# Multimeter behaving strangely on AC current measurement

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
I connected an incductor in parallel with a capacitor, supposed to resconate at a certain frequency. The meter was connected in series with those two components and the voltage source, to monitor the current. As the frequency got closer to the resconance point the meter showed a decrease in current as expected, but then past a certain point it started showing a current bouncing up and down without displaying an exact value. (It was powered by a square wave, can the higher frequency harmonics that do not resconate somehow confuse the meter?)

#### wayneh

Joined Sep 9, 2010
16,102
The meter is probably expecting DC current and is 'confused' by any AC component in the current. Watching the voltage waveform with an oscilloscope would tell you a lot more about what's happening.

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
The meter was set to measure AC current only. Can the higher frequencies have a similar effect? Thanks for the help!

#### dendad

Joined Feb 20, 2016
2,937
What frequencies are you talking about?
Generally a multimeter is not designed or calibrated for high frequency operation. You may well be getting RF interference in it too.
Check the data on your meter.

#### Picbuster

Joined Dec 2, 2013
982
Your meter is part of that loop.
At a certain frequency the capacitor has an impedance close to zero.
The full current at that frequency will be put into your meter.
Your meter will rectify the AC and feed it into a mechanism to read this Dc as an voltage.
The input circuit will carry a bunch of diodes resistors and capacitors.
when the frequency goes above the spec's caps in that circuit will have an low impedance hence measurement is incorrect.

As stated before: consult meter's specifications.

Picbuster

#### Reloadron

Joined Jan 15, 2015
4,468
(It was powered by a square wave, can the higher frequency harmonics that do not resconate somehow confuse the meter?)
Purely a guess but the leading edge of your square wave applied contains an infinite number of odd harmonics and with that in mind:
–When we apply an input square wave voltage of frequency of resonance (w0) to the parallel LC circuit, we are in essence applying frequencies w0, 3w0, 5w0, etc. simultaneously with relative amplitudes 1, 1/3, 1/5, etc. (respectively)
–The LC circuit is a “detector” of its resonance frequency f0, including contributions from the harmonics of the input fundamental frequency
•“Mini-resonance” peaks will occur in the output voltage at driving frequencies of f0 / 3, f0 / 5, etc.

The above was taken from a portion of a Power Point Presentation which for some reason my browser does not see as safe. However, a Google of "parallel resonant circuit square wave applied" should give you several results. You do not mention the values of C and L in your circuit?

Additionally some of what you see as to current may depend on if your meter when measuring AC signals is Average Responding RMS Indication or True RMS Responding. Since you are looking at a square wave an average responding AC meter will not give an accurate representation of square wave current.

So you have a few things to consider.

Ron

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
The meter was set to measure True RMS, and the fundamental frequency (square wave) was at about 150 kHz (well within the meters capabilities). It displayed an exact value above and below the resconance frequency, but around the resconance point it started behaving strangely (bouncing up and down without showing an exact value). Can the higher harmonics be the problem? The peak value of the square wave was 4.5 V.

#### Reloadron

Joined Jan 15, 2015
4,468
True RMS, and the fundamental frequency (square wave) was at about 150 kHz (well within the meters capabilities).
Well at 150 KHz the harmonics would be high, my guess is too high to see on the meter. You got me on this one. Hopefully another forum member has some thoughts to share. Really peculiar.

Ron

#### MrChips

Joined Oct 2, 2009
19,160
There are too many unknowns to be able to come to a conclusive answer.

What is the make and model of the current meter?
What is its frequency range for measuring true RMS current?
What shape waveform is the meter designed to measure?
How does it measure true RMS current?
What is the internal impedance of the meter?
Does the meter account for power factor?

#### GopherT

Joined Nov 23, 2012
7,983
The meter was set to measure True RMS, and the fundamental frequency (square wave) was at about 150 kHz (well within the meters capabilities). It displayed an exact value above and below the resconance frequency, but around the resconance point it started behaving strangely (bouncing up and down without showing an exact value). Can the higher harmonics be the problem? The peak value of the square wave was 4.5 V.

What meter are you using? A Fluke87, for example, can only read True RMS up to 20 kHz. I would love a meter that can comply at 150kHz or more.

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
What meter are you using? A Fluke87, for example, can only read True RMS up to 20 kHz. I would love a meter that can comply at 150kHz or more.
Didn't know that! I based my fact on its capabilities of handling frequencies up to 10MHz (when measuring frequency, which seems to be irrelevant when measuring current). If the frequency is above the 'True RMS' range, is it displaying it in average instead? Btw its a UNI-T UT61D and im pretty sure it has a lower frequency limit than the fluke.

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
An oscilloscope is probably whats needed here to get a good measurement. Thanks for the help!

#### GopherT

Joined Nov 23, 2012
7,983
Didn't know that! I based my fact on its capabilities of handling frequencies up to 10MHz (when measuring frequency, which seems to be irrelevant when measuring current). If the frequency is above the 'True RMS' range, is it displaying it in average instead? Btw its a UNI-T UT61D and im pretty sure it has a lower frequency limit than the fluke.

From your description, it sounds like you have your circuit and meter connected as follows...
Is that correct?

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44

#### GopherT

Joined Nov 23, 2012
7,983
Yes. The 4.5V square is the meter right?
Yes, the 4.5 is the meter.

The meter is equivalent to a 10M resistor in most cases (most meters).

Your circuit is not really anything more than an antenna of sorts. What are you hoping to do with the circuit when the meter is not present? A complete circuit will not exist without the meter completing the connection to ground.

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
Yes, the 4.5 is the meter.

The meter is equivalent to a 10M resistor in most cases (most meters).

Your circuit is not really anything more than an antenna of sorts. What are you hoping to do with the circuit when the meter is not present? A complete circuit will not exist without the meter completing the connection to ground.
When the meter is not connected, a jumper is shorting it. (It was set to measure current, equivalent to a short in that case) sorry if i was unclear

#### BR-549

Joined Sep 22, 2013
4,936
You could measure the DC current feeding the oscillator.

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
You could measure the DC current feeding the oscillator.
Did that, i was more curious about why the multimeter behaved in such a way. Also, will not the 90 degree lag of the inductor current cause a problem with the measurement then? Anyway once the resconance point has been reached, there will be next to no current drawn.

#### BR-549

Joined Sep 22, 2013
4,936
I would think trying to directly measure current at resonance will disturb the resonance.

What are you driving this with?

Have you a schematic?

Thread Starter

#### AngryGecko

Joined Jul 7, 2017
44
I would think trying to directly measure current at resonance will disturb the resonance.

What are you driving this with?

Have you a schematic?
It's a 555 timer that together with a not gate feed an h-bridge.