Elementary resistance detection of load and powering the load in the same circuit.

Hi all,

The main problem I'm having is being able to detect the resistance of a load AND power the load within the same circuit. In the image I've posted (which is somewhat simplified), the load is R2 and is in fact a heating coil. Although it displays a resistance of 2 ohms, it can be changed by the user and range from anything between 0.2 ohms (for nutters) to 10 ohms. The +5V rail is the output from an Atmega digital output pin which I'm pretty sure I'm not overloading (should be around 20mA). R4 is used to bias the load to increase the low voltage as it isn't directly fed into an ADC it goes through an op-amp first to amplify it a bit, R4 is needed to ensure the voltage at the op-amp input is over the threshold voltage for the op-amp. The resistance is determined and the Atmega works out using ohms law, what the power of the heating coil will be when it is turned on. When the switch is pressed (which is a simplification of a PWM power mosfet switching circuit controlled by the Atmega) the full 12V (PWM) flows into the coil. What I'm unsure about is if the 12V will cause any problems for the LM317 and/or cause current to feedback into the 5V rail (arduino output pin).

Cheers,
MuD
**broken link removed**

That 12 volts will DEFINITELY be a problem. I assume you are trying to use the LM317 as a constant current source? Why not just use a diode in place of R4? This will:
1) Give you your 'bias' voltage
2) Protect the 317 AND the ADC.