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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Current Sensing Questions
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Offline DjKiDD

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Current Sensing Questions
« on: December 15, 2013, 12:15:52 AM »
For my MCU VV/VW build I plan to go with the TI INA226 http://focus.ti.com/docs/prod/folders/print/ina226.html for current sensing because it does the calculations on the chip and I just have to read the registers then I can calculate the atty resistance easily. I am having trouble deciding on suitable current sense resistor capable of 50W.
Does the resistor have to have to be over 1Ohm? I don't want to introduce too much inefficiency...
Can I double up resistors to get the power rating I need? 
Right now my top choice is the MP2060 line from Caddock Electronics.



Offline CraigHB

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Re: Current Sensing Questions
« Reply #1 on: December 16, 2013, 01:05:33 PM »
Looks like a nice part to use.  Taking a quick look at the data sheet for that part, you need to caclulate the value of the CS resistor using the specified formula based on desired resolution.  Since those types of resistors are normally very low resistance, a 1 Watt resistor is usually sufficient.  A 10 mOhm resistor rated for 1 Watt can handle currents up to 10 Amps.  CS resistors are typically rated for 1 or 2 Watts.  It does cost a little efficiency to measure current with a sense resistor.  It depends on maximumal currents, but for 10 Amps, 10 mOhms would be reasonable.  If you go higher than that, it starts wasting too much power.

Offline CraigHB

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Re: Current Sensing Questions
« Reply #2 on: December 16, 2013, 09:04:21 PM »
That's a great looking part so I read the data sheet all the way through on it.  Maximum input voltage from the shunt is 80mV so if you use a 5 mOhm current sense resistor, you'll be able to measure up to 16 Amps.  A 10 mOhm current sense resistor will allow you to measure up to 8 Amps.  The shorter the swing the better the resolution since it's always covered by 16 bits.

I was actually thinking of using that part myself, but it has some issues for me.  For one it has a ghastly 40mS recovery from sleep mode.  Man, I could power up my MCU 100 times in that eternity.   What's weird is it has a 500ksps ADC but the shortest programmable sample time is 140uS which is another limiting factor.  My MCU's ADC is also 500ksps and only takes 2uS to take a sample.  Too bad, the 16 bit ADC in that part is awesome, but otherwise it's too slow for my preferences.  Don't understand why they throttled it like that.  It's pretty expensive too, costs about 10 times more than the differential amp I'm using now.

Offline DjKiDD

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Re: Current Sensing Questions
« Reply #3 on: December 17, 2013, 06:38:37 PM »
Thanks for the replies! I am still looking at parts but I chose these because free samples are available... this whole mod will be made out of free samples. Eventually I want to post it as OpenHardware so others can build it

Offline CraigHB

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Re: Current Sensing Questions
« Reply #4 on: December 18, 2013, 01:53:11 PM »
I take of advantage of samples whenever I can as well, but you still have to consider the cost on parts in the event you want to build the project in some number.  The BOMs I end up with for these projects are usualy about a page long.  50 cents or a dollar doesn't sound like much for a component, but it adds up fast for bigger parts lists.  When I see chips with costs of several dollars it makes me cringe.  The 10 bit digital pot I'm using for my latest project is about $5 and the converter controller is about $7, yikes!  At only $3, the MCU is cheap in comparison. 

The one thing that's always really cheap is SMD capacitors and resistors, but the caps get expensive when you get into the higher values.  I'm really pleased I found a 22uF MLCC cap in a 3225 size that will work well for only 50 cents, but still, I have to use 3 on the output and 2 on the input for the converter.  That's $2.50 in capacitors.  Not cheap compared to the one or two cents a piece you usually pay for those parts.  The MLCC caps are great, but the higher values can get expensive and can have issues with DC bias.  If you can get around that, they have huge advantages over other types of capacitors.

Offline DjKiDD

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Re: Current Sensing Questions
« Reply #5 on: December 19, 2013, 01:48:18 PM »
Not to derail my own thread but what digital pot are you using? I found a 256 tap 1k pot from Maxim a little under $5 that I plan on using

Offline CraigHB

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Re: Current Sensing Questions
« Reply #6 on: December 19, 2013, 03:20:49 PM »
I was using the Microchip MCP4152 on my last project which is a 10 kOhm 8 bit pot.  For the wider voltage range of my latest project I need a 10 bit pot.  The ~only~ one I found was the Maxim MAX5482, but that one has some variation in the base part number depending on resistance and type.  The one I'm using is a 50K pot, but it's also available in 10k.  Interstingly, a logarithmic taper would allow me the use of 8 bits, but it's impossible to find a digital log pot with more than 100 taps.

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