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From: "Frank Bemelman"
Subject: Re: Using The Parts You Have
Date: Tue, 10 Sep 2002 22:29:01 +0200
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Organization: EuroNet Internet
NNTP-Posting-Date: 10 Sep 2002 20:29:09 GMT
"Dr. Ed Edmondson" schreef in bericht
> I have some LM358 op amps and some NTC 10,000 ohm (at 25 degrees C)
> thermistors. I am trying to build a small thermal controller and am
> lost. I scoured the Net for a circuit which would use what I have on hand
> have found nothing which might work.
> I saw a circuit which might work but I don't seem to get the results I am
> looking for. I used one half of the 358 and connected Vcc (pin 8) to plus
> volts and the ground (pin 4) to ground. I connected the plus input (pin 3)
> through a 10 meg ohm resistor to the output (pin 1). I then created a
> divider made of two 10,000 ohm resistors with a 10,000 ohm pot in the
> connected the wiper of the pot to the plus input (pin 3) and each end of
> series string to plus 5 volts and ground. I then connected the 10,000 ohm
> thermistor in series with a 10,000 ohm resistor. I connected the junction
> between the resistor and thermistor to the minus input (pin 2) on the op
> The thermistor end to ground and the resistor end to plus 5 volts.
> By turning the pot I can turn the output on and off but when I heat or
> thermistor I get no output response. Exactly what am I doing wrong? All I
> is to turn a relay on (resistive load) when the temperature falls below 20
> degrees C and turn off the relay when the temperature reaches about 30
> C. I wanted to use a pot to set the trip point to turn on the relay and
> thermistor change to turn the relay off.
> What am I doing wrong? From what I have read an op amp (I have these)
> work as a comparator (I do't have these).
It seems that your circuit is okay. You could remove the 10M resistor, to
remove the hysteresis, see if that makes a change. Also measure the voltage
at the potentiometer (should be about 1/2 supply voltage at the trip point)
and measure the junction of the thermistor/resistor, should also be at 1/2
supply voltage, at room temperature. Check, check & double check.
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