A circuit for temperature control of the atomizer coil

[kinabaloo]

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.

[KoS]

Nice and simple. I like it. I was wondering if a simle logic circuit would work but needed some refreasher cources. I think you are right to include the voltage regulator. If you get away with out it I will be surprised but weirder things have happened when thery meets reality lol. When ya going to breadbord and test? Might need tweaking now and then due to gunk build up causing a higher resistance in the coil.

[kender]

Very nice design Kinabaloo
I'm impressed, it looks like it will work with very little power consumption.

[kinabaloo]

Very nice design Kinabaloo
I'm impressed, it looks like it will work with very little power consumption.

Yes indeed, a high quality but still very cheap op amp comparator will need little power. It would be no more than lighting an LED I think.

[kinabaloo]

Nice and simple. I like it. I was wondering if a simle logic circuit would work but needed some refreasher cources. I think you are right to include the voltage regulator. If you get away with out it I will be surprised but weirder things have happened when thery meets reality lol. When ya going to breadbord and test? Might need tweaking now and then due to gunk build up causing a higher resistance in the coil.

I was thinking of using a zener in the reference voltage part of the circuit but then realised that this would definitely require that the supply be voltage regulated. Because the comparator is working on relative values from the same supply, I still think that the voltage regulator is not needed.

Re need to tweak as atomiser ages: I think here you might have a point. Will think about it. Wouldn't it be nice if it self-calibrated

[kinabaloo]

I have realised since that the circuit will also maintain performance as the battery's voltage diminishes as it is discharged (to a large extent); achieving a constant voltage effect without needing to start out with a higher voltage battery

[surbitonPete]

It doesn't seem to make any sense to me how the gunk building up on the outside of the coil could change the resistance. Can anybody explain why?

[kinabaloo]

It doesn't seem to make any sense to me how the gunk building up on the outside of the coil could change the resistance. Can anybody explain why?

Morning Pete - nice day up in the north ?

A deposit might affect the resistance - would be a very small change but might be enough to worry. Will ponder over breakfast ...

It cant change the resistance of the nichrome wire (I don't think) but could the col as a whole - for example, if the deposit is slightly conductive.

And what about the juice - if that is slightly conductive ??
Need some testing ...

[surbitonPete]

Morning kinaba....I guess it could make it 'less' resistance if the gunk is electrically conductive

Hey yes it actually is a great morning up here today......sunshine and no clouds....wow.

[kinabaloo]

Pete - I am near Heathrow airport - pollution clouds building now

Added to my previous post.