v ... .also between virtual ground and pin 4 gives 4.32v and pin 11 gives - 4.32 v so I believe the voltage divider and U1A are working correctly.

S
 

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JimB

Super Moderator
Most Helpful Member
Stuart
Backing up a few steps more:

Is the 0v side of the bench PSU connected to mains earth, ie the case of the PSU ?
I think that this may be your basic problem, the PSU earth and the scope earth* are effectively shorting out the negative side of the supply.

also I've used a DMM between virtual ground and the emitter of Q1 and found 4.35v ... .also between virtual ground and pin 4 gives 4.32v and pin 11 gives - 4.32 v so I believe the voltage divider and U1A are working correctly.
Did you do this with the scope connected to the circuit?
If not, re-connect the scope and measure it again. This will prove/disprove my hypothesis.

* Disconnecting mains earths from scopes is generally a bad idea.
There are those here who will disagree, I do not wish to repeat the debate here.

JimB
 

AnalogKid

Well-Known Member
Most Helpful Member
Circuit comments IMHO:

At 100 kHz the TL084 has an open loop gain of only 30, or 30 dB. That's not much. Also, it appears to have very asymmetrical input overload recovery times. These combine to distort severely what should be a 100 kHz symmetrical square wave at pin 14. Not that there's anything wrong with that. The original designer might have taken it into account in the meter calibration. Still, looks bad.

Also it appears that Q1 is being starved by R7. I would expect a transistor running wide open to saturate, but Q1C peaks at 1.5 V, 3 V below saturation. Again, that might be intentional on the part of the circuit designer. But the linear region gain of a transistor varies significantly with temperature, collector current, base current, and the phase of the moon. Not good for calibration stability.

Pin 7 is not fluctuating at all. First, even though it is noisy it exactly represents the output of the excitation oscillator (which as noted is not a sinewave, either by design or accident). U1C functions as a differential amplifier, but it's common mode rejection is critically dependent on the matching of the four feedback resistors. Same for the null balance of the bridge. Seat of the pants: 1% resistors mean that 1% of perfectly balanced inputs will get through. You have about 2 mVpp out and a gain of 27, so all you need is a total input error of 75 uV. The exciting waveform is 1.5 Vpp, the bridge attenuates that to 21 mV, and 1% of that is 100 uV. So you're over budget. Now, those are worst case first order estimates, and I'm sure a real calculation will yield better error margin numbers. But the circuit basically relies on one calibration adjustment to clean up all errors. That might be fine for large output values, but the errors assume a larger percentage of the output as the output decreases.

The link in post #5 is to a very similar circuit that does not have nearly as much error sensitivity, and has a minimum signal cutoff below which there is no meter reading. If you don't want to mess with a 100 kHz small signal transformer, it is easy to get around with a circuit change.

ak
 

large_ghostman

Well-Known Member
Most Helpful Member
Stuart
Backing up a few steps more:

Is the 0v side of the bench PSU connected to mains earth, ie the case of the PSU ?
I think that this may be your basic problem, the PSU earth and the scope earth* are effectively shorting out the negative side of the supply.


Did you do this with the scope connected to the circuit?
If not, re-connect the scope and measure it again. This will prove/disprove my hypothesis.

* Disconnecting mains earths from scopes is generally a bad idea.
There are those here who will disagree, I do not wish to repeat the debate here.

JimB
I agree Jim but given my level of competence it would be wrong of me to suggest someone did this when its against the manufactures advice, mine is disconnected but I am not of a level to recommend it if you see what I mean :D
 

Musicmanager

Active Member
Hi Guys

I think the answer is Clear & Present - as Harrison Ford once said !

However, to answer the questions .. ..

1. The issue of the unwanted effects of the PSU came up in an earlier post, related to current limit not the earthing potential, but as a result I have been powering the circuit from a 9v battery for a while.

2. I hadn't realised how critical the tolerance of the resistors is to the function of the circuit. AK is talking about an over budget situation based on 1% tolerance resistors. The resistor components I'm using for this circuit are cheap chinese imports and I've just got one out of a packet marked 10k @ 1% and it meters at 9332 which is almost 7% .. .. .. I'm not just over budget, more up the cut in a punt !!

Time to abandon the project in favour of the alternative in post #5 .. you are quite right about the small signal transformer, but I'll look into that .. ..

As a matter of course, I would not ever consider disconnecting a mains earth from anything .. .. for two reasons ... firstly, if the manufacturers put it there I've no doubt they have good reason; also, this 'scope is a brand new gift from my two sons .. ... there would be hell to pay if I made such an alteration and did some damage to either the instrument or myself in the process .. .. .. secondly, and much more important to me .. ... 30 years ago, my business partner was killed in a substation in Warwickshire because some metal thieves had removed busbar effectively disabling the safety earth device .. no chance!

Thanks for all the information and help; AK particularly grateful for your detailed explanation, makes a lot more sense of it for me .. ..

I'll be in touch when I can't get the other one to work !! :D

S
 

Nigel Goodwin

Super Moderator
Most Helpful Member
2. I hadn't realised how critical the tolerance of the resistors is to the function of the circuit. AK is talking about an over budget situation based on 1% tolerance resistors. The resistor components I'm using for this circuit are cheap chinese imports and I've just got one out of a packet marked 10k @ 1% and it meters at 9332 which is almost 7% .. .. .. I'm not just over budget, more up the cut in a punt !!
Resistors are only rarely that critical, it shouldn't be a problem at all - with the exception of the bridge components of course :D

As a matter of course, I would not ever consider disconnecting a mains earth from anything .. .. for two reasons ... firstly, if the manufacturers put it there I've no doubt they have good reason
In the absence of a full understanding of why it's earthed, and how it can help in 'some' circumstances - then don't consider disconnecting it. Using a 9V battery has eliminated that aspect anyway.

However, when you understand the implications of earthing, you soon realise that it's more dangerous than not earthing under many circumstances - but legislation is geared towards the 'seriously thick' :p, hence the common use of earthing on many items.

Incidentally, my nice new Rigol scope is still earthed - none of my other scopes are - either at home or at work. But I'm not planning using the Rigol on anything where it been earthed makes it more dangerous - I'll use one of my older scopes for anything that requires not been earthed.
 

Musicmanager

Active Member
Hi Nigel

but legislation is geared towards the 'seriously thick' :p, hence the common use of earthing on many items.
Don't beat about the bush ... tell it like it is !! I admit it willingly :)

In the world I've been used to, earth bonding has always been critically essential .. .. maybe not so in this environment, but that is a new and somewhat contradictory experience to me.

Changing the subject, I've spent most of the last week building the circuit boards for your PIC tutorials .. I've got a couple left to do, and then a batch of interconnecting leads and I'm equipped ! If you want to get the first plane to South America in anticipation of the questions now would be a good time to start !! :D

I've also got a Sony TV problem I need your help with .. .. but I'll raise that when I've got more info to hand.

S
 

AnalogKid

Well-Known Member
Most Helpful Member
Since this basically is an ohmmeter project, I been a thinkin. First, why not go with a current source excitation to linearize the result. And second, does anyone with instrumentation experience have a recommendation for the excitation frequency? The two internet circuits are 50 kHz and 100 kH z square(-ish) waves, but I think you would get more accurate results with sinewave excitation. Also, how does this thing know the difference between ESR and Xc? At 100 kHz, a 0.1 uF cap has Xc = 15.9 ohms, but the ESR has to be less than 1% of that. So what is this circuit actually measuring? Or am I missing something...?

ak
 

Musicmanager

Active Member
Hi AK

My expertise is nowhere near up to that question ... .. :(
However, I've attached the document I used of which that schematic is based which may answer some for you ..

I've just been looking at Amidon ferrite cores .. .. looks laden with trickiness to me !

S
 

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