2N7000 as VCR

The FET is going to follow the OPAMP output -~4V with a threshold of about 2V (the V difference between gate and source where it starts to turn on)

If the FET was to be turned fully on then you have a resistor divider between the 220 and 33 resistor so the max that will be output is about 1.5V as the resistance of the FET is negligable so you will never get the full 5V no matter what you do, perhaps switching around the 33 and 220

what is the resistance of the heater ??

with regard to the buffer opamp, it will try to get its - input the same as the + so if you have 5V on the +, the output should try to go to about 7-9V depending on the load (AKA the 33ohm + Heater), the 220ohm will severely limit the current flow which is why it wont ever get to the desired voltage and the opamp will or should be going to the + rail (12V)

if the fet is hard on then your current will be about 50mA without the heater connected but even if it is connected it will never exceed 68mA due to the 220 Ohm  (Remember I=V/R)

If you tell me / the community what the heater needs then I can tell you what resistor values you need

Last question, how are you planning on getting the 5V and 1.5V outputs, the current design will only give one volt setting. not two

Ok, regarding the heat, 50mA at 12V is 576mW which if this was across the FET it would be at its limit but in the schematic you show, all the volts will be dropped across the 220ohm resistor so it will be or should be getting hot, not the FET.

like I said, swap the 220 and 33 ohm resistors, it should work much better (This is without knowing your real load parameters), with this design you could also remove the 220 altogether and connect the top of the fet directly to 12V, I would still put a 220 or more instead of the 33 Ohm though.

Hope this helps for now, provide the feedback and I can help more

I'll add a couple of points to Peter's comments:

1) this configuration of a FET is usually described as a  source follower, as Peter describes in his first paragraph.

2) when you put a buffer (the FET in your case) on the output of an op amp and inside the feedback loop it will very often be unstable. The solution is often to put a resistor between the output of the buffer (FET source in your case) and the op amp -ve input, and a small capacitor from the op amp output to the op amp -ve input. Adding a resistor between the FET gate and the op amp will often make basic stability worse but may be needed to stop HF oscillation with some FETs and op amps. If you are laying out a pcb add all three extra parts, then all your options are open !

3) download a cop of LT Spice  from Linear Technology's website -it's free and it's much quicker to resolve some of these issues in simulation.

4) If you are pulse width modulating PB1 you will need a capacitor from the op amp end of R3 to ground - and be careful of noise.

MK

Peter/Michael, you do not see the 220 on the drain, it doesn't exist... (Jedi hand wave)  I don't know how that little guy stayed on the schematic but it doesn't exist on the breadboard after it smoked out.  I realized my mistake and omitted it.

The heater is 33 ohm, so around 120 mA is what I'm looking at for that guy.  What I suspected was the FET pretty much had no resistance and when there was no sensor heater and no 33 ohm resistor I could bake brownies on the ATTiny85, even setting one on fire by not paying attention (silicon and marshmallows DO NOT MIX!).  So I added the 33 ohm to provide the same load that the heater would while I was probing everything and decided to leave it in there in case the heater ever decided to fail and create an open circuit there, I wouldn't start melting and smoking things again.  My suspicion is that without the resistance on the output I don't have any current limiting going on and the ATTiny pays the price because the opamps and FET don't offer any resistance.

The way I'm getting the two voltages is by driving PWM to the G1 V+ pin, 25% duty cycle for 1.4V out and 92% duty cycle for the 5V output.  So the amp is there because at most I could push from the ATTiny is 4.9V, which would not turn the FET on enough to allow for 5V to go through so I put the amp in there with about a 10% amplification to compensate.

The buffer is there becasue I noticed the FET would not stay stable and would sometimes go a little crazy, even hitting 6V on the Source, so I am using the buffer to make sure that my FET stays where I want it.  It's kinda like a comparator I guess.

I was wondering if a lot of my issue is HF noise on the PWM line being transmitted through to the FET and causing my issues, was thinking about a .1 uF ceramic on there to ground to filter those frequencies out, which the ATTiny85 lowest PWM frequency is 20 kHz.

I was thinking about the resistor from the source of the FET to V- on the opamp but wasn't sure what it would do and what resistance I should put in there.  The cap between Vout and V- of the opamp is brilliant, is that to act as a HF filter?  So would .22 uF or .1 uF be appropriate?

I do want to add, outside of the FET wanting to try and go nova everything in the circuit is working how I planned it would; the FET is able to switch between 1.4V and 5V, and all that jazz.  honestly I'm suprised I only got it on the third try, and the first try with the opamps.  I tried one with two mosfets, one in an inverted fashion and that did nothing, and I tried to drive the FET directly with PWM and ya know, that kinda just sucked.

Using a power fet as a heater is not that great of an idea in practice as a warm fet has lowered Zin with low gain Switching resistors is a better idea as they are made to be heaters a bipolar Q cheap and basic like a  2N30552N3055 makes a better choice positive(conductance tempco

so if only a 33 ohm in circuit then there will be 363mA flowing, and that's about 4.4W of total power , now if your using PWM and limiting the output through the feedback of the opamp to 5V then your left with 7V @ 363mA dissipated in the FET, thats 2.5W, the device is only rated at 200mW so i'm surprised it is even surviving, double all these figures if you have the heater and the 33 ohm

put the heater between the 5V and the fet drain, connect the source to gnd, and drive the FET directly from the uController

It should turn on completely in this configuration and as such will not dissipate much heat and the max volts is still 5, then use PWM to simulate the 1.x volts you need

In your current design, your using the FET in its linear region and trying to get rid of way too much heat

Re: 2N7000 as VCR

Absolutely. Make it as simple as possible. I too am surprised that the FET didn't let out it's magic pixie smoke  right away. Another idea might be to use an adjustable Switching Mode Power Supply (SMPS) and just control the voltage output with the Arduino and  resistive divider. Way less power wasted and simpler. I do have to say I love the little 2n7002. I have use many of them.

I guess I could replace the 2N7000 with a FQP9N08, don't think that guy would have an issue!  On a serious note, I'm going to layout what my interpretation of the suggestion is and hope I'm on the mark.

One thing I've been thinking, I want this to be battery powered and having 8 AA's in a case is a lot of weight, so I'm pondering getting rid of the LM7805 and replacing the 12VDC with 6VDC.  I have used the MCP1253 from Microchip in past items and I friggen love the guy but it's able to only handle 150 mA at the most so it may be a good drop-in replacement for the LM7805 to power the ATTiny85 with and run the sensor/heater off the battery directly.  I would have to definitely see what the amperage of the circuit as a whole is to make sure batteries will last for a while.

Anyway I'm gonna focus on getting the thing working as is and figure out the portable items later and use my MCP1253 on it's breakout board after things are working at 12V.

Ok, so here the Version 4 of the design with the new configurations and elements.  I'm gonna breadboard it tomorrow and see what happens.

Sensor Heater HA to +5V

Sensor heater HB to Mosfet Drain (Top Side)

NO 220Ohm resistor

Assuming this is a valid configuration for the actual heater sensor

what is the part number for the sensor so I can check or you can

I'm using the MQ series of sensors, the MQ-9 to be specific (https://www.pololu.com/file/download/MQ9.pdf?file_id=0J314).  I won't be able to connect the sensor to the +5V line directly because I'm limited to 120 mA on that line, but what if I put the opamp buffer/voltage follower back in with a zener diode on the V+ line so it can't go above +5V then make sure the two resistors are equal at 220k ohm.  Just a thought...