- Station for PTC based soldering irons -
A P-Channel driver FET (Q1) that is driven by a NPN bipolar transistor (Q2) switches the solder iron on and off. The PWM frequency is 1 Hz with a resolution of 1ms. It is controlled via the ATMEL ATMega8. Every one second the driver FET switches off to allow temperature measurement. A constant current source (U3) always puts a current of 50mA thru the iron. The voltage drop on the irons PTC is measured in the off state using the AD converter of the Atmel ATMega8 that uses its internal 2.56V reference. The value measured is fed into a PID loop to calculate the next PWM value. A watchdog timer is used for safety reasons.
To calculate the slope and offset of the resistance versus temperature characteristic, I measured some values for the ERSA Micro Tool:
The PTC shows an overall linear characteristic. This allows easy calculation of the current temperature with the Atmel ATMega8.