I had a bit of spare time so designed a circuit that can limit the temperature of the atomiser coil, which may help prevent burn-outs.
Just limiting the current would slow the heat-up time. This will heat up as fast as normal but not overheat when dry. No need for thermocouples or PWM. In effect, the coil itself is the temperature sensor: the resistance of the nichrome wire increases slightly as the temperature increases. Once the cil is up to normal temperature, the circuit behaves much like a classic PWM circuit.
The coil will heat up fast to the operating temperature but not higher; power is automatically disconnected and reapplied to maintain the optimum temperature. The 10nF capacitor (and the right-most 100nF) and the heat capacity of the coil control the switching speed (these capacitors introduces a little hysteresis). The 2 x 100nF capacitors I just included for extra stability and are probably not essential.
The values shown should about right (I haven't tried it). Adjust trimming potentiometer till the atty runs at optimum temperature.
Initially the coil will heat up when button pressed. The voltage across it is compared with the reference and as soon as the voltage is greater, the mosfet is switched off (comparator output will go close to zero). When the atty cools and the resistance decreases, the voltage will decrease and the power applied again. In a 5 second power on this may occur 10-20 times, all autmatically controlled by this circuit.
One of the 10K resistors may need to be omitted (shorted) / replaced by a very low value such as 100ohm. (Once the pot position is known for a given mosfet/atty combination, the side resistor values can be upped and changed to make the pot more sensistive in the correct range; or the whole reference bit changed to simply 2 resistors as the potential divider.)
Actually, as the comparator is relative, the voltage drop of the battery under load should not affect the circuit (much) so the voltage regulator can probably be omitted.
Don't forget to connect power to the comparator IC.
Just limiting the current would slow the heat-up time. This will heat up as fast as normal but not overheat when dry. No need for thermocouples or PWM. In effect, the coil itself is the temperature sensor: the resistance of the nichrome wire increases slightly as the temperature increases. Once the cil is up to normal temperature, the circuit behaves much like a classic PWM circuit.
The coil will heat up fast to the operating temperature but not higher; power is automatically disconnected and reapplied to maintain the optimum temperature. The 10nF capacitor (and the right-most 100nF) and the heat capacity of the coil control the switching speed (these capacitors introduces a little hysteresis). The 2 x 100nF capacitors I just included for extra stability and are probably not essential.
The values shown should about right (I haven't tried it). Adjust trimming potentiometer till the atty runs at optimum temperature.
Initially the coil will heat up when button pressed. The voltage across it is compared with the reference and as soon as the voltage is greater, the mosfet is switched off (comparator output will go close to zero). When the atty cools and the resistance decreases, the voltage will decrease and the power applied again. In a 5 second power on this may occur 10-20 times, all autmatically controlled by this circuit.
One of the 10K resistors may need to be omitted (shorted) / replaced by a very low value such as 100ohm. (Once the pot position is known for a given mosfet/atty combination, the side resistor values can be upped and changed to make the pot more sensistive in the correct range; or the whole reference bit changed to simply 2 resistors as the potential divider.)
Actually, as the comparator is relative, the voltage drop of the battery under load should not affect the circuit (much) so the voltage regulator can probably be omitted.
Don't forget to connect power to the comparator IC.
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