2N7000 as VCR

Question

Ok, so going to start off by stating the issue I am facing, Q1 (2N7000) is getting hot enough to cook with. Seriously, I'm measuring 50C on the sucker. What I can't figure out is why. Now I will state, I am far from an engineer, I am easily classified as a noob. Anyway, I have attached my schematic. The sensor has a resistive heater that is driven by Q1 on the HA pin, and the trick is it has to cycle between +5VDC and +1.4VDC at regular intervals. So since I am using an ATTiny85 (I love these little guys!) I'm looking at max Vout on the IO pin of +4.85VDC @ ~20mA. So, I came up with using the LM324 as a small amp to bring my IO up to a full +5VDC and then as a buffer to ensure that even as I put load on Q1 Source I will maintain the desired voltage at the sensor's HA pin. This may not be the best way to do this, but like I said, noob here. So, if my calculations are correct, G1 of the LM324 should represent about a 10% amplification of Vin. So at most I'm pushing something in the neighborhood of +5.5VDC into V+ of the LM324 G2, which should be well within specs. As far as Q1, since I'm using it as a voltage controlled resistor I'm not running Q1 into saturation, which I don't think is bad but I'm almost wondering if my heat issue is coming from the fact that on the drain of Q1 I have +12V and on the source I have +1.4-5VDC, so that's a lot of Vdrop which I'm sure turns into wattage and dissipated heat. I am struggling with this one and hoping someone could get my head straight on it. Oh and yes, I have a lot of filtering caps and zener diodes for over-voltage protection. It's cheap insurance in my mind having the zeners, and the caps, well it can't hurt to filter out any in-rush current . 
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D_Hersey · Answer

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 2N3055 2N3055 makes a better choice positive(conductance tempco

Robert Peter Oakes · Answer

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

tibmeister · Answer

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.

Robert Peter Oakes · Answer

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

Robert Peter Oakes · Answer

your using the FET as switch, not a linear device, so think of it as a mechanical switch, it is either open or effectively a short (A few milliohms anyway) so you wont be able to use the ADC to measure the voltage reliably, what you could do is measure the supply voltage and then derive a pulse width to give you the 1.5 or 5 volts

Robert Peter Oakes · Answer

If your measuring with a DMM then it may not be accurate as (Depending on the DMM) it may not be correctly interpreting the PWM based voltage You can however correctly measure the supply voltage, you may need a resistor divider for the Heater and supply it it is relatively simple Using the internal Band Gap reference voltage instead of the supply (1.1V vs maybe 5 ish volts and also provides more accuracy) and hooking up one analog input to the supply via a divider, same for the Sensor Voltage you can adapt the PWM with some simple math. right now as your using the Supply to the ATTiny as the VREF is is going to drift around anyway, this will make the measurements more stable and therefor accurate. You can adjust any inaccuracies in the resistors using software

2N7000 as VCR

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