[moved] Measuring high speed changing currents

I am using a 7.4V battery (2200mAH),

AT some instance when the circuit draws more current, I see drop in this voltage. As my knowledge goes, it is because of internal battery resistance and the total resistance of the circuit where the voltage gets divided.

I tried connecting a DMM to measure current and a CRO for voltage.

Since I am using this to run a thermal printer, I see a lot of variations in current drawn and obviously its not constant and the current drawn depends on what is being printed.

These current changes are very fast and cannot be read by DMM or probably it changing way too fast for me to see any difference.

I did try with current resistor and using CRO to check voltage change. But again too fast and too many variations to know that maximum or average current.

I guess max current drawn is what i need to know at various battery conditions (100%, 75%, 50% so on)

How do I really check the maximum current my circuit is drawing.

Re: Measuring high speed changing currents

Hi,

I guess max current drawn is what i need to know

Click to expand...

You guess..

You should know instead of guessing.

What´s your goal of measuring the current?
* Estimate battery lifetime?
* Estimate battery health satus?
* switch OFF printer?
* activate an alarm?

*****
Often the peak current is not very informative.
But a scope should give you a clue.

You could easily "slow down" the peaks by connecting a large electrolytic capacitor parallel to your printer.
(We don´t know currents nor the timing, so I assume at least a 1000uF capacitor should smooth the current peaks.)

Klaus

Re: Measuring high speed changing currents

"Guess" i think was too much to add there klausST

Whats happening is when I am trying to print a image, or something where in all the strobes need to be turned ON (6 in total), the current drawn is more and hence it stucks and doesnt print anything.

but if the complete line isnt black then the printing is fine.

I am not sure if I am not getting enough current from the battery or is it something else. Battery shouldnt be a problem

I want to know why I am not able to print when I am trying to print the whole line black.

Note: The code and schematic is correct. I connected the board to a external supply that can handle 5 amps and it kind of works fine but randomly sometimes it does stuck too when I am trying to print image or rather say a complete black print.

Re: Measuring high speed changing currents

Hi,

FIRST: as recommended use a capacitor. A battery is not suitable for high speed current peaks.

Then use a low ohmic shunt to measure the voltage drop (current) with a scope.
With a digital scope or a PC scope you should show at least some seconds during printing. Find out the peak magnitude. With installed capacitor.
Then setup the scope trigger "DC mode" t rigger only the high currents. adjust the timing of the scope to find out the timing of the peaks.
Then we can recommend an appropriate capacitor (higher or lower value than the recomended 1000uF)

****
I assume the current peaks are one problem.
But an additional problem may be the wiring.

If you show a picture of your wiring, we could give recommendations.

Klaus

Re: Measuring high speed changing currents

I don't see why it shouldn't be possible to determine the maximum current during printing with an oscilloscope. Just a problem of reasonable setup, e.g. trigger conditions.

I guess the printer and other electronics is not directly supplied by the 7.4 V battery. You didn't yet tell any details about the power supply circuit.

Re: Measuring high speed changing currents

Power supply I am using is as below



VDM being battery voltage

Re: Measuring high speed changing currents

If the battery worked properly when powering the printer before but does not work properly now then probably the battery, its charger or the power supply needs replacement.
A "7.4V" lithium battery should be 8.4V when fully charged.
A healthy Lithium rechargeable battery should be able to provide a very high current that is enough to cause an explosion or catch its wiring on fire if it is shorted.
A Lithium rechargeable battery is "killed" if discharged below about 3V per cell (6V for your battery or if left fully charged for 6 months or more.

Simple question: Do you see drop of 5V supply during printing of all-black area? If so, what's the Vdm level at this moment?
What's the printer peak current specification?