Help With Electrical Circuit And MOSFET Question

[jyl]

Any electrician/electronics guys here?

I am trying to understand how Circuit 1, below, works. I got this design off the Internet. http://www.heatsink-guide.com/content.php?content=control.shtml

It is supposed to allow temperature control of a CPU cooling fan. I have crudely re-drawn the circuit diagram and added my understanding of how the components behave in text boxes, and for convenience I added three reference points A, B, and C.



First, I'm confused by the role of the potentiometer part of the circuit.

Suppose the potentiometer part of the circuit were eliminated. Then my understanding is that, when temperature at the thermistor is high, resistance across the thermistor is low, so voltage at point A is high. Then voltage at point B, the MOSFET gate, would also be high. Once the voltage at the gate exceeds the MOSFET's threshold voltage, the MOSFET switches to "on-state" and resistance from source to drain goes to a low value. So the MOSFET passes its max current and the fan runs at high speed.

The website where I took the design says the potentiometer allows you to prevent the cooling fan from running all the time.

"Before you start using the temperature control, the temperature at which the CPU (or case, etc...) should be kept must be set. This is done by adjusting the spindle trimming potentiometer. A good strategy to do this is: Start with a cool CPU (or case). Turn potentiometer until the fan doesn't spin at all. Then, watch the temperature rise. As soon as the temperature gets close to the maximum it should reach, adjust the potentiometer so that the fan just starts to spin."

Okay, suppose at a given temperature the thermistor resistance is low enough that voltage at the MOSFET gate would normally be above the threshold voltage. But you don't want the fan to be running. So the potentiometer somehow lets you control voltage at the gate?

I am confused. Why wouldn't you instead place the potentiometer in series between the thermistor and the gate, instead of as a parallel bypass from gate to source?

Second, is it correct that in this circuit, if the temperature is high enough that the fan is not running, current is nonetheless flowing through the potentiometer so that the whole circuit is consuming power?

I want the circuit to consume no (or very little) power when temperature is high. This is to preserve the life of the battery that will be driving all this.

Third, I'm actually trying to control not just a fan, but also a Peltier cooling element - this combined load that can draw several amps.

The cheapo ($3) MOSFETs I find at Radio Shack are rated for only 3 amps current from source to drain.

So I am thinking, instead of connecting the MOSFET directly to the load, I should connect the MOSFET to the control circuit of a relay, and connect the fan/Peltier to the relay's load circuit?

Kind of like Circuit 2, below?



Sorry if I'm not being clear about all this.

[Hendog]

First off, in electronics we look at current flowing from ground up to source. I'm looking at the diagram as if +12VDC is at the Source, and Ground (not -12VDC) is at the bottom of the diagram.

The pot and thermistor form a voltage divider. You set the pot so that the voltage at the Gate is correct to drive the transistor while the thermistor's resistance is low (hot). The current path is from ground, thermistor, pot, +12VDC; does that look like it's in series now? You're just tapping the voltage between the thermistor and the pot and appying it to the Gate.

Build it like circuit #2; I like that one since the load current is running through a relay; your FET will stay cool.

[jyl]

Thank you!

I looked up "voltage divider" and now I understand. I even sort of - dimly - recognize the formulas from college days.

Well, now it look like the components that I bought, i.e. the ones spec'd in the website, may not work. Which kind of makes sense, the website is for CPU cooling so they're want fans to turn on around 35C or so. I want fan and Peltier to turn on around 7C or so. I looked up the spec for the thermistor I was going to use and at 7C it is 24K ohm. The MOSFET I bought has 4.0v threshold gate voltage. So I'd need the potentiometer to be >12K ohm just to get the MOSFET to switch "on". And sure enough I bought a 10K potentiometer instead of a 50K one. (Edit: duh, I can just add a 10K resistor.)

Whatever, the stuff is cheap.

The most important thing is, with your tip I can now calculate which components I need instead of guessing. Thanks again.

[jyl]

Ah, I see, current flowing when MOSFET "off" will be like 0.3mA, so my second concern is moot.

[rick-l]

I think you drew that wrong.

There is a thermistor in series with a 10k pot between +12 and ground. When the thermistor resistance is low enough that the voltage on said divisor exceeds Vgs the mosfet turns on.

12 volts goes into the fan, the black wire on the fan goes to the drain and the source goes to ground.

http://www.irf.com/product-info/datasheets/data/irfz24vpbf.pdf

[jyl]

I thought more about the alternative of putting the themistor, the potentiometer, and the gate-source of the MOSFET in series.

Reading about MOSFETs, I found that the resistance between gate and source is very high, essentially infinity.

Input resistance: The electrically insulating Gate oxide layer prevents any flow of current from Gate to Source. In a common-Source configuration with Gate as the input and Drain as the Output, the input resistance is infinity. This is because the input current (the Gate current Ig) is zero regardless of the value of the input voltage (the Gate-Source voltage Vgs). http://jas.eng.buffalo.edu/education/mos/mosfet/mosfet.html

So the voltage that the gate sees will be nearly equal to the 12v voltage drop over the entire circuit. E.g. if the thermistor is 27K ohms, the potentiometer resistance is 15K ohms, and the gate-source is ten million ohms (just to pick a big number), the voltage drop over gate-source will be 11.99v. This is higher than the threshold voltage 4.0v, so the MOSFET is "on". To get V(GS) below threshold voltage, the potentiometer resistance has to be close to the gate-source resistance. Which seems impractical. So with a series circuit, it is hard to turn the MOSFET "off".

Does that make sense?

(Sorry that you guys are having to give me a remedial lesson in electrical circuits)

[Hendog]

Try this.



Sorry about the poor quality hand drawn circuit but I'm old school...

[Hendog]

You may want to reduce the value or even eliminate the fixed 10K resistor which I added, depending on the resistance value of the NTC at the "turn on" temp.

[jyl]

Thank you! Your hand-drawn diagram is actually readable unlike my diagram.

Here's my math -

Suppose "turn on" temp is 41F.
Thermistor should be 27K ohm at 5C/41F. I got the specs (B value) and the formula that is supposed to calculate the thermistor curve (hope its right)
V(GS) threshold is 4.0v for this MOSFET per specs
To get V(GS) 4.0v I need resistance of potentiometer plus resistor 13.5K ohm, because

4.0 v = 13500 ohm * 12 v / ( 27000 ohm + 13500 ohm)

So if use the 10K resistor, a 10K potentiometer is set at 3.5K, nicely in the middle of its range.

Setting the potentiometer at 0K gives V(GS)=4.0v at 50F where thermistor is 20.7K ohm
Setting the potentiometer at 8K gives V(GS)=4.0v at 32F where thermistor is 35.2K ohm

So potentiometer seems nicely sized for desired range of "turn on" temps.

Well, I'll get the parts whenever they arrive (the average Radio Shack doesn't stock much anymore, had to order) and we'll see if this works.

Oh, I see V(GS) threshold is actually spec'd at "min 2.0v max 4.0v". If 2.0v I guess I'll need a smaller resistor. Maybe I'll just use a trimmer.

[jyl]

Hey Henri, FYI we had talked in an earlier thread about camping in BC. I'm going to be camping in some provincial parks on Vancouver Island in July. That's what's motivating this cooler project.

[jyl]

I'm now reading about Peltiers - how they work, what the specs mean. Very interesting.

[Hendog]

If your thermistor value is to be around 27K, then move that fixed resistor down on the other side of that divider (on the pot side). Or you can eliminate the fixed resistor and get a 20K pot.

[Hendog]

I never heard of Peletiers before; you learn something every day!

We've not done much camping. We have all the gear but we have pets that make it hard to camp; two dogs (not to hard) and a parrot (hard to take camping). I want to see more of this province myself. I've been here 13 years. We bought a F150 so that we can tow a trailer and take the zoo, or just go rent a cottage somewhere and still take the zoo. This year we're spending two weeks at an ocean fron cottage near Powell River in August. Next year I'd like to pull a trailer and go up to Williams Lake, then head west through Tweedsmuir Provincial Park, Bella Coola, 12 hour ferry ride to Port hardy (on the island), down to Nanaimo and back to Vancouver. Might as well do this stuff while we're alive and well!

Provincial parks are great! You'll enjoy them!

[rick-l]

Originally Posted by Hendog

Try this.

Maybe not

download this http://www.linear.com/designtools/software/spice_models.jsp

This isn't that difficult

[Hendog]

Hey Rick, the current flowing through R1 (fan or coil) also flows through the drain. Does it matter then if R1 is on the source leg or drain leg?

[Hendog]

Ah, actually it is better to put the load in the Source leg as Rick suggests.

If you did it my way, the voltage difference between the drain and gate would vary which would then affect your turn on voltage (Gate bias), thus affecting your S-D current flow; unstable!

[rick-l]

Originally Posted by Hendog

Hey Rick, the current flowing through R1 (fan or coil) also flows through the drain. Does it matter then if R1 is on the source leg or drain leg?

Remember what turns it on is voltage from the gate to the source to get carriers in the channel.

For example if you put 3 volts on the gate referenced to ground and there is something in the source leg that raises the source voltage above ground.... Vgs goes down.

The other thing is you really need to add some hysteresis in the circuit to make sure that the mosfet is not in the active region very long or it sits and oscillates.

[rick-l]

note that the circuit above only dissipates significant power when it switches. You don't want to hold it there.

[jyl]

So does the polarity of V(GS) matter? I noticed that the original circuit I got off the web had negative (ground) connected to the source, while you guys are connecting positive to the source.

And, okay, I'm now reading that V(GD) matters, if it is in a certain range relative to V(GS) then the MOSFET is not truly "ON", it is in something called "triode" mode and the I(D) curve is sloped not flat. I found a cool little Java applet that helped me see this. I think that is what you guys are saying.

SPICE looks cool. I will try to learn it. Looking for a Mac-friendly version, ideally w/ a graphical or browser front end. My only PCs are work machines and they are locked down, can't install software.

I took a circuit design class in college (decades ago). Wow, there was no SPICE back then. Pencils and calculators.

[jyl]

Originally Posted by Hendog

I never heard of Peletiers before; you learn something every day!

We've not done much camping. We have all the gear but we have pets that make it hard to camp; two dogs (not to hard) and a parrot (hard to take camping). I want to see more of this province myself. I've been here 13 years. We bought a F150 so that we can tow a trailer and take the zoo, or just go rent a cottage somewhere and still take the zoo. This year we're spending two weeks at an ocean fron cottage near Powell River in August. Next year I'd like to pull a trailer and go up to Williams Lake, then head west through Tweedsmuir Provincial Park, Bella Coola, 12 hour ferry ride to Port hardy (on the island), down to Nanaimo and back to Vancouver. Might as well do this stuff while we're alive and well!

Provincial parks are great! You'll enjoy them!

Plan is, ferry from Port Angeles WA to Victoria then camp on West coast of island, later ferry to Southern Islands and camp awhile there. Taking the Westy and an extra kid, an exchange student from France. Should be fun. Especially if my beer is cold.