Simple current sense circuit - Page 1
Forum section: electronics / projects

author: kuon
author_rank: Regular Contributor
timestamp: March 08, 2019, 10:29:45 pm
content: I built my own µCurrent device based on Dave's schematic. It works, but the output range is a bit limited and I have to monitor some current in specific range (like 5ma) which are a bit hard to monitor with the µCurrent.I came with the following design, which is more expensive, but should have the ranges and performances I need.As this is my first current sensing circuit, I was wondering if you could comment it.

author: DaJMasta
author_rank: Super Contributor
timestamp: March 08, 2019, 10:55:21 pm
content: Do you need the very low current ranges? Using lower value shunts lets you sense higher currents on similar hardware, but Dave's design is optimized for the very low currents - there are some other considerations (and in some cases some cheaper parts) that will work well on higher ranges if you don't need the very low ones.Also worth mentioning: when you start getting shunts a few ohms or less, the contact resistance and repeatability of your switch will start to matter.. If your shunt is 1k Ohms, 50mOhms variation in your contacts is a tiny amount in the noise of your measurement, but if your shunt is 500mOhm, +-50mOhm means +-10% to your reading's accuracyjust from the switch.

author: kuon
author_rank: Regular Contributor
timestamp: March 08, 2019, 11:29:13 pm
content: Quote from: DaJMasta on March 08, 2019, 10:55:21 pmI don't need the 1k, 5k and 10k range, but as you pointed, the switch is important. So I picked a 10mOhm rotary switch, but it is only available in 10 positions, so I added the 1->10k range to fill it.

author: radix
author_rank: Contributor
timestamp: March 09, 2019, 01:14:28 am
content: Just get a 2x10 switch. Use one wiper to switch the input and the other to switch the sense connector to the amplifier. The contact resistance then doesn't play a role in accuracy. Although I'm still not sure that you need all these ranges...

author: kuon
author_rank: Regular Contributor
timestamp: March 09, 2019, 08:51:22 am
content: Quote from: radix on March 09, 2019, 01:14:28 amWell, I don't need all those ranges, I just filled the 10 positions of the switch I could find in stock.I am not sure I get how to wire the 2x switch. You mean to also switch the gain of the amplifier?

author: JuanGg
author_rank: Regular Contributor
timestamp: March 09, 2019, 10:34:23 am
content: QuoteI think he means using one wiper to send the current through the appropriate resistor, and the other wiper to switch the sense line from the op-amp to that same resistor, so that you can measure voltage across the resistor, and not across the resistor + contact resistance. Since ideally no current will flow to the op-amp, the contact resistance of the second wiper won't matter. See diagram attached.Juan

author: HendriXML
author_rank: Super Contributor
timestamp: March 09, 2019, 10:54:26 am
content: Nice idea to make your own version!Quote from: DaJMasta on March 08, 2019, 10:55:21 pmMaybe R12 could be duplicated (10x) so the sensing is behind the switch?

author: radix
author_rank: Contributor
timestamp: March 09, 2019, 04:51:55 pm
content: Quote from: kuon on March 09, 2019, 08:51:22 amJuanGg described exactly what I meant.

author: kuon
author_rank: Regular Contributor
timestamp: March 10, 2019, 12:50:43 am
content: Quote from: radix on March 09, 2019, 04:51:55 pmThat is very neat, I indeed didn't think about that. I will look into a dual switch.