Elektor Electronics

On Sat, 17 Sep 2005 12:37:44 +1000, "Phil Allison" put finger to keyboard and composed:

Using a "perfect" cap demonstrates how the meter responds to the C component in a real cap. The designer's assumption is that the C component does not contribute significantly to the measurement process. This is demonstrably true for 1uF caps, but not for smaller ones.

The OP wanted to know the range of an ESR meter. I answered the question, both by actual testing, and by analysis of the design methodology.

Nope. Read the datasheet and apply the formula. If you still don't like the result, then complain to the manufacturer. Maybe he will give a damn. I don't.

The OP's question was in regard to the *limits* of measurement of the ESR meter. In the absence of explicit specs from the manufacturer or designer, the answer to such a question requires an understanding of the measurement process ... or access to perfect caps. Therefore, the question of itself *demands* a pedantic answer.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Sat, 17 Sep 2005 12:22:05 +1000, "Phil Allison" put finger to keyboard and composed:

Whether or not the test signal needs to be sinusoidal is irrelevant. The fact is that it isn't. It is in fact an 8us 2kHz *pulse*. The principle of operation of the meter involves driving such a pulse into a pure C in series with a pure R. A discussion of a real capacitor's performance at 100kHz is just pointless obfuscation.

All you need to know is that I = C * dV/dt. From this it follows that a constant current produces a voltage ramp.

That's elementary circuit theory. Quite simple really.

Irrelevant. You were referring to electros. Anyway, nobody cares about the behaviour of the test cap at 100kHz.

The meter does not read Xc. It reads *ESR*, or at least it thinks it does. In any case, the fact that the reading is "far from negligible" for a "pure" capacitance demonstrates that the meter has limitations, albeit acceptable ones, which was essentially the point of the OP's question.

Capacitors C5 and C6 have no effect since they are in series with a current source.

Capacitor C7 also has no impact because the input impedance of the meter (neglecting R23) is appropriately high.

You can quite easily calculate the expected degradation of the pulse at the junction of C7 and R12. Assume you are testing a pure resistor, and that it produces a voltage, Vt, at the meter's test leads.

The voltage developed across C7 by the test pulse would be:

dV = Iin . dt / C7

where Iin is the current flowing through C7 and R12.

Now the impedance looking into the junction of R13 and R14 is 69K.

This gives:

Iin = Vt / 69K

So the degradation, dV, of the pulse, Vt, is given by:

dV/Vt = (Vt /69K) . (dt / C7) / Vt = 8us /(69K . 33nF) = 0.35%

Hardly worth worrying about.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

"Franc Zabkar" "Phil Allison"

** Strange then how YOU raised the issue.

Go back and read the first para re the PRINCIPLE of operation.

** Which is AC coupled to the cap under test AND is a signal with energy concentrated in the range above 100kHz. ** You are DEAD WRONG !!

The PRINCIPLE of operation is that measuring an electo's IMPEDANCE at a high frequency finds its ESR - because they are essentially the same number.

** Completely irrelevant to has Bob's meter works.

The voltage across the cap under test is assumed to be entirely due to ESR - and so it is for electos above a few uF.

** Piss off - d*****ad. ** YOUR comment above refers to a film cap - d*****ad. ** WRONG - that is how the ESR is being measured.

At that frequency, electro cap impedance and ESR are the same or similar.

** WRONG - it reads Xc.

The PRINCIPLE of operation is that Xc = ESR over a range of high frequencies.

** Gobbledegook.

** The OP never asked any such thing - d*****ad.

** The remove any DC component - you stubborn bloody ass.

The cap under tests cannot "charge up"

You are so UTTERLY wrong, Zabkar - there is no way to get anything RIGHT into your fat head.

............ Phil

"Franc Zabkar" "Phil Allison"

** WRONG !!!!!!!!!!!!!!!!!!!!!

Here is the post:

" Anyone get Elektor Electronics magazine http://213.222.12.205/Default.aspx?tabid=27&year=2005&month=9&art=52895&PN=O Hopeless NZ bookshops have not put it on the shelf yet, Does anyone know what capacitors range the ESR meter does? "

The new, Elektor meter is a COMBINED capacitance and ESR meter.

The OP was asking what range of capacitors it could test for capacitance.

Bob Parker's ESR meter has NOTHING to do with the OP's question !!!!!!!!!!

** But for the WRONG meter and you got it all WRONG as well.

** But NOT Bob Parker's one.

** You are an autistic lunatic - piss off.

......... Phil

"Franc Zabkar" "Phil Allison"

** Please do - look at the impedance curves on page 7.

A 470nF cap has a resistance of **40 milliohms** at 2 MHz.

This represents the intrinsic, series resistance of the cap.

Bob Parker's ESR meter indicates the same parameter for an electro.

The big difference with electros is their impedance minimum is very broad - ie the curve has a low Q.

......... Phil