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Big Current Source

Posted: Thu Jan 17, 2013 2:02 am
by basicchip
>from the Yahoo Forum
>looking for a 1A controlled current source

A current source can be built with a DAC and a resistor divider in your case to get a 0 to 0.1 V reference, an op amp for feedback with a big MOSFET with a low resistance and small current sense resistor (remember it's dissipating a lot of heat)

Values are not exact, but the general idea
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Re: Big Current Source

Posted: Thu Jan 17, 2013 2:43 am
by danlee58
I was thinking along those lines. I'll ned an Op Amp that works on a single supply, and a way to mount & heat sink the FET, also a DIP or SIP DAC.

I got my ARMmite board working. I cleaned the solder splashes and flux. I plan to use it to test my Interrupt Proceedure, but I'll do the job on the Pro board. I have changes to make to the circuitry on the ARMmite board, and I don't think it will survive removing more components. Unlike software, you can't simply download new circuitry.

Re: Big Current Source

Posted: Thu Jan 17, 2013 1:02 pm
by danlee58
I'm looking at a Maxim MAX517ACPA DAC, and an NXP BUK75144-55A,127 for the MOSFET. I need through hole mounting for the DAC. I still need to identify a single supply Op Amp.

Re: Big Current Source

Posted: Thu Jan 17, 2013 1:50 pm
by basicchip
Even if you don't buy from them Digikey, they have the BEST searchable site for electronic parts.

I entered op amp, and it gave me a long list of categories, scrolled down to integrated circuits, op amps...

Still lots to choose from, but then selected thru hole mounting and 1-5 V single supply.

And it returned


Digi-Key Part Number NJU7014D-ND
Price Break Unit Price Extended Price
1 0.78000 0.78
25 0.58520 14.63
100 0.48750 48.75
500 0.46800 234.00

Re: Big Current Source

Posted: Thu Jan 17, 2013 9:03 pm
by danlee58
The parts that I selected are from Mouser. I'll try Digikey next. I have purchased from both in the past. I will look for another DAC, because the Mouser unit is a Dual and priced at $3.73 ea for 1 to 25 units.

In the Dual DAC setup each DAC uses a seperate External Reference, since there are two DACs in a package, I could use VCC as a reference for one, then connect the 1st output to the Reference of the 2nd DAC. Then I would program the 1st output to be x/256, and I wouldn't need a Divider for the OpAmp input. The 2nd DAC would directly drive the OpAmp.

Re: Big Current Source

Posted: Sat Jan 19, 2013 12:00 pm
by danlee58
I have run into a problem. The I2C bus outputs are on PO(2), and PO(3). These are not available on the PRO or ARMmite boards.

The DAC that I have on order rquires SDA & SCL for the I2C data transfer.

Re: Big Current Source

Posted: Sat Jan 19, 2013 1:35 pm
by basicchip
If you stick with the 2103 on the PRO, then use the bit banged I2C

Re: Big Current Source

Posted: Sat Jan 19, 2013 2:55 pm
by Pompey2
danlee58 wrote:I have run into a problem. The I2C bus outputs are on PO(2), and PO(3). These are not available on the PRO or ARMmite boards.

The DAC that I have on order requires SDA & SCL for the I2C data transfer.
Access to the I2C lines IS provided on the PRO. The 2 lines are brought out as P17 and P18 on J6 - just above the LED on the right side of the board.
http://www.coridiumcorp.com/files/Schem ... proSCH.pdf

I've installed 2x4 headers on all my PROs for easy access to SDA,SCL,Gnd and V3.3.
Hardware implementation is much more reliable, and smaller code, than bit banging.

Chris

Re: Big Current Source

Posted: Sun Jan 20, 2013 10:55 am
by danlee58
Chris,

Thanks!

I'll jumper P!(17) & P1(18) to IO(12) & IO(13). They are unused and will be tri-state. This way they will be available on the Prototype Shield thriugh the Header.