The digital PWM signal is converted to an analog voltage using a low-pass filter formed by the 1kω series resistor and a 4.7μF tantalum capacitor. By varying the PWM duty cycle and therefore the DC signal level out of the filter, the program can indirectly vary the current flow through the transistor. A 100 resistor in series with the emitter of Q1 converts the loop current to a small voltage, which is fed into the micro on input1. The program uses this feedback signal to zero in on the desired current level with the aid of the micros analog-to-digital converter. Details of this can be seen in the accompanying program listing.
If the PICAXE senses an open circuit, it shuts down the output and goes into an alarm state, to alert the operator and protect the circuit under test. The switch can be pressed to reset operations to the start once the open circuit has been rectified. The LED flashes a code for various milestones, as follows: one flash at 4m and one flash to confirm a switch press two flashes at 12m when ramping up (for the first 5 cycles); three flashes at 20m and continued fast flash sequence for open-circuit alarm. For portable use, the circuit can be powered from two 9V batteries, whereas for bench testing, a 12V DC plugpack will suffice.
