Current Measurement using oscilloscope

danadak

Joined Mar 10, 2018
3,879
The integral method, if done over the entire period, if scope has enough
resolution, will result in more accurate method than the approximation using
duty cycle I advised earlier. Thats because there is a min baseline of current
that is ignored when using the DC calc. Buts its a reasonable approximation
in most work.



Regards, Dana.
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
I'm not sure what you mean by "250mA rated D battery." Any reasonable D cell, even a plain "carbon-zinc" type, will easily deliver a quarter of an ampere until it is nearing end of life. A bulk capacitor in parallel with a high frequency capacitor, both placed near the motor driver, are generally good practice. They work to overcome both the source impedance and the connecting wire resistance and inductance and keep fast switching edges off the wires which is important to controlling radio frequency interference (RFI). Be sure to twist the wires between the battery and the driver into a pair to minimize "loop area" which minimizes inductance and the ability radiate noise.

For such small peak current I would try about 220 to 470 µF. The voltage rating should be at least equal to your maximum supply voltage. Higher voltage rating will yield lower equivalent series resistance (ESR) and higher ripple current rating, but you don't need to derate the voltage (e.g. if your supply were 15 volts max, a 16 volt cap would be fine). If weight and space aren't issues, there is no harm in using greater capacitance. Calculating the "optimum" capacitance requires that you know the source (power supply/battery) impedance. Assuming the capacitor will supply 100% of the pulse current will result in a much larger cap than necessary. If you can, use a "low ESR" capacitor. They are quite readily available from most larger vendors. The types people use as replacements for computer motherboards are good. Add a ceramic capacitor of somewhere in the 100 nF to 10 µF range in parallel (this is the high frequency capacitor to combat RFI).
.
For rectangular waveforms, the RMS current is the square root of the duty cycle times the peak current.
RMS current is important for assessing resistive losses in the motor windings, connecting wires, FETs (assuming that is what is used) in the driver and source (battery) resistance and capacitor ESR. Unless those resistance are unusually large, they will be quite small relative to the current required to deliver the mechanical power (in high power situations, even though they are fractionally small they may be very significant to each of the things mentioned). The battery mostly cares about the average current.


With regard to oscilloscope current probes: They are extremely useful for the class of work for which they are ... useful. They are also expensive. If you have a higher end top-brand oscilloscope with built-in interface for "exotic" probes, they aren't too bad - you might even find a good used one for under a thousand dollars. If I recall, mine, which has its own control and amplifier box) was about $3500.
 

MrAl

Joined Jun 17, 2014
7,101
Hello,

I use a 0.1 ohm 25 watt precision resistor for measurments. If the current is low, i'd use a 1 ohm resistor.

It also helps to use a low pass RC filter across the resistor to help reduce noise.
 

Thread Starter

abc14

Joined Oct 15, 2017
117
I'm not sure what you mean by "250mA rated D battery." Any reasonable D cell, even a plain "carbon-zinc" type, will easily deliver a quarter of an ampere until it is nearing end of life. A bulk capacitor in parallel with a high frequency capacitor, both placed near the motor driver, are generally good practice. They work to overcome both the source impedance and the connecting wire resistance and inductance and keep fast switching edges off the wires which is important to controlling radio frequency interference (RFI). Be sure to twist the wires between the battery and the driver into a pair to minimize "loop area" which minimizes inductance and the ability radiate noise.

For such small peak current I would try about 220 to 470 µF. The voltage rating should be at least equal to your maximum supply voltage. Higher voltage rating will yield lower equivalent series resistance (ESR) and higher ripple current rating, but you don't need to derate the voltage (e.g. if your supply were 15 volts max, a 16 volt cap would be fine). If weight and space aren't issues, there is no harm in using greater capacitance. Calculating the "optimum" capacitance requires that you know the source (power supply/battery) impedance. Assuming the capacitor will supply 100% of the pulse current will result in a much larger cap than necessary. If you can, use a "low ESR" capacitor. They are quite readily available from most larger vendors. The types people use as replacements for computer motherboards are good. Add a ceramic capacitor of somewhere in the 100 nF to 10 µF range in parallel (this is the high frequency capacitor to combat RFI).
.
For rectangular waveforms, the RMS current is the square root of the duty cycle times the peak current.
RMS current is important for assessing resistive losses in the motor windings, connecting wires, FETs (assuming that is what is used) in the driver and source (battery) resistance and capacitor ESR. Unless those resistance are unusually large, they will be quite small relative to the current required to deliver the mechanical power (in high power situations, even though they are fractionally small they may be very significant to each of the things mentioned). The battery mostly cares about the average current.


With regard to oscilloscope current probes: They are extremely useful for the class of work for which they are ... useful. They are also expensive. If you have a higher end top-brand oscilloscope with built-in interface for "exotic" probes, they aren't too bad - you might even find a good used one for under a thousand dollars. If I recall, mine, which has its own control and amplifier box) was about $3500.

Thanks for your detailed reply. This is how my circuit looks like , I have place 470uF polarized cap alongside 10uF ceramic cap.

upload_2018-4-17_18-32-40.png
 

ebp

Joined Feb 8, 2018
2,332
I didn't even think about a primary lithium cell. Many don't do too well at high current.

Adding capacitors the way you have shown should work well. Because the voltage is so low you could increase the capacitance of the electrolytic to 1000 µF or even more and still use a cap that is physically quite small, but I don't think you'll need to. You can use the same sort of method you've used to measure the current between the cell and the rest of the circuit (including the capacitor) to confirm if the capacitor is doing an acceptable job or if you might want to increase the value.

It is good to see you experimenting with methods to make somewhat unusual measurements with your scope. Doing such experiments and thinking through the issues will serve you well.
 

Thread Starter

abc14

Joined Oct 15, 2017
117
Hi Guys,


I have put a Multimeter as ammeter to measure the servo current(without any capacitor next to servo), my results are attached in the video. It
seems peak draw of .158mA per actuation. This is only opening of servo. Its about roughly the same draw on closing operation.

So .158mA for opening and .158mA for closing of servo. Now my application requires this actuations 6 times a day.

To calculate mAh...

3s is average time for Servo to actuate.
.158mA *3s /3600s = 0.00013166666667 mAh.

it will be doing this 6 times a day so that would be
0.00013166666667 * 6 = 0.00079000000002 mAh a Day

or
.288mAh a year

or 1.44 mAh for 5 years ?

Am I on right track with my calculations ?

 
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