Puzzling result from esr meter i'v just built - Page 1
Forum section: electronics / projects

author: Zener57
author_rank: Newbie
timestamp: January 12, 2012, 09:08:28 pm
content: Hi I'v just finished an ESR meter based on Ludens simple analog circuit, which seems to work very well. I did a small mod (one of the 10 ohms changed to 4.7 to broaden the range a bit), so a 4.7 ohm resistor measures below mid scale great. Works fine.Here's the Puzzle!.I short the test leads out to zero the meter fine.Then If I measure a 'NON Polarized' 1.5uf polycarb capacitor it takes the meter about 5mm Past the zero point!, It also goes over the zero on another 2.2uf non polar cap I have?How can this be?. If a short circuit limits the meter to Zero FSD. how can measuring a non Polar cap send it further over the Zero?As for electrolytics the meter works fine.Any Info would be much appreciated.

author: Psi
author_rank: Super Contributor
timestamp: January 12, 2012, 09:17:57 pm
content: Just a stab in the dark, do you have the two diodes across the capacitor that you see in most esr meter circuit?

author: jimmc
author_rank: Frequent Contributor
timestamp: January 12, 2012, 09:34:31 pm
content: I suspect that the output impedance of the ESR meter is slightly inductive (caused by he leakage inductance of the transformer and the inductance of the test leads), the 1.5uF capacitor partially resonates resonates with the inductive component to give a greater current than the short circuit.At the fundamental frequency of 50kHz, 1.5uF has a reactance of 2.1ohm which will resonate with 6.8uH.Electrolytic capacitors are much more lossy than polycarbonate and this will mask this effect.Jim

author: Zener57
author_rank: Newbie
timestamp: January 12, 2012, 10:10:17 pm
content: Hi Psi and Jimmc,Yes I do have the protection diodes across the test cap.?Also i'm running at 100 Khz and germainium diodes feeding the meter (movement from an old car dwell angle test meter) but theres plenty of gain for it.Thanks for the replyscheers

author: Zener57
author_rank: Newbie
timestamp: January 12, 2012, 10:40:46 pm
content: Hi Jimmc,Thanks for your comments (nice & deep I like it).When you say, "I suspect that the output impedance of the 'ESR meter' is slightly inductive"I take it you mean the source side, ie the output from the transformer feeding the Cap under test.When I reduced the value of one of the 10 ohm to 4.7, I chose the one where the mesurment is taken across, leaving the one across the TX as 10 ohm, If I do the opposite and drop the one across the TX to 4.7, would this help in dampening the TX further to improve the Inductance you speak of?, and if so would I loose the spread across the meter for the low ohms I have?Thanks againcheers

author: jimmc
author_rank: Frequent Contributor
timestamp: January 13, 2012, 09:11:06 pm
content: QuoteYes, as I said, mainly caused by the leakage inductance of the transformer and the inductance of the test leads.QuoteAgain yes, if the transformer were perfect the resistor across its output would have no effect since the primary is fed from a low impedance (saturated output of OP-Amp).To be honest, the effect you're seeing is hardly worth worrying about, however you could turn it to your advantage by building a cap into the meter in series with one of the test leads. (Try a few values either side of your 1.5uF and see which gives the largest reading.)Doing this gives two advantages...1/ It protects the meter from DC voltages up to the rating of the capacitor.2/ It reduces the susceptibility of the meter to 50 / 60Hz pick-up since the reactance of the capacitor is much higher than 4.7ohm (at approximately 2k)Jim

author: Zener57
author_rank: Newbie
timestamp: January 14, 2012, 12:22:41 am
content: Hi Jimmc,"To be honest, the effect you're seeing is hardly worth worrying about, however you could turn it to your advantage by building a cap into the meter in series with one of the test leads. (Try a few values either side of your 1.5uF and see which gives the largest reading.)"Brilliant.!!! just Brilliant.!!!Thanks Jimmc, that's an excellent solution, that never crossed my mind.I tried a few things tonight ,1: changed 4.7 ohm in series with the cap under test back to 10 ohm, which improved the over reading from the NON polar cap greatly but I lost my extra spread of low ohms.2: tried lowering the resistor across the secondary of tx but now change to over shoot.3: removed protection diodes, no change4: eliminated chance of charge pump action of c3 back to back caps (another group) . No change5: scoped across the sense resistor thus eliminating the rest of the circuit, got .56mv short circuit and 0.57mv with the non polar cap!.So it seems as you said it's reacting with the tranformer and the 100Khz in a way where it's boosting the signal. So 'Using it to my advantage' is an excellent solution.I'll test a few polycarbs and find the the best value for the highest signal, and build it in to the circuit as a permenent mod. Sorted.!!Many thanks Jimmc for the excellent insite, tips and sound logicCheers

author: jimmc
author_rank: Frequent Contributor
timestamp: January 17, 2012, 01:06:23 pm
content: I forgot to mention that it would be wise to include a discharge resistor across the capacitor.This will ensure that it does not retain a charge after blocking a large DC voltage.A few hundred kilo-ohm should be do the job and have no effect on the ESR measurements.Jim

author: BravoV
author_rank: Super Contributor
timestamp: January 17, 2012, 01:52:43 pm
content: Thanks Jim, really appreciate the knowledge and info you've posted here in this thread.

author: Zener57
author_rank: Newbie
timestamp: January 18, 2012, 01:47:23 am
content: Hi Jim,Yes good point I'll do that,I'v also got the protection diodes across the test cap, even so I suppose 0.6V or so could still get through and be stored in the feed cap, and effect the reading. I'll try a 220k-470K as disscharge.Many Thanks again Jim, and I echo BravoV.All the bestPhil.

author: jimmc
author_rank: Frequent Contributor
timestamp: January 18, 2012, 11:21:11 am
content: Quote from: Zener57 on January 18, 2012, 01:47:23 amYou would be better off putting the diodes before the 'feed' cap to get the full (protection) benefit from the capacitor.See reply #49 of this threadhttps://www.eevblog.com/forum/index.php?topic=4476.msg67379#msg67379As an aside I did wonder when I originally saw this circuit if the added reactance of the capacitor would cramp the lower end of the scale but your results suggest that the effect might not be as bad as I thought.Finally if you're interested in a bit of the background to my thoughts see my reply #5 of this threadhttps://www.eevblog.com/forum/index.php?topic=3228.msg43138;topicseen#msg43138.Jim

author: Zener57
author_rank: Newbie
timestamp: January 19, 2012, 12:24:06 am
content: Quote from: jimmc on January 18, 2012, 11:21:11 amHi Jim, yes I did see that method. But I my case that would leave my carefully chosen 1.5uf Polycarb which is only rated at 100v exposed to potentialy destructive high voltages where it may break down, so I thought with the diodes across the test cap only 0.7v could get though and it and the rest of the circuit would be safe.Given your comments I also now wonder about the protection to the 'Circuit under test' and any rapid discharge condition that may occur from the diodes, is this what you mean by 'Full protection'. If not, then there is also the possibility of placing protection diodes both before and after the feed cap, so protecting my feed cap and double protection to the meter overall?"added reactance of the capacitor would cramp the lower end of the scale"I wondered about that too, but fortunately I did not have to recalibrate my scale after the inclusion of the cap.My cap was also chosen to maximize tuning with 100khz with my TX, maybe this helps.?Quote from: jimmc on January 18, 2012, 11:21:11 amWell would you beleve it.!!!I'm new here as such and don't know names etc.Jim, before I built this circuit I looked at quiet a few circuits around the net, trying to find out all about this ESR meter concept and the different and similar designs, I ended up homing in on a couple one was a more simple design which I desided to build first 'This one!', just to see how it performed. the other impressed me more as I agreed with the high frequency gain limitations with the op-amps and I liked the transistor aproach, and the all together professionalism of the ciurcuit & explination, I was also blown away with the Gorgeous meter scale and construction. I do all my construction on vero board, so that was a hit. So I planned to make this after as the icing on the cake so to speek, In fact today I have just recieved from china a 100ua 2.5% panel meter!, So Phase two! might be to build this 555 transistor version with my new panel meter, if I do build it, nothing less than that pretty scale dissplay will surfice.To the point, the circuit to which i refere just happens to be yours.!!!http://kripton2035.free.fr/analog%20esr/esr-jimmc.htmlI'v only just made the connection!Cheers,Phil

author: jimmc
author_rank: Frequent Contributor
timestamp: January 20, 2012, 08:34:36 pm
content: Thanks PhilI understand your concerns about stored charge but remember that...ESR meters are normally used or 'in circuit' testing onunpoweredcircuits however accidents happen and it is best to try and minimise any damage caused.Take three possibilities:Accidentally connecting the meter to a powered circuit could cause damage to the circuitIn this case the placing capacitor before the protection diodes helps.Connecting the meter to a large value capacitor that is still charged.Again placing capacitor before the protection diodes helps.Connecting the meter to a sensitive circuit after either of the above.Here placing the capacitor before the protection diodes could cause problems, but remember that neither of these scenarios should occur in normal operation.Even then, if you use a 100k discharge resistor (it's the dissipation in the resistor at maximum voltage that sets the lower limit on its value, so 100k at 100v is fine) the time constant with the 1.5uF capacitor will be 150mS.Taking the worst case of the capacitor charged to 100v, it will discharge to 127mV in one second (6.667 time constants).Thus in order to cause damage you have to make a mistake to charge the capacitor in the first place and then transfer the probes to the sensitive circuit less than one second after this.With regard to the 100v rating,it's not ideal but if you're working on this sort of voltage (or higher) you really need to be sure everything is off and discharged for safety's sake.Of course it's your choice, but I hope I've explained why I prefer the capacitor where it is.Jim

author: Zener57
author_rank: Newbie
timestamp: January 23, 2012, 01:22:27 am
content: Hi Jim,QuoteWell yes i agree completely, and I for one would never dream of testing a component in circuit without voltage testing first.But as you rightly pointed out mistakes and oversights can happen.So I will be adding a protection capacitor with disscharge resistor and series inductor to my ESR meter's.Thank's Jim for 'tuning me in' more to AC theory, reactance , inductance & capacitance, I'v learned a lot, it's an area i'v not studied a great deal since college 35 years ago!.I can see from experimenting with these projects at frequency's of 100 Khz, how low resistance components coils resistors etc, and capacitors can have masive reactions with each other, and how it's possible to tune components to work together and enhance throughput at there resonant frequency's. I'm learning a lot about inductance and coils here too, a bit of a grey area for me in the past. So ESR projects are a superb learning vehicle for AC theory, and so I encorage all electronic hobbyists & Service engineers to jump in and build one Period.Jim,I built your ESR meter today from the notes found at,http://kripton2035.free.fr/analog%20esr/esr-jimmc.htmlIt worked great first time!, I did have a few bugs to Iron out though,I did not have the cmos 555 so used a HA17555 hitachi 555 so I get 10ma total (I have just ordered TS555 so will replace asap)I noticed the detector is very sensitive, I built the whole circuit on a sigle piece of vero board, but I noticed you built the detector on a seperate board (if I had thought). I had a very slight deflection of half a unit from zero (left 0) when powered up, I noticed if I touched the wires going to the meter (twisted pair) I got quater schale deflection. This I have sorted out by wiring the meter with twin screened to earth, I'v also played about with earthing points to get the lowest bleed to the detector, so now it seems ok.the scale range is really good about the same as yours in the picture. I chose the 'Simplyfied Source circuit', but then went about adding my own protection.So I had a 400V 1uf Polycarb which I placed after the protection diodes in series with RS which took me up to about .5 ohms or so now by using my new found knowledge I knew that I would need to find an Inductor to react with this 1uf cap at 100Khz for lowest impeadance of tuned circuit. As I don't have much in the way of coils, I decided to 'roll my own', so using a small ferrite bar (tiny one) I wound a few turns of Enamaled copper wire round the bar whilst watching the meter, and after a few turns the meter reading started moving more towards FSD!, Then I got to the point where it started to come back, so it was easy to find the peak, so I ended up with about 8 turns on this former, which I then superglued in place. So this is the best tuned coil for my 1uf cap at 100khz.I ended up only giving away about 0.2 ohms deflection, similar to to what you discribed with your 0.47 & 5.4uf.So 4.7 ohms is well below midscale and I can measure 0.1 ohm as 1 devision of the 100uA scaleI think also I may need a screened enviroment maybe (colmans mustard can may surffice!)So now with a disscharge resistor added the circuit functions great.!Of course The hard part will be that fancy scaleThanks again, for all the insite,cheersPhil