Experimental dummy load hit the skidders.

[Colin hoof]

I am currently working on a dummy load project.It is limited only by my choice of components.I wanted to use a pic 16 c84 to read a rotary encoder.This in turn controls a 12 bit dac mcp4921 via spi ,which outputs a buffered voltage to an op amp.This drives a power mosfet which forms with a 1 ohm power resistor, the constant current part of the circuit.I would like an oled for the display, instead of an lcd display .I have realised that after starting to build this project ,the major drawback is having zero experience, with source code and having no code or hex file doing anything constructive, this project is dead in the water.

I would like to keep with the components chosen if possible. Just need some code or at least some information, on how to implement code with the circuit I have described. This isn't a mega accurate or laboratory standard .Just a way of learning something useful, about circuit building blocks and how microcontrollers can help, in the design process.

 

[cowboybob]

A block diagram of your concept would be helpful.

... This in turn controls a 12 bit dac mcp4921 via spi ,which outputs a buffered voltage to an op amp.This drives a power mosfet which forms with a 1 ohm power resistor, the constant current part of the circuit. ...

This description of your concept is confusing. My reading of it is that of a dac controlled variable voltage output that feeds into a 1Ω resistor, which implies a constantly changing current, not a constant current.

 

[Nigel Goodwin]

First off, drop the idea of the 16C84 - this is an incredibly old and obsolete device, long since replaced by far more modern devices.

My personal favourite now if the 16F1827, an enhanced 14 bit device, pin compatible with the 16C84, and a fraction of the price of the 16F84 (which replaced the16C84 last century, and was in turn replaced by the 16F628).

You might find my tutorials helpful, which were based on the 16F628

 

[AnalogKid]

Your description is ok up to the part that is missing. True constant current regulation requires feedback of some kind, and your description does not include this. Typically, the DAC output drives a transistor gate or base, with the emitter or source grounded through the 1 ohm sense resistor and the collector or drain going off to the load. The top end of the sense resistor is the pick off point for feedback to whatever is controlling the circuit. That controller compares the voltage across the resistor to a reverence value, and adjusts the transistor drive to maintain the resistor voltage (and hence its current) at a constant value. Your description relies on the MOSFET gate voltage - drain current transfer function to maintain the output current, and this function varies wildly with temperature and from part to part. Do you plan on having the PIC monitor the resistor voltage with an A/D input?

ak

 

[Colin hoof]

I do apologise for being a bit vague, with what my expectations are for the circuit !! The feedback from the power resistor was going back to the op amp driving the gate of the mosfet. I am not averse to having the feedback from the resistor fed into the pic however. This is a fantastic opportunity for learning code , especially mixing analogue and digital elements, in the same circuit.

 

[AnalogKid]

Keep the feedback connected to the opamp. Running the feedback loop through a microcontroller is very tricky programming, not beginner's stuff. Can you post a sketch or schematic of your circuit so far?

ak

 

[atferrari]

Keep the feedback connected to the opamp. Running the feedback loop through a microcontroller is very tricky programming, not beginner's stuff. Can you post a sketch or schematic of your circuit so far?

ak

And maybe too slow, no matter how fast your code is...

 

[Colin hoof]

I am starting work shortly, so I won't have time to post the schematic until later on . The idea for using an old obsolete pic such as the 16 c 84, was only to try and proof the concept, also to try and put it to use in a circuit.It must have been lined up for another circuit previously, and was swapped out for a more updated powerful chip.

Thanks to all for the replies, feedback and positive encouragement !!