Author Topic: question in "Safety and proper usage" section  (Read 103 times)

apku04

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question in "Safety and proper usage" section
« on: October 13, 2020, 02:43:56 PM »
Hello, I have just purchased a CurrentRanger.
And running through the  "Safety and proper usage" section from the documentation: https://lowpowerlab.com/guide/currentranger/.

Here i dont understand what it means by: "never connect a low impedance voltage source to the inputs" dose this simply means dont connect
the voltage source directly to the input, but instead connect the DUT in series (the conventional way of measuring current ?)

secondly, what dose it means by "A maximum differential of 33mV is allowed before overloading starts to occur (vBat > 3.4V)." ?
(i think i understand the concept of overloading in terms of current measurment, but how can i prevent this?...perharps by not connecting any DUT that can/may draw unexpected high current?)
The scenario of this measurement is mainly on measuring current on 3v3 voltage devices that can draw at most few hundreds of mA while active and then go back to sleep.
So perhaps in my case I dont need to be afraid of blowing things up.

Sry for these stupid question, my background is "computer science"  hence my knowledge in electrical engineering is limited :) ..trying to self learn electrical engineering by simply doing and blowing stuff up :D

Hope some one will take their time and explain this in terms like: "dont connect power source directly, but only in series" and no more than xxx mili ampaire , and yy voltage :)..think you got my point
before i blow up this equipment :)
« Last Edit: October 13, 2020, 02:52:26 PM by apku04 »

Felix

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Re: question in "Safety and proper usage" section
« Reply #1 on: October 13, 2020, 08:21:07 PM »
1) "never connect a low impedance voltage source to the inputs"
It means don't connect a PSU + and - to the input of the CR. Low impedance means low resistance in DC current, meaning that you would pass a lot of current at a high voltage difference (differential) between the inputs. Which would far exceed the 33mV max that is expected, and that would grossly overload the opamps in the CR.

2) "A maximum differential of 33mV is allowed before overloading starts to occur (vBat > 3.4V)." ?
The input current is measured as a voltage differential (created with the internal shunts) that is amplified 100x times.
IOW when you pass in 1AMP of current through the internal shunt of the CR, that shunt creates a 10mV burden voltage across that shunt (Ohm's law: current passed through resistor = voltage diff across the resistor), which is then amplified 100x times to result in 1000mV at the output of the CR.
Consequently, if you pass 3.3Amps through the inputs, that would create a 33mV differential across that shunt, resulting in 3.3V at the output. Anything over that, and the opamps are overloaded. If you had an ideal PSU that is set to 1V, and you connect PSU+ to CR_input+ and PSU- to CR_input-, then that would result in 1000mV across that shunt, overloading/saturating the second opamp.

So the bottom line is that this is a high side current meter, and you should always connect the CR_input+ to the PSU+ first, then have the DUT connected to the CRinput_-. So always have a DUT in there, because that's how it's supposed to work. Then heed the warnings about ground loops.

This is another reminder to try to find some time and create some diagrams with DOs and DONTs that should help explain all this verbiage with more visuals.  ::) ::) ::) ::) ::) ::) ::)

apku04

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Re: question in "Safety and proper usage" section
« Reply #2 on: October 14, 2020, 02:00:52 AM »
Great thanks for the fast response !! :)
It makes perfect sens, I just want to be clear in my doubts.
Some time a picture tells more than word :) so please if you have time perhaps there are more like me out there confused :)

Thanks for the reminder of the ground-loop, this part I understand :)

once again thank you !