DMM to measure in-rush current issues/recommendations - Page 1
Forum section: electronics / testgear

author: marcusone
author_rank: Contributor
timestamp: November 08, 2015, 01:44:45 pm
content: I find that I keep running into a problem with inrush currents exceeding my DMM's 400mA current measurement, so resets my AVR chip, and the cycle repeats so that I can not get an accurate reading of the current used during operation.If I switch to the 10A setting on my meter (Fluke87 or EX330) then I can get the AVR to startup, but then I can't get a reading as during sleep and operation it only uses a few mA or uA in sleep mode (which I want to know).Any sugestions on a meter that won't cost me a fortune that has perhaps a 1000mA setting were it can read up to that and down to uA?or is there some other way to suppress the in-rush current so I can get a reading during operation?Thanks!

author: TiN
author_rank: Super Contributor
timestamp: November 08, 2015, 01:54:51 pm
content: It's unlikely that inrush is causing issues in your case, but it's likely burden voltage, which is too high at low ranges of most DMMs.You can try using external shunt, for example good 10 ohm resistor as a current sensor, and read voltage from this resistor using your DMM in mV range.1uA will be equal to 0.1mV in such case. If you can bodge up simple opamp circuit with gain 10, you can get 1uA = 1mV.

author: marcusone
author_rank: Contributor
timestamp: November 08, 2015, 02:01:00 pm
content: Quote from: TiN on November 08, 2015, 01:54:51 pmThanks, I just found someone refering to using a "shunt resistor", so now i'm trying to find a good guide on how to setup such a circuit. If an opamp is better, can you send me some links to resources of setting up either of these?

author: DimitriP
author_rank: Super Contributor
timestamp: November 08, 2015, 02:23:40 pm
content: Power up using the 10A scale.Once it's powered up short the DMM leads together so no current will flow through the DMM and switch the range to 400 mA.Unshort the leads and measure the current.tada.wav

author: marcusone
author_rank: Contributor
timestamp: November 08, 2015, 03:11:29 pm
content: I have figured out how to use the shunt resistor; which works great. However, could someone help me understand (or point to resources) as to why when my circuit is active it shows 3.5v, and in sleep is 6.5mV.Circuit information:I have a Lithium Ion battery (currently reading 3.8v output) to a boost convert which boosts to 4.35v, then connected to an ATMega328p Circuit that does a bunch of things, then goes into sleep mode for 1min (custom arduino basically).I know its the boost converter causing the issue (because if I connect 4.35v directly to the "Arduino", I can measure the current use, even during startup, with the 87v or EX330 inline)... I have tried different values of capacitors before and after the boost converter to try and minimize "spikes" in draw on the battery.

author: SeanB
author_rank: Super Contributor
timestamp: November 08, 2015, 03:41:07 pm
content: Buy or build Dave's uCurrent, which will help by reducing the voltage srop of the current sense resistor.

author: TiN
author_rank: Super Contributor
timestamp: November 08, 2015, 04:38:01 pm
content: What if you add 1k resistor to ground on your boost converter output, just before your AVR circuit?Can you show us photo of your setup?

author: marcusone
author_rank: Contributor
timestamp: November 08, 2015, 04:57:18 pm
content: Here is the circuit:

author: TiN
author_rank: Super Contributor
timestamp: November 08, 2015, 05:00:02 pm
content: Do you have caps on your ATmega pins?What is boost converter chip / schematics?

author: marcusone
author_rank: Contributor
timestamp: November 08, 2015, 05:06:08 pm
content: Quote from: TiN on November 08, 2015, 05:00:02 pmI just have the standard ceramic cap on the ATmega (this also occurs when connected to an Arduino pro Mini).https://www.pololu.com/product/2560boost converter set to output 4.35v.(there are lots of reasons why I did this; right now I'm just trying to understand why I see such a spike in "current" drawn during wakeup).would larger caps on either side of the boost regulator help? I'm still not even sure what they do, just that I've read in several places that putting them in helps "smooth" voltage (keeps the input/output voltage stable as current draw changes).Thanks!