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How much stray inductance in leads? How accurate are LCR meters?

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szzuk

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I have some fairly short leads on a piezo that appear from resonance measurements to have an inductance of 20uH or so. Is this typical? I mean the leads are in length about 20cm or 8 inches in total and multicore of 1mm diameter. They travel through a breadboard too. It may be that the LCR meter I'm using isn't so accurate.

On LCRs, I had 20 individual parts sent with individually measured capacitances. For example the datasheet says Part 1: Capacitance 683nF. Part 2: Capacitance 692nF. However I measure the values with my fairly cheap LCR meter much lower typically 50-100nF lower. THe LCR meter is a Tenma 72-8155 model that cost around £30 or $50. I presume the answer is get a better LCR meter?
 
Why would you be measuring it anyway?.

Connecting things via a breadboard isn't a good idea either, it will change both capacitance and inductance values.
 
I have some fairly short leads on a piezo that appear from resonance measurements to have an inductance of 20uH or so. Is this typical?
Using a calculator on the internet, it gave the inductance of 20cm of 1mm wire as about 250nH. So no, 20uH is not typical.

What are the "resonance measurements"?
How did you do them?

I mean the leads are in length about 20cm or 8 inches in total and multicore of 1mm diameter. They travel through a breadboard too. It may be that the LCR meter I'm using isn't so accurate.
Wires "travel through a breadboard" ??
Do you mean that there is one pair of wires going into the breadboard and another pair going out to the piezo (or whatever)?

On LCRs, I had 20 individual parts sent with individually measured capacitances. For example the datasheet says Part 1: Capacitance 683nF. Part 2: Capacitance 692nF. However I measure the values with my fairly cheap LCR meter much lower typically 50-100nF lower. THe LCR meter is a Tenma 72-8155 model that cost around £30 or $50. I presume the answer is get a better LCR meter?
Measuring 690nF with that meter means that it will be using the 2uF (2000nF) range.

The specification of the meter states +/-4% accuracy on that range, it does not state if this is a percentage of full scale or percentage of reading.

Lets take a worst case and assume percentage of full scale.
4% of 2000 is 80, which would make your 50 to 100nF lower readings reasonably on the edge of the meter specification.

My guess is that if you want more accurate measurements, you need a better (more accurate) meter.

JimB
 
Are you aware that the word "piezo" is a generic term, and there are numerous types of devices that could be called a "piezo"? It is easier to help you if you describe your "piezo" with more detail.

JimB already asked how you made your "resonance" measurements, and I repeat the request.

Piezoelectric elements generally are designed to be resonant at some particular frequency, and their equivalent capacitance varies a lot in the vicinity of the resonance frequencies.

Your Tenma meter only measures capacitance at either 1 kHz or 100 Hz, depending on the range you have selected. The supplier of your "piezo" elements has measured the capacitance at some particular frequency; you should find out what frequency they used.

If you have determined the apparent capacitance with a "resonance" measurement and then calculated the apparent inductance of the leads using the capacitance given to you for the device, you are overlooking the fact that the "piezo" has its own resonance that has almost nothing to do with the leads.

I have measured the apparent capacitance of a piezoelectric bender of the type used to make the beep sound in portable electronic devices and attached a couple of plots.

The first plot shows the apparent capacitance over a frequency range of 1 kHz to 100 kHz. You can see two major resonances, and a number of minor resonances. Notice that the apparent capacitance varies from 1 nF at the bottom of the plot, to 200 nF at the top. Actually, the variation is greater than that.

The second plot zooms in on a narrower frequency range of 3.5 kHz to 4 kHz. The apparent capacitance varies from about 43 nF at the left edge of the plot (3.5 kHz) to 2 µF at the peak (approx 3.67 kHz).

You say "For example the datasheet says Part 1: Capacitance 683nF. Part 2: Capacitance 692nF. However I measure the values with my fairly cheap LCR meter much lower typically 50-100nF lower."

Your Tenma measures the capacitance at a relatively low frequency; it is to be expected that the apparent capacitance rises with increasing frequency as you approach resonance. You should find out at what frequency those capacitances were measured.
 

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Using a calculator on the internet, it gave the inductance of 20cm of 1mm wire as about 250nH. So no, 20uH is not typical.

What are the "resonance measurements"?
How did you do them?

Wires "travel through a breadboard" ??
Do you mean that there is one pair of wires going into the breadboard and another pair going out to the piezo (or whatever)?

I feed the output of an amp into a breadboard and hot swap inductors to see which inductance works best with the piezo at the resonance frequency of the piezo.

Measuring 690nF with that meter means that it will be using the 2uF (2000nF) range.

The specification of the meter states +/-4% accuracy on that range, it does not state if this is a percentage of full scale or percentage of reading.

Lets take a worst case and assume percentage of full scale.
4% of 2000 is 80, which would make your 50 to 100nF lower readings reasonably on the edge of the meter specification.

My guess is that if you want more accurate measurements, you need a better (more accurate) meter.

JimB

It was the cheapest LCR meter I could find, I guess you get what you pay for. I will need to take some more measurements based upon the comment below too.
 
Are you aware that the word "piezo" is a generic term, and there are numerous types of devices that could be called a "piezo"? It is easier to help you if you describe your "piezo" with more detail.

JimB already asked how you made your "resonance" measurements, and I repeat the request.

Piezoelectric elements generally are designed to be resonant at some particular frequency, and their equivalent capacitance varies a lot in the vicinity of the resonance frequencies.

Your Tenma meter only measures capacitance at either 1 kHz or 100 Hz, depending on the range you have selected. The supplier of your "piezo" elements has measured the capacitance at some particular frequency; you should find out what frequency they used.

If you have determined the apparent capacitance with a "resonance" measurement and then calculated the apparent inductance of the leads using the capacitance given to you for the device, you are overlooking the fact that the "piezo" has its own resonance that has almost nothing to do with the leads.

I have measured the apparent capacitance of a piezoelectric bender of the type used to make the beep sound in portable electronic devices and attached a couple of plots.

The first plot shows the apparent capacitance over a frequency range of 1 kHz to 100 kHz. You can see two major resonances, and a number of minor resonances. Notice that the apparent capacitance varies from 1 nF at the bottom of the plot, to 200 nF at the top. Actually, the variation is greater than that.

The second plot zooms in on a narrower frequency range of 3.5 kHz to 4 kHz. The apparent capacitance varies from about 43 nF at the left edge of the plot (3.5 kHz) to 2 µF at the peak (approx 3.67 kHz).

You say "For example the datasheet says Part 1: Capacitance 683nF. Part 2: Capacitance 692nF. However I measure the values with my fairly cheap LCR meter much lower typically 50-100nF lower."

Your Tenma measures the capacitance at a relatively low frequency; it is to be expected that the apparent capacitance rises with increasing frequency as you approach resonance. You should find out at what frequency those capacitances were measured.

The piezo is an ultrasonic atomiser, the capacitance measurments of the supplied parts were at 1KHz according to the manufacturer. So it appears the cheap LCR meter accounts for this discrepancy. I will use the supplied cap values and not my measured ones.

I had overlooked the fact that the piezo capacitance changes with frequency. Based upon a piezo cap of around 1400nF and resonance at 25KHz I calculated that theoretically I should have inductance of around 40uH but the part was actually measured at most volts with 20uH. So I attributed the extra 20uH inductance to 'stray' inductance from the breadboard or leads but I think that your comments about the piezo cap changing with frequency are on the mark.
 
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