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Hysteresis needed in 741 Op-Amp circuit

R

Rich Grise

Jan 1, 1970
0
Terry Given said:
I printed it to a PDF - it will be useful in the future, as I often find
myself working with people who dont understand this sort of thing, and John
kindly saved me from writing it myself :)

- besides, it will frighten the hell out of my year 12 electronics
students

I _hope_ you don't mean 12 years _of_ electronics!!?!?!?!!

;-)
 
R

Rich Grise

Jan 1, 1970
0
John Fields said:
---
Check the same subject on seb.

What the **** is it with some of you clowns, anyway?

I do a guy a favor with a little tutorial to which he refuses to
respond and then I get flak from some fucking idiot about wasting
bandwidth, from you with your snide goddam "paper" bullshit, and from
Givens with his "I was gonna write the same thing myself, but"...

To hell with all of you niggardly sons of bitches.

Now _There's_ the John I've come to know and love! Didn't recognize
you there for a minute, John, but then, I didn't pick a fight this
time. :)

I picked my nit, and as usual, three posts later in the thread,
I found out it (my nitpick) was extraneous anyway, but the treatise
took more work than I'd have done, at least on thermistors. ;-)

Cheers!
Rich
 
T

Terry Given

Jan 1, 1970
0
Rich Grise said:
students

I _hope_ you don't mean 12 years _of_ electronics!!?!?!?!!

;-)

I'm way past that....:)

no, 6th form students at my local high school (2nd to last year of high
school - I cant remember what grade that is in USian). I teach electronics
there for free, for fun. turns out they were doing it by video conference,
with some twit who knows nothing (and kept making mistakes). I offered to
tutor them, and after a month the students asked if I could be their teacher
as they learned nothing from the VC class (1hr per week VC, 3 hrs left on
their own - no teacher, no resources, not even a room allocated for them -
no wonder they werent learning).

yesterday I chopped down a VERY large old pine tree - it had partly fallen
over years ago, and slowly descended onto one of my fences. So out came the
chain saw and a gang of mates who wanted the firewood. Accidentally cut into
a possum which was hiding in a hollow branch - oops, poor bugger. So I
grabbed it by the tail & brained it - I hate to see animals suffer.

cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
Rich Grise said:
I found myself trying to figure out what kind of transfer function
would be appropriate to map onto those 3 points ... since it's
essentially impossible to guess, I looked it up from your reference,
http://www.epcos.com/inf/50/db/ntc_02/01470178.pdf
and decided, yup, I never would have got that one, nosireebob!

Cheers!
Rich

I just recently completed an over-under temperature lockout for a Li-poly
battery charger, using a window comparator and an NTC thermistor as the
temperature sensor. The exponential curve fit is quite accurate, but if you
want a really scary equation, look up the Steinhart-Hart equation, its a lot
nastier.

Cheers
Terry
 
T

Terry Given

Jan 1, 1970
0
John Fields said:
---
Check the same subject on seb.

What the **** is it with some of you clowns, anyway?

I do a guy a favor with a little tutorial to which he refuses to
respond and then I get flak from some fucking idiot about wasting
bandwidth, from you with your snide goddam "paper" bullshit, and from
Givens with his "I was gonna write the same thing myself, but"...

To hell with all of you niggardly sons of bitches.

Sorry John, I was not trying to upset you - I thought it was an excellent,
well written dissertation - like I said, I have kept a copy. I WAS going to
write much the same sort of paper, as part of a circuit description for an
NTC temperature window comparator I just designed, but I preferred YOUR
paper, as it is an in-depth tutorial. my circuit description is now quite
terse, and refers to your paper :)

I dont care what question you were answering - your document is
self-supporting, lucid and coherent. Even if I had read none of your many
posts, I would conclude from this one alone that you both know what you are
doing, and are a good communicator.

I was perplexed by the bandwidth comment - clearly Baer didnt understand it,
perhaps thats why he got snippy (pun intentional).

Cheers
Terry
 
C

clive

Jan 1, 1970
0
Could you please explain what 'seb' is? Interesting post, thanks.

Thanks, Clive
 
J

John Fields

Jan 1, 1970
0
Could you please explain what 'seb' is? Interesting post, thanks.

---
I will if you'll please refrain from top posting...

It's sci.electronics.basics

Thanks,
 
B

budgie

Jan 1, 1970
0
---
Check the same subject on seb.

What the **** is it with some of you clowns, anyway?

I do a guy a favor with a little tutorial to which he refuses to
respond and then I get flak from some fucking idiot about wasting
bandwidth, from you with your snide goddam "paper" bullshit, and from
Givens with his "I was gonna write the same thing myself, but"...

As a fly on the wall so far in this thread, and as a retired (from full-time
employment) EE, I commend you for the effort you have put in to assist and
educate someone. Although I didn't see the original question/request in another
group, I appreciate your post and have learned from it. In fact, I'd be very
surprised if the majority of s.e.d. lurkers haven't.
To hell with all of you niggardly sons of bitches.

Just don't let them dampen your enthusiasm.

Peter
 
Y

YD

Jan 1, 1970
0
------------- SNIPped ------------
... a *VERY* long-winded paper that might be called a thesis (or
mini-novel?).
If you had a ?questiion? concerning hysteresis, it would have been
nice to make a *BRIEF* synopsis of the problem.
Else, do not waste bandwidth!

I liked it. A good exposition on how to tackle the problem. I put a
copy in the saved folder for future referral. Only one thing, where
does the 741 come into it?

- YD.
 
Y

YD

Jan 1, 1970
0
I sure did as with all your posts. Absolutely brilliant. I love these
exercises and I benefit from nearly all them. This NG is one of the
most valuable sources I have ever found. Many thanks to you and the
few other Titans in here that take the time to educate and advance the
art through your unselfish sharing of your knowledge.

Thanks again!

<bandwagon>Hear, hear!</bandwagon>

- YD.
 
J

John Fields

Jan 1, 1970
0
I liked it. A good exposition on how to tackle the problem. I put a
copy in the saved folder for future referral. Only one thing, where
does the 741 come into it?
 
F

Fred Bloggs

Jan 1, 1970
0
John said:
---
Check the same subject on seb.

What the **** is it with some of you clowns, anyway?

I do a guy a favor with a little tutorial to which he refuses to
respond and then I get flak from some fucking idiot about wasting
bandwidth, from you with your snide goddam "paper" bullshit, and from
Givens with his "I was gonna write the same thing myself, but"...

To hell with all of you niggardly sons of bitches.

It is possible to achieve some semblance of precision with the Radio
Shack parts. The OP's relay driver is garbage. An example of a 3oC
hysteresis t-stat shown below- make Rh adjustable for other -5% carbon
film is good enough:

View in a fixed-width font such as Courier.



RADIO SHACK THERMOSTAT 3oC HYSTERESIS


Rb
V+ >-/\/\--------+-----+------+
| | |
/ D --- /
R \ / Rthermistor
/ --- /
\ | \ LM741 V+
| | | |\ |
+---+ | +-----------|-\
| | | | | >-----+->Vout
| / Vz | / +---|+/ |
| R --- / Rpot | |/ | |
| / // \ / | --- |
| \ --- \ | /// |
Rb | | | | | |
+---/\/\-------+-----+------+ | |
| | | |
| | | Rh |
--- +------------------------+----/\/\----+
///



V+=12V Rb=220 Vz=1N4733A 5.1V D=1N914 R=10K


Temperature Hysteresis=3oC==>

for Ro=10K thermistor and R(35o)=6.8K and R(15o)=14.9K

BETA=4088 where R(T)=Ro*EXP(BETA*(1/T-1/To))

TCR R(T)= -0.39K/oC @25oC ( TCR=Ro * -BETA/T^2 )

-2.925V
TCR Vin(-)= ------- x TCR R(T), assuming zener +diode=5.85V
2 x R(T)

==> TCR Vin(-)= +57mV/oC or 170mV for 3oC hysteresis

6V
so that Rh= -------- = 180K for required hysteresis
170mV/5k

Actual Vhyst=160mV or 2.8oC at 25oC


At T=15oC TCR R(T)= -0.62K/oC so that Hysteresis= 2.6oC

At T=35oC TCR R(T)= -0.25K/oC so that Hysteresis= 3.0oC

T-Hyst=2.8oC +/-0.2oC 15oC < T < 35oC
 
F

Fred Bloggs

Jan 1, 1970
0
Fred Bloggs wrote:
[...snip...]

Continuing to fiddle with this thermostat- a circuit with unipolar
comparator feedback makes for independent trip point adjustment and
hysteresis. Since the 5.1V zeners come in a two-pack, the second one can
be used for a simple speed-up level shift and single transistor relay
driver- complete ckt sans bypass capacitors shown:

View in a fixed-width font such as Courier.



V+
V+ |
| +----+
/ / |
Rb 1K |
/ / |
\ \ e
| 680 | |/
+-----+------+ +--/\/\-+--| 2N2907A
| | | | |\
/ | / | c
R | Rthermistor Vz | |
/ | / --- / |
\ Vz | \ LM741 V+ // \ |
| --- / | |\ | --- |
+---+ // \ +-----------|-\ | |
| | --- | | >------+ +---------+
| / | / +---|+/ | | |
| R | Rpot | |/ | | +-+-+ |
| / | / | --- | relay | ) | |
| \ | \ | /// | 360R | ) | D ---
| | | | | | coil | ) | / \
| +-----+------+ | | | ) | ---
| | | | +-+-+ |
| | | Rh D | | |
+------------------------+-/\/\--|>|---+ +---------+
Rb | |
+----/\/\--+ ---
| ///
---
///


V+=12V Rb=220 Vz=1N4733A 5.1V D=1N914 R=10K Rh=130K

Thermistor Rnom @25oC=10K

Rpot adjusted= Thermistor R(TRIP_TEMP)

Relay actuated at TEMP= TRIP_TEMP


ON + +---<----+------
| | |
| | |
RELAY | |<- 3o ->|
STATE | | |
| | |
OFF +------+--->----+------
^
TRIP_TEMP


- Vz
TC Vin(-)= -------- x TCR(T) (=thermistor TC)
4 x R(T)


V = T x TCVin(-)
hyst hyst


(V+)/2-1.4V-VD R
Rh= ( -------------- - 1 ) x -
Vhyst 2


TRIP_TEMP adjustment and TEMP hysteresis are independent.
 
T

Terry Given

Jan 1, 1970
0
Fred Bloggs said:
Fred Bloggs wrote:
[...snip...]

Continuing to fiddle with this thermostat- a circuit with unipolar
comparator feedback makes for independent trip point adjustment and
hysteresis. Since the 5.1V zeners come in a two-pack, the second one can
be used for a simple speed-up level shift and single transistor relay
driver- complete ckt sans bypass capacitors shown:

View in a fixed-width font such as Courier.



V+
V+ |
| +----+
/ / |
Rb 1K |
/ / |
\ \ e
| 680 | |/
+-----+------+ +--/\/\-+--| 2N2907A
| | | | |\
/ | / | c
R | Rthermistor Vz | |
/ | / --- / |
\ Vz | \ LM741 V+ // \ |
| --- / | |\ | --- |
+---+ // \ +-----------|-\ | |
| | --- | | >------+ +---------+
| / | / +---|+/ | | |
| R | Rpot | |/ | | +-+-+ |
| / | / | --- | relay | ) | |
| \ | \ | /// | 360R | ) | D ---
| | | | | | coil | ) | / \
| +-----+------+ | | | ) | ---
| | | | +-+-+ |
| | | Rh D | | |
+------------------------+-/\/\--|>|---+ +---------+
Rb | |
+----/\/\--+ ---
| ///
---
///


V+=12V Rb=220 Vz=1N4733A 5.1V D=1N914 R=10K Rh=130K

Thermistor Rnom @25oC=10K

Rpot adjusted= Thermistor R(TRIP_TEMP)

Relay actuated at TEMP= TRIP_TEMP


ON + +---<----+------
| | |
| | |
RELAY | |<- 3o ->|
STATE | | |
| | |
OFF +------+--->----+------
^
TRIP_TEMP


- Vz
TC Vin(-)= -------- x TCR(T) (=thermistor TC)
4 x R(T)


V = T x TCVin(-)
hyst hyst


(V+)/2-1.4V-VD R
Rh= ( -------------- - 1 ) x -
Vhyst 2


TRIP_TEMP adjustment and TEMP hysteresis are independent.

clever bugger, eh wot.

cheers
Terry
 
F

Fred Bloggs

Jan 1, 1970
0
I can see where that 741 output can float down without kicking over- so
a small amount of bipolar feedback will still be necessary- maybe 0.05oC
off the trip-point adjust:

View in a fixed-width font such as Courier.


V+
V+ |
| +----+
/ / |
Rb 1K |
/ / |
\ \ e
| 680 | |/
+-----+------+ +--/\/\-+--| 2N2907A
| | | | |\
/ | / | c
R | Rthermistor Vz | |
/ | / --- / |
\ Vz | \ LM741 V+ // \ |
| --- / | |\ | --- |
+---+ // \ +-----------|-\ | |
| | --- | | >------+ +---------+
| / | / +------|+/ | | |
| R | Rpot | |/ | | +-+-+ |
| / | / | --- | relay | ) | |
| \ | \ | /// | 360R | ) | D ---
| | | | | | coil | ) | / \
| +-----+------+ | | | ) | ---
| | | | +-+-+ |
| | | Rh D | | |
+---------------------+----/\/\---|>|--+ +---------+
Rb | | | |
+----/\/\--+ | 10M | ---
| +------/\/\------+ ///
 
T

Terry Given

Jan 1, 1970
0
Fred Bloggs said:
I can see where that 741 output can float down without kicking over- so
a small amount of bipolar feedback will still be necessary- maybe 0.05oC
off the trip-point adjust:

Im impressed - IMO one of, if not THE, most important thing to do with a
design, is rigorously ask the question "how can this screw up, why, and what
can be done about it"

alas many do not do this; many take such actions as personal attacks, and
get all shitty. Me, I am HAPPY when someone points out a problem - I just
learned something, and the circuit just got better.

cheers
Terry
 
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