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Registered Member #211
Joined: Sun Feb 19 2006, 05:33PM
Location:
Posts: 27
Howdy folks
I haven't been around here for a while, tied up with various things including work. Work, as it happens, is what brings me here now - I'm stuck on a project I am designing - a battery analyser system. My problem lies in that I need a way to produce a bunch (8 of each at least) of current sources (up to 250mA) and sinks (up to 1.3A) to be controlled from a MCU likely PIC16F887. I really cannot find any adaptable OTS solution. Here's the catch : It can't be raw PWM or otherwise, for PWM to work, the peak has to be higher than the average, this is not permissible. Another reason is that I am continuously monitoring current and voltage via a bank of ADC chips, PWM would upset it. Going to such extent I need to be able to set the currents with reasonable precision, as the device will compensate for changing battery voltage on the fly, but can adapt to differing numbers of cells and chemistries. So, my question to the audience is: How would you go about an array of current sources/sinks that have a smooth and stable output, but can be controlled via a PIC ?
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
For a current sink, either use a smoothed PWM signal or a DAC IC to create a reference voltage that represents the current you desire. The use it as the reference in this circuit
Values are not correct but the concept works - I've used it for a similar thing. You'll need +ve and -ve rails on your op-amps. An instrumentation amplifier might help on the current sense resistor
Registered Member #211
Joined: Sun Feb 19 2006, 05:33PM
Location:
Posts: 27
Hey James! Cheers for the information and the reference there, much appreciated. With regards to smoothing out PWM, I'm assuming I could cheat and use a GPIO pin as a digital output, add a diode (to avoid complication of triple state output) somewhat like my attached diagram, from MCU on the left, to + at the top, via load, to ground at the bottom terminal. I tried this with a BJT on the desk and it sucked, I am sure FETs are far more suited to this application, could even JFETs be used? It need not regulate itself, as I have a big ADC array monitoring current, fed back to the PIC, a hysteretic control loop should sort that out.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Hi Pete,
Just to throw in another design... This is my favouritest of all current sources:
If you want to make the current adjustable you just replace the zener diode with a variable voltage source. This can be the output of a DAC (or smoothed PWM stream) if you want the current level to be programmed digitally by a microcontroller.
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Digitally controllable current sources and sinks
