temperature controller ?

C

Chan

Guest
Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

Thanks
 
"Chan" <dsadsdddads@btinternet.com> wrote in message
news:bsnf2u$ra5$1@titan.btinternet.com...
Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

Thanks
Click to expand...
The ratio of R6/R8 defines the amplifier gain. If the circuit gain is too
low, there will be a large hysteresis around the balance point of the
temperature bridge input. If the the gain is too high, the relay will tend
to chatter around balance.
 
On Sun, 28 Dec 2003 20:39:59 +0000 (UTC), "Chan"
<dsadsdddads@btinternet.com> wrote:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?
Click to expand...
---
It's 'jitter'.
It doesn't, and can actually add to it.

The circuit is shown incorrectly in that the end of R6 going to IC1-2
should be going to IC1-3.

In this application the 741 is being used as a voltage comparator and
for a slowly-changing signal like the one which will appear at the +
input, when the signal gets close to the switching point (the voltage on
the - input) the slightest noise on the signal coming from the
thermistor can cause the output of the 741 to chatter wildly between its
full-on and full-off states, easily causing the relay to chatter.

To get rid of this very real problem, the output of the 741 is coupled
back to the + input through a rather large-valued resistor so that when
the output goes high it forces a tiny extra current through the
thermistor, which forces the voltage on the + input to go a little bit
higher than it would if the resistor wasn't there. This little increase
in voltage causes the + input to quickly 'step over' any noise that may
be on the input line, forcing the output to stay high. When the
temperature rises, the same thing happens in reverse and the output's
going low forces the + input through the noise threshold, keeping the
output low and the relay off. This action is called 'hysteresis' and is
almost universally used in circuits like this.

Some other things which wouldn't hurt to do would be to put an
electrolytic and a ceramic cap across the Zener to minimize noise on the
Zener supply as well as from IC1-2 to ground in order to minimize on
the - input, which is just as bad as noise on the +input. I'd use about
a 10ľF electrolytic paralleled with a 0.1ľF ceramic in both places.

--
John Fields
 
"Chan" <dsadsdddads@btinternet.com> wrote in news:bsnf2u$ra5$1
@titan.btinternet.com:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

Thanks
Click to expand...
I'm sorry - this is probably a really basic question [but hey we're in SEB
so it's OK :) ] But what is RL1? I'm not familiar with that term. Thanks,

Michael
 
On Sun, 28 Dec 2003 23:18:36 GMT, Michael
<nleahcimathotmaildotcom@nowhere.com> wrote:

"Chan" <dsadsdddads@btinternet.com> wrote in news:bsnf2u$ra5$1
@titan.btinternet.com:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

Thanks



I'm sorry - this is probably a really basic question [but hey we're in SEB
so it's OK :) ] But what is RL1? I'm not familiar with that term. Thanks,
Click to expand...
---
It's a relay coil.

--
John Fields
 
John Fields <jfields@austininstruments.com> wrote in
news:lhpuuvs00g4plr28fugue5rre8p26mid3p@4ax.com:

On Sun, 28 Dec 2003 23:18:36 GMT, Michael
nleahcimathotmaildotcom@nowhere.com> wrote:

"Chan" <dsadsdddads@btinternet.com> wrote in news:bsnf2u$ra5$1
@titan.btinternet.com:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

Thanks



I'm sorry - this is probably a really basic question [but hey we're in
SEB so it's OK :) ] But what is RL1? I'm not familiar with that term.
Thanks,

---
It's a relay coil.
Click to expand...
oooooohhhhhhh... It all makes so much more sense now... thanks :)
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:7giuuvog29odug8lr8fb2oef7hkd0ckkud@4ax.com...
On Sun, 28 Dec 2003 20:39:59 +0000 (UTC), "Chan"
dsadsdddads@btinternet.com> wrote:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

---
It's 'jitter'.
It doesn't, and can actually add to it.

The circuit is shown incorrectly in that the end of R6 going to IC1-2
should be going to IC1-3.

In this application the 741 is being used as a voltage comparator and
for a slowly-changing signal like the one which will appear at the +
input, when the signal gets close to the switching point (the voltage on
the - input) the slightest noise on the signal coming from the
thermistor can cause the output of the 741 to chatter wildly between its
full-on and full-off states, easily causing the relay to chatter.

To get rid of this very real problem, the output of the 741 is coupled
back to the + input through a rather large-valued resistor so that when
the output goes high it forces a tiny extra current through the
thermistor, which forces the voltage on the + input to go a little bit
higher than it would if the resistor wasn't there. This little increase
in voltage causes the + input to quickly 'step over' any noise that may
be on the input line, forcing the output to stay high. When the
temperature rises, the same thing happens in reverse and the output's
going low forces the + input through the noise threshold, keeping the
output low and the relay off. This action is called 'hysteresis' and is
almost universally used in circuits like this.

Some other things which wouldn't hurt to do would be to put an
electrolytic and a ceramic cap across the Zener to minimize noise on the
Zener supply as well as from IC1-2 to ground in order to minimize on
the - input, which is just as bad as noise on the +input. I'd use about
a 10ľF electrolytic paralleled with a 0.1ľF ceramic in both places.

--
John Fields
Click to expand...
Excellent, thank you !!!
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:7giuuvog29odug8lr8fb2oef7hkd0ckkud@4ax.com...
On Sun, 28 Dec 2003 20:39:59 +0000 (UTC), "Chan"
dsadsdddads@btinternet.com> wrote:

Hello all,
I have a question regarding the following circuit

http://www.geocities.com/x_file_space/tcontroler.jpg

How does R6 prevent gitter ?

---
It's 'jitter'.
It doesn't, and can actually add to it.

The circuit is shown incorrectly in that the end of R6 going to IC1-2
should be going to IC1-3.

In this application the 741 is being used as a voltage comparator and
for a slowly-changing signal like the one which will appear at the +
input, when the signal gets close to the switching point (the voltage on
the - input) the slightest noise on the signal coming from the
thermistor can cause the output of the 741 to chatter wildly between its
full-on and full-off states, easily causing the relay to chatter.

To get rid of this very real problem, the output of the 741 is coupled
back to the + input through a rather large-valued resistor so that when
the output goes high it forces a tiny extra current through the
thermistor, which forces the voltage on the + input to go a little bit
higher than it would if the resistor wasn't there. This little increase
in voltage causes the + input to quickly 'step over' any noise that may
be on the input line, forcing the output to stay high. When the
temperature rises, the same thing happens in reverse and the output's
going low forces the + input through the noise threshold, keeping the
output low and the relay off. This action is called 'hysteresis' and is
almost universally used in circuits like this.

Some other things which wouldn't hurt to do would be to put an
electrolytic and a ceramic cap across the Zener to minimize noise on the
Zener supply as well as from IC1-2 to ground in order to minimize on
the - input, which is just as bad as noise on the +input. I'd use about
a 10ľF electrolytic paralleled with a 0.1ľF ceramic in both places.

--
John Fields
Click to expand...
YIKES! I'm going to have to look at schematics more carefully before
commenting in the future. Do I hear a New Year's resolution in the making?
 
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