Re: op amp output

On Sat, 16 Apr 2005 07:03:23 +0000 (UTC), "frogfot"
<frogfot@xxxxxxxxxxx> wrote:


>John Fields wrote:
>>You don't need Q1 and, assuming that your thermistor has a resistance
>>of around 1000 ohms at your setpoint, this should work:
>>...
>>...
>
>Mine NTC is 10k at 25*C and 500 at 100*C. I don't understand why you've
>used an MOSFET (I'm more used to transistors :P ).. and what's the
>purpose of Z15V, is it a zener diode?

---
I used a MOSFET because the gate doesn't draw any current, so you
don't have to worry about the comparator output going out of
saturation when it's sinking the transistor's base current. If you
used a transistor you'd want to force beta to about 10, which means
that for a relay with a 100mA coil the base current would need to be
10mA, which could pull the LM319 output out of saturation. The Zener
(15V) is to make sure that any spikes which appear on the MOSFET's
gate will be clamped to 15V in order to keep the MOSFET from getting
damaged.
---

>I was going to use this circuit to control temp in a water-bath at
>0-100*C.

---
If you mean from 25 to 100°C, then I suggest you do something like
this:

+12V>-----+-------+---------+--------+----+-----+
| | | | | |
|R1 |R2 | [10k] |K | O------C
[10k] [10k] | | [CR1] [COIL]- - -|
| | +---|--[1M]--+ | | O--> |<--O--NC
| | | | | +-----+ |
| | | | | | +-----------NO
+---+---------+--|+\ | D
| | |R3 | >------+--+-----G 2N7000
[RTH] | [10k]<--+--|-/ LM393 | S
| | | | | | |
| | +-----+ | |K |
| | | | | [Z15V] |
| | [R4] | | | |
| | | | | | |
|[0.1µF]| [0.1µF]| | |
| | | | | | |
GND>------+---+---+-----+---+-----------+-------+

Notice that I changed the comparator to an LM393, which is a better
comparator for this application, I believe. (and cheaper,too) The
value of R4 is going to depend on the the end point resistance of R3
and should be set so that with the pot (R3) rotated fully toward GND
the voltage on the comparator - input is the same as the voltage on
the + input with the water bath at 100°C. Probably close to 500 ohms,
the 100°C resistance of the thermistor.

--
John Fields
Professional Circuit Designer
.

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