Simple (I think) transistor question

this:

R3/

Just killfile "it". I did so about a week ago. ...Jim Thompson

--
                  [On the Road, in New York]

| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.     Sometimes I even put it in the food.

Of course. It is a Spice directive to cause a step in temperature using the following list of temperatures in degrees C. Another way of doing it without the "list" word is to specify the temperature min, max, delta T as in .step temp -55 55 10 which will cause 10 plots over 100 C.

In this circuit, I think only the characteristics of the transistor change. There are ways to change the other components as well, as I recall.

John S

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Apology accepted. :-)

That circuit lacks what is called dynamic range which describes the range of thermistor resistance it is capable of handling. Adding transistor amplification only makes matters worse, much worse. And this especially makes no sense when you consider the resolution of even a weak PIC A/D at 8 bits of 20mV. What good is the 5V input clamp if the PIC no longer sees the circuit response?

Using the basic thermistor equation R=3DRo x exp[beta x ( 1/T- 1/To)] and plugging in some real world beta of 3468 for a 750R at 25oC and allowing the thermistor core temperature to vary over 0-100 oC gives a resistance range of approximately 80- 2500 ohms. The nice thing about a simple resistance voltage divider is that the thermistor power dissipation increases substantially with temperature, then since the temperature differential between the airflow cooled and uncooled thermistor scales directly with power dissipation, this compensates for the otherwise flattening of thermistor resistance versus temperature slope at higher temperatures, keeping the voltage divider differentail well above the A/D resolution by something like a factor ten. So the simplest approach for the values given as an example is a simple voltage divider like so: Please view in a fixed-width font such as Courier.

. . . . . . 12V . | . | . | . T . H_ . E/| . /R . M . | . | . | . +--------> A/D . | . | . | . [56R,1W] . | . | . --- . . .

So change the value of R4 to get the appropriate ratio. There's no reason to stick in a whole nother regulator just to power a thermistor.

Hope This Helps! Rich

As your using a micro anyway you can simplify the hardware at the expence of a little software. Connect the thermister and equal resistor in series across the micro supply, thermistor in lower half. Connect ad/digital port input to junction. Set port low allow thermister to reach ambient. Switch to hi z, read voltage. Set port high, heat thermister. set high z read voltage, set low, subtract voltages to get result, ie. the smaller the difference the faster the airflow. For a match head size thermister 0.5 sec period should be enough. This method also has the advantage of ambient temperature compensation.

I'm certainly glad, John, that you "seem interested in learning"...

Bwahahahahahahahahaha ;-) ...Jim Thompson

-- [On the Road, in New York]

| James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

a 3/

In response to you, Rich and cbarn specifically, this did cause me to go back and make sure I didn't overthink this. Putting a low value like 56 in place of R4 seems like a good idea, but this will cause the thermistor to dissipate lots of power. It's not very big and I don't feel very comfortable making it this hot. Making R4 large enough will cause the voltage output to be too high. Powering it from 5v won't make it warm enough. Short of obtaining a different thermistor, I see no other way out.

this:

Have you considered using a pizo type circuit for wind detection or does this have to be very small? You can use a pizo buzzer for the element.

Jamie

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Do have any data for the thermistor or do you know the manufacturer
and part number?

n a

R3/

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Using your original voltage divider, you could split it up like so to get approximately the same effect.

Please view in a fixed-width font such as Courier.

. . . . . +12v . | . | . [270] . | . | . [THERM] . | . | . +--------> A/D . | . [220] . | . | . | . --- . . . . . . . .

n a

R3/

w /

When I did this the thermister didnt get noticably warm at all, it still worked very well though.

To Jamie, I did contemplate using a microphone, not quite the same thing, but similar idea. But yes, the piezo element setup will be too big and too fragile. Good idea though.

John, I put the @25C resistance of 750 into Digikey and there's only one it could be. It seems to match the physical description too. I got mine from All Electronics, but based on what Digikey brought up I believe it's a GE RL2005-434-95-K.

Surprisingly it has a max temperature of 150C (ouch). I'm going to try two things later: run it off 5V in series with lower resistance, and see if it heats up enough, and also try it at 12V as Fred suggests. According to my calculations the thermistor is dissipating about 70mW at steady-state, so it might work at 40mW or so when running off 5V. However, on 12V it's looking more like 500mW. I'm not too comfortable with how hot I'm assuming it'll get. It might even exceed 150C in that case. Also not too safe for little kids (she'll want to play with it and I want her to be able to).

Oh, how complicated "simple" projects become! I could just build it as originally planned but I also want to make sure I've considered everything.

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For that particular component, something like this will get you an uncooled component temperature differential of 10oC or less above ambient over the range 10-38 oC, device dissipation less than 65mW. Please view in a fixed-width font such as Courier.

. . . . 12V >---[180R, 1/4W]----. | . T . H . R GE 2005-434-95-K . M . S . T . R . | . A/D ------------------' . .

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Ok, I've tried powering the thermistor from 5V, and there's no way it'll get warm enough. I've also tried the circuit you've suggested with various values for each resistor (mostly using Excel and knowing what the approx thermistor R spread will be) and it would work, but the voltage output delta isn't enough. I also have to worry about exceeding 5V to the micro. I think I'm sticking with the transistor circuit. It's impossible to exceed 5V to the micro A/D, and I can use

2.5V for the A/D reference to increase the A/D deflection since the transistor output doesn't go that high once settled. t Once I get this built I'll put it on my website. It's definitely one of my simplest projects but since I insist on PCBs and enclosures for practically everything, laying that out and fabricating it will take a little longer.