# Measuring charging current of the Capacitor

#### t.haliulin

Joined Nov 3, 2021
24
Hello All,

I am trying to get a circuitry that will measure charging current for the capacitor.
Given parameters are: Functional generator: 20Vpp with 1kHz frequency. Capacitor value is 1nF
I know that the current will be extremely low - 87uA RMS.
Is there any solutions how to measure it?

#### neonstrobe

Joined May 15, 2009
190
Put a resistor in series and measure the voltage across it.
e.g. 1k
You will need an a.c. millivoltmeter (or an amplifier with a known gain) to measure.
And check your maths on the expected current.

#### t.haliulin

Joined Nov 3, 2021
24
Put a resistor in series and measure the voltage across it.
e.g. 1k
You will need an a.c. millivoltmeter (or an amplifier with a known gain) to measure.
And check your maths on the expected current.
Thank you. If I am going to implement resistor do I need to place to two amplifiers on each end of the resistor and measure difference?
Expected current was achieved from the simulation in LTspice, it is wrong?

#### BobTPH

Joined Jun 5, 2013
8,971
If one side is at ground you need to measure only one voltage. Otherwise a differential amp is used.

Bob

#### t.haliulin

Joined Nov 3, 2021
24
If one side if s at ground you need to measure onky one voltage. Otherwise a differential amp is used.

Bob
Thank you, resistor would be in series with the capacitor, therefore differential amp is considered using, but from the experience would it be sufficient to use resistor with diff amp to measure that low currents, as I have no experience for low current applications?

#### BobTPH

Joined Jun 5, 2013
8,971
There are special current sense amplifiers.

Bob

#### t.haliulin

Joined Nov 3, 2021
24
There are special current sense amplifiers.

Bob
I have used previously current sense amplifier for the motor control circuitry, but its resolution and other parameters not really suitable for me.
Are you aware of any AC current sense amplifiers for low current measurements? I was struggling to find any

#### ericgibbs

Joined Jan 29, 2010
18,849
hi t.h
What is the resistor value of the series resistor.?

R

#### Ian0

Joined Aug 7, 2020
9,819
I have used previously current sense amplifier for the motor control circuitry, but its resolution and other parameters not really suitable for me.
Are you aware of any AC current sense amplifiers for low current measurements? I was struggling to find any
An AC current sense amplifier doesn’t actually know the current that is being measured, all it knows is the voltage across the shunt resistor. It will work at any current value, with an appropriately chosen shunt value.

#### t.haliulin

Joined Nov 3, 2021
24
hi t.h
What is the resistor value of the series resistor.?

R
Hi,
I haven't decided yet, wanted to know first if it will work or not

#### t.haliulin

Joined Nov 3, 2021
24
An AC current sense amplifier doesn’t actually know the current that is being measured, all it knows is the voltage across the shunt resistor. It will work at any current value, with an appropriately chosen shunt value.
I know that it is measuring voltage difference, but with such a low current would it be able to actually see the voltage difference between across resistor?

#### ericgibbs

Joined Jan 29, 2010
18,849
hi t.h.
This is the most basic circuit of your project, so that we have a reference circuit to discuss.
Note: the Capacitor current.

E

BTW: Which type/construction is this 1nF...

Updated image to show Irms through Cap.

#### Ian0

Joined Aug 7, 2020
9,819
I know that it is measuring voltage difference, but with such a low current would it be able to actually see the voltage difference between across resistor?
Yes, if you use a larger value resistor!
The limiting factor is the offset voltage of the amplifier, which will be in its datasheet.
 included TS’s question.

Last edited:

#### t.haliulin

Joined Nov 3, 2021
24
hi t.h.
This is the most basic circuit of your project, so that we have a reference circuit to discuss.
Note: the Capacitor current.

E

BTW: Which type/construction is this 1nF...

Updated image to show Irms through Cap.View attachment 252134
Hello,
Yes this is what I have simulated as well, but voltage should be 20Vpp.

#### ericgibbs

Joined Jan 29, 2010
18,849
Yes, if you use a larger value resistor!
Is this for my attention.???

#### ericgibbs

Joined Jan 29, 2010
18,849
but voltage should be 20Vpp.

hi t.h
That Vx is 20V ppk already,!!

Do you mean 20Vpk....?
E

#### t.haliulin

Joined Nov 3, 2021
24
Yes, if you use a larger value resistor!
The limiting factor is the offset voltage of the amplifier, which will be in its datasheet.
If I will use larger value resistor, then the voltage waveform across the capacitor will reduce its amplitude and will be shifted, probably that is fine, but ideally I want to keep certain amplitude of the voltage for a device

#### t.haliulin

Joined Nov 3, 2021
24
but voltage should be 20Vpp.

hi t.h
That Vx is 20V ppk already,!!

Do you mean 20Vpk....?
E
Hi eric,

Sorry, yes its 20Vpp. Your waveform is correct

#### ericgibbs

Joined Jan 29, 2010
18,849
hi t.h,
This plot is for 20Vpk or 40Vppk.
E
What's your next problem in the project.?

#### Attachments

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#### t.haliulin

Joined Nov 3, 2021
24
hi t.h,
This plot is for 20Vpk or 40Vppk.
E
What's your next problem in the project.?
Hello eric,
it is 20Vpp, so your previous waveform was correct.
The problem is how to measure current? Using differential amplifier across resistor?