The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: John Popelish
Organization: This space not available for advertising.
X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Using The Parts You Have
Date: Wed, 11 Sep 2002 01:17:03 GMT
NNTP-Posting-Date: Tue, 10 Sep 2002 21:17:03 EDT
"Dr. Ed Edmondson" wrote:
> 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 totally
> lost. I scoured the Net for a circuit which would use what I have on hand and
> 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 5
> 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 voltage
> divider made of two 10,000 ohm resistors with a 10,000 ohm pot in the middle. I
> connected the wiper of the pot to the plus input (pin 3) and each end of the
> 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 amp.
> 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 cool the
> thermistor I get no output response. Exactly what am I doing wrong? All I want
> 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 degrees
> C. I wanted to use a pot to set the trip point to turn on the relay and the
> thermistor change to turn the relay off.
> What am I doing wrong? From what I have read an op amp (I have these) would
> work as a comparator (I do't have these).
I think I follow your description, and it sounds good. Before you
worry about the whole circuit, you should be able to test the
thermistor by looking at the voltage from the divider. Does it change
as you heat or cool the thermistor? After all, the opamp is just a
high gain amplifier of this voltage change.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup