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temperature controller ?

B

Baphomet

Jan 1, 1970
0
Chan said:
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

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.
 
J

John Fields

Jan 1, 1970
0
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 10µF electrolytic paralleled with a 0.1µF ceramic in both places.
 
C

Chan

Jan 1, 1970
0
John Fields said:
---
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 10µF electrolytic paralleled with a 0.1µF ceramic in both places.

Excellent, thank you !!!
 
B

Baphomet

Jan 1, 1970
0
John Fields said:
---
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 10µF electrolytic paralleled with a 0.1µF ceramic in both places.

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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