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Simple current sense circuit

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kuon:
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.



 

DaJMasta:
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 accuracy just from the switch.

kuon:

--- Quote from: DaJMasta on March 08, 2019, 10:55:21 pm ---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 accuracy just from the switch.

--- End quote ---

I 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.

radix:
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...

kuon:

--- Quote from: radix on March 09, 2019, 01:14:28 am ---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...

--- End quote ---

Well, 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?

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