Cyrustek based LCR meters - IN CIRCUIT ESR measurements

[baldsoprano]

Hi all,

I purchased a Voltcraft LCR-300, which is a Cyrustek ES51919-51920 based LCR meter (this is a big family including the not quite identical looking DER EE DE-5000, but my LCR has many identical clones as well - see post #4:)
https://www.eevblog.com/forum/testgear/voltcraft-lcr-300-micro-review/

My first project with the LCR meter is to check the ESR of the electrolytic caps in my vintage transistor radio (ITT TOURING 103) and replace the bad ones.

This is also a good opportunity for me to check the IN CIRCUIT capability (ESR measurements) of the Voltcraft LCR-300 (if this meter can't do this, I will return it).

So could you please help me out with the following questions:


1. is it possible to measure the capacitor ESRs IN-CIRCUIT with these LCRs? The common answer to this question is "Yes it is possible because the 0.6Vrms test voltage will drop strongly when testing at 100kHz". The problem is that these LCRs can only test/measure bigger caps at LOWER frequencies (measuring range changes). That is, I can only measure my 1mF cap in my radio @ 1Khz (that would be only 120Hz if I had a 10mF cap). Will the test voltage drop enough at these lower frequencies?

2. is it safe to use the LCR meter in-circuit (no harm to components and speakers)? Also, is there any danger of blowing the tweeter when measuring the high-pass cap's ESR? (please see pic in the attachment)

3. The Voltcraft LCR-300 manual warns that components must be discharged when connected to the meter (I have several 1000uF/16V and 2.5uF/35V caps in the radio). Is it enough to simply unplug the radio's power chord (and press the power button) some hours before I take the measurements to be sure that all caps will discharge? (offtopic: this radio has a 12W AC to DC power supply (230v) - touching the inside of the radio would not pose any danger right?)

Sorry for the novice questions (I am quite a beginner in practical electronics). I posted this in "Test Equipment" because this is the place where all the relevant info on the LCR subject is located.

Thanks for your help,
b

[omgfire]

This is also a good opportunity for me to check the IN CIRCUIT capability (ESR measurements) of the Voltcraft LCR-300 (if this meter can't do this, I will return it).

No meter can provide reliable in-circuit measurement for all possible circuits. So, you can return it.
Proof: imagine 2 capacitors in parallel: one cap is good with low ESR, and another cap is bad with high ESR. If you measure ESR you will get ESR of good cap. You cannot tell which one is bad without disconnecting them.

1. is it possible to measure the capacitor ESRs IN-CIRCUIT with these LCRs? The common answer to this question is "Yes it is possible because the 0.6Vrms test voltage will drop strongly when testing at 100kHz". The problem is that these LCRs can only test/measure bigger caps at LOWER frequencies (measuring range changes). That is, I can only measure my 1mF cap in my radio @ 1Khz (that would be only 120Hz if I had a 10mF cap). Will the test voltage drop enough at these lower frequencies?

Voltage goes down for low DUT resistance (big capacitors). https://www.eevblog.com/forum/testgear/review-and-tear-down-of-uni-t-ut612-lcr-meter/msg198457/#msg198457

[baldsoprano]

This is also a good opportunity for me to check the IN CIRCUIT capability (ESR measurements) of the Voltcraft LCR-300 (if this meter can't do this, I will return it).

No meter can provide reliable in-circuit measurement for all possible circuits. So, you can return it.
Proof: imagine 2 capacitors in parallel: one cap is good with low ESR, and another cap is bad with high ESR. If you measure ESR you will get ESR of good cap. You cannot tell which one is bad without disconnecting them.

1. is it possible to measure the capacitor ESRs IN-CIRCUIT with these LCRs? The common answer to this question is "Yes it is possible because the 0.6Vrms test voltage will drop strongly when testing at 100kHz". The problem is that these LCRs can only test/measure bigger caps at LOWER frequencies (measuring range changes). That is, I can only measure my 1mF cap in my radio @ 1Khz (that would be only 120Hz if I had a 10mF cap). Will the test voltage drop enough at these lower frequencies?

Voltage goes down for low DUT resistance (big capacitors). https://www.eevblog.com/forum/testgear/review-and-tear-down-of-uni-t-ut612-lcr-meter/msg198457/#msg198457

Yes I am aware about the parallel cap problem (nonetheless, we still call them "in-circuit capable" LCR meters)

------ (didn't figure out how to quote part of your post yet  )

Thank you for the link, this is new info for me, and it really makes me less concerned. I still wonder though what was the Capacitance value (the tested Cap had 0.24 Ohm ESR, and the test freq was 1kHz if I understand correctly which resulted in a 0.014V RMS test voltage which is very good).

On the other hand if we encounter a bad Cap with a high ESR wouldn't it cause a much higher test voltage and an unpredictable in-circuit behavior (that is possible false negatives)?.  Probably not as the ESR for bigger (bad) caps would not raise so high anyway. (but this is just my speculation)
UPDATE: I just found the answer to this (correct me if I'm wrong): there is the measurement for a 47 Ohm resistor (inside your link) which is equivalent to a 47 Ohm ESR (bad) Cap and it still gives a 0.159V RMS  test voltage which is still acceptable.

I would be very grateful if there would be somebody out there who could answer my other safety-related questions (need for cap discharge; tweeter safety).

Thanks again

[omgfire]

On the other hand if we encounter a bad Cap with a high ESR wouldn't it cause a much higher test voltage and an unpredictable in-circuit behavior (that is possible false negatives)?.

I'm not a EE, so I might say stupid thing:
suppose meter impedance 120 Ohms
suppose test voltage without DUT 0.6V
suppose real cap's ESR infinite (very broken)
suppose voltage drop 0.3V due to in-circuit silicon semiconductors
That could result in measuring cap's in-circuit ESR as 120Ohms (instead of infinity). But 120Ohms is about 3 orders of magnitude bigger than expected from most of electrolytic capacitors (<0.1Ohms).
Note: on sine AC voltage drop due to diode (clipping) looks different from voltage drop due to resistor (attenuation). So, above calculation is not valid.

(need for cap discharge; tweeter safety).

If in doubts, discharge with beefy resistor. Destroying speaker with 5mA source?

[baldsoprano]

Thank you omgfire, this was a great help.

I now understand that there is probably no need to desolder/disconnect the tweeter when doing the in-circuit ESR measurements.

Can I be absolutely sure that the few milliamps testing current always stays in the few milliamps region and there are no current peaks before it "sets" to few milliamps (5mA in our case)?

(no more reassuring questions i promise )

[baldsoprano]

Also (this might be a dumb question) can I be sure that my Voltcraft LCR 300 only performs AC tests when doing the measurements and no DC ones? (i think it can recognize components automatically for instance)

no more reassuring questions really

[omgfire]

Also (this might be a dumb question) can I be sure that my Voltcraft LCR 300 only performs AC tests when doing the measurements and no DC ones? (i think it can recognize components automatically for instance)

Read the manual
AUTO LCR - AC
DCR - DC
These might worth reading:

http://www.produktinfo.conrad.com/datenblaetter/100000-124999/103677-an-01-ml-VOLTCRAFT_LCR_300_LCR_MESSGE_de_en_fr_nl.pdf

http://akizukidenshi.com/download/ds/deree/DE-5000_manu_en2p.pdf

http://conradhoffman.com/capchecktut.htm

https://www.eevblog.com/forum/projects/impedance-lcr-esr-meters/

http://www.ietlabs.com/pdf/application_notes/030122%20IET%20LCR%20PRIMER%201st%20Edition.pdf

I now understand that there is probably no need to desolder/disconnect the tweeter when doing the in-circuit ESR measurements.

One touch with iron and you can get accurate measurement. It is faster than reading this post. If you find dozen caps glued together in PSU, then in-circuit measurement comes in handy.

[baldsoprano]

Thank you very much for the AC vs DC measurements clarification and for the links. You helped me a lot.

About the tweeter: I just realized that I can simply short the tweeter connectors using a wire and this offers 100% protection during the in-circuit measurements,

it is clear by now that this is not needed anyway