measure ESR with L/C/R only meter

[ddavidebor]

hi

i've a question

all the lcr meter measure inductance, capacitance and resistance, and they measure them with an AC current.

the most new ones have ESR function, wich is pretty userful.

do you know if i measure resistance of an electrolitic cam with an LCR i'm in fact measuring ESR or something else?

thanks to all

[PA4TIM]

Depends on the meter but my IET DE-5000 measures ESR in Rac mode. In theory all AC meter should be able to measure ESR this way. But if your meter as no ESR, tan d, d or phase it probably measures Z in Rac mode and not ESR

[ddavidebor]

The one i use is a old escort wich measure at 1khz max.

For you, in the measure will be more important the inductance of the cap or is ESR?

[PA4TIM]

http://www.pa4tim.nl/?p=3775 my page about ESR ( and ESL and other cap specs, i you look through my site you find much mor about caps)

If the meter does 100 KHz and the SRF ( self resonance frequency) is below 100 KHz you should be able to measure ESL. From C measurement at 1KHz and L at 100 KHz it is possible to calculate SRF but there is nomally not much use for this.

ESL is only important in high speed designs. In those cases also things like the selfinductance of a via are important. ESL is a positive reactance, so it adds to the negative reactance of a cap. If a cap has an impedance of ( 2-j100) Ohm and the ESL is (0.1+j100)the net result is (2.1+j0) Ohm, so to the circuit it behaves as a pure 2.1 Ohm resistor. A reactance does not dissipate power, a resistance does. That is the bad part of ESR. But in some cases ESR is a good thing.

ESR on its own is a rather useless number. Most datasheets only give D or Z, from most caps that have a high failure rate in consumerstuff the datasheet is not available.

A cap has 3 failure modes,
- capacitance loss
- DC leakage
- ESR
It can fail on only one mode, or on two or all three ( but that is a rare case)
If ESR is real high, like 15 Ohm for a 100 uF cap you can be sure it is dead, so real bad ones you will find. ( if measured out of circuit) .

DC leakage is a fault on its own.  What happens most times is that because of heating ( due to location, design, leakage and ESR) the electrolyte changes /vapourates and capacitance decreseases. This increases the dielectric loss that is part of ESR. So ESR increases to. But before ESR is real high, Capacitance will show what is going to happen. A real bad ESR cap will most times have a low capacitance too. Most ESR meters measure Z at 100 KHz, if Capacitance decreases, Impedance (Z) increases too, so the ESR meter will spot this. If ESR increases and capacitance stays the same, impednce increases too. So that is why impedance instead of ESR is not a bad thing.

Problem is there is a huge grey area where D and C will tell you what is going on and Z or ESR does not. But nobody cares about replacing a cap that did not fail but only looks like it was a failure due to wrong measurement because.