Author Topic: Simple current sense circuit  (Read 1758 times)

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Offline kuonTopic starter

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Simple current sense circuit
« on: March 08, 2019, 10:29:45 pm »
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.



 
 

Offline DaJMasta

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Re: Simple current sense circuit
« Reply #1 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.
 

Offline kuonTopic starter

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Re: Simple current sense circuit
« Reply #2 on: March 08, 2019, 11:29:13 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.

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.
 

Offline radix

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Re: Simple current sense circuit
« Reply #3 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...
 

Offline kuonTopic starter

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Re: Simple current sense circuit
« Reply #4 on: March 09, 2019, 08:51:22 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...

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?
 

Offline JuanGg

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Re: Simple current sense circuit
« Reply #5 on: March 09, 2019, 10:34:23 am »
Quote
 
I am not sure I get how to wire the 2x switch. You mean to also switch the gain of the amplifier?

I 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
« Last Edit: March 09, 2019, 10:43:38 am by JuanGg »
 

Offline HendriXML

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Re: Simple current sense circuit
« Reply #6 on: March 09, 2019, 10:54:26 am »
Nice idea to make your own version!

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.
Maybe R12 could be duplicated (10x) so the sensing is behind the switch?
“I ‘d like to reincarnate as a dung beetle, ‘cause there’s nothing wrong with a shitty life, real misery comes from high expectations”
 

Offline radix

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Re: Simple current sense circuit
« Reply #7 on: March 09, 2019, 04:51:55 pm »
I am not sure I get how to wire the 2x switch. You mean to also switch the gain of the amplifier?
JuanGg described exactly what I meant.
 

Offline kuonTopic starter

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Re: Simple current sense circuit
« Reply #8 on: March 10, 2019, 12:50:43 am »
JuanGg described exactly what I meant.

That is very neat, I indeed didn't think about that. I will look into a dual switch.
 


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