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Registered Member #211
Joined: Sun Feb 19 2006, 05:33PM
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Posts: 27
Hi folks! I am currently researching a project I will be building in the near future, an automated battery analysis rig, datalogging with a micro and probably a PC. The type of micro will probably be a PIC, but it is somewhat unimportant, the part I am stuck on is how to make a micro controlled current source and sink. So far I have the following ideas: Analogue port to logic level FET gate, resistive current sense into analogue port. A very similar setup using a BJT and base resistor could be used. A simple control loop in pseudo; IF Icharge < Itarch increment gate voltage (or base) IF Icharge > Itarch decrement gate voltage (or base) Of course with this simplicity the state will rapidly just bounce up and down by a minute amount but it keeps the current constant. I searched for current source ICs, but they are sub mA range, where I will need at most, 1.1A. Most of this circuit: http://www.discovercircuits.com/DJ-Circuits/100ma-current-source.htm seems like an epic waste of parts. There is http://www.maxim-ic.com/appnotes.cfm/an_pk/3464 but it does not lend itself to external control. I must be able to measure the load voltage. Being able to measure load current is useful for open circuit detection.
Can anyone think of a better way to control current from a micro? Many thanks, Pete
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
My choice would be to connect an I²C DAC to the pic to produce a programmable variable voltage, then use something similar to the circuit you linked to, to convert that to a large variable current source.
Another similar active current source circuit is shown here in figure 10:
If you don't mind a bit of residual ripple on the constant current, you could do away with the DAC, and use one of the PIC's built in PWM channels. Heavily smoothing the PWM output should give you a nice smooth reference for you active current source.
You could also make a current-mode controlled switching converter that would do the job more efficiently if you wanted to go down that route!
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Microcontrolled battery anaylser design
