# Hysteresis needed in 741 Op-Amp circuit

C

#### CF

Jan 1, 1970
0
Well, I did end up putting a voltage regulator in there, though I went for
a 7812 1 amp unit because of the fan that I want to power too. It seems to
make the relay very jumpy when it's switching off. Something about the
addition of the 7812 seems to sensitize the circuit more. I can tune it out
a bit with more feedback, but this widens my hysteresis zone beyond 1
degree C, which I don't really want. So I'll ditch it and go for resistors
on the relay coil and fan circuits, which is neat enough anyway. By my
measurment, the 7812 draws 3.7mA just to power itself, so I'm not overly in
love with it (speaking in perfectionistic terms, for the sake of it).

Measurements are as follows:

------------------

AC from transformer: 14.75V

DC with no load from fan: 17.9V

DC with fan drawing: 16.0V

Current of entire circuit, quiescent: 2.3mA AC, 1.9mA DC

Current of entire circuit, relay coil on: 66mA AC, 57mA DC

Current of entire circuit, relay & fan: 236mA AC, 220mA DC

Casualties: One 0.5A multimeter fuse, 6km round-trip

-----------------

Voltage at all times: 11.9V DC

Current of entire circuit, quiescent: 5.0mA DC

Current of circuit minus 7812, quiescent: 1.3mA DC

Current of entire circuit, relay coil on: 9.5mA DC (strange!)

Current of entire circuit, relay & fan: 171mA DC

Casualties: Another 0.5A fuse, another 6km round-trip tomorrow.

CF

J

#### John Fields

Jan 1, 1970
0
John, this is outstanding stuff. I think what I'll end up doing is letting
the bulk of the circuit run at the higher voltage, which is within range,
as you point out, for the 311 and even the current 741. I'll just drop the
voltage for the relay coil, which is what I'm most concerned about. This
way when the relay is off (most of the time), there's no extra component
making unwanted heat, etc.

---
Something I should have pointed out, (but didn't, because I didn't
think about it) was that the LM311 (or your 741, for that matter)
_can't_ run on unfiltered DC, since it'll get unhappy once the output
from the rectifier falls below its minimum allowed positive supply
voltage, duhh...
---
My power supply circuit does use a full-wave bridge rectifier, but the
transformer is actually putting out 14.75 volts AC according to my latest
reading on my (non-true-RMS) multimeter. The reservoir cap is 2200uF. When
I spoke of the voltage under load, I was actually including a small fan
too, rated 12v, 0.11A. The transformer is not marked, but looks like a
typical 150-ish mW unit.

---
Milli_watts_? Surely you meant milli_amps_ of secondary current,
which would make the transformer rated for something like 12V * 150mA
= 1.8VA. Plus, there's the cost of running the capacitive input
filter, which can run up to about 60% of the load, which means you
should be using about a 3VA transformer.

With a 2200µF cap and a 160mA load (the relay _plus_ the fan) you can
expect a ripple voltage across the cap of about +/-0.35V on top of the
DC out of the filter.
---
Talk about electronic detective work! I can't believe you could deduce so
much from so little info, though the quality of the info was bad.

---
Yes.

A good idea, when you're asking questions, is to include as much
information as possible about what your goal is, what you've done, so
far, to get there, and what you've measured in order that as little
ground as possible has to be covered more than once. That saves us
_all_ time, effort, and heartburn.

J

#### John Fields

Jan 1, 1970
0
On Thu, 17 Jun 2004 14:11:20 -0500, John Fields
---
Oops... I forgot about the hysteresis. You'll have to refigure it
because of the increase in the relay driver's peak voltage, and you'll
need to add a cap to the comparator's output to smooth the 120Hz
pulses when the relay isn't turned on. Something like this this:

12VAC>-----------------------+
|
12VAC>--------------------+ |
| |
+-[1N4002>|]--+--|--[1N4002>|]-+
| | |
+-[1N4002>|]-----+--[1N4002>|]-+
| |
GND>--+-----+---[C1+]--------+--[|<1N4002]-+
| | |
| IN |
+-------------------[78L12] |
| OUT |
| | |
| | |
| | |
| +--------+--------+ +-------+
| | | | |K | O---> |
| [R1] [10.7K] | [1N4001] [COIL]- -|- - -|
| | | | | | | O--->COM
| | +-[634K]-------+-------+ |
| | | | | +--------->NO
| | | +------+ |
| | +----|+ C|----------+
| | | |LM311 | |
| [1000]<-----|----|- E|--+ |
| | | +------+ | |+
| | | | | [10/35]
| [R2] [RT1] | | |
| | | | | |
+----+--------+--------+-----+-------+

OK John, very nice. Any special type of capacitor? I presume I'd be getting
100Hz pulses where I live (50Hz mains).

---
Since the side of the 634k resistor connected to the thermistor will
be sitting at about 6V and the other side of it will be connected to
essentially the unloaded peak voltage out of the bridge when the relay
is off, (19V, I think you said) the load on the cap will be about 20µA
so, at 100Hz, the ripple voltage across it will be about 21mV, which
shouldn't interfere seriously with your hysteresis. Also, since the
impedance of the source charging the cap is negligible in this
instance, you could use a vanilla aluminum electrolytic. Any 10µF cap
with a voltage rating greater than the unloaded output voltage from
the bridge would work. 25V would probably be OK, but for a few
pennies more I'd use 35V just to be safe.

J

#### John Fields

Jan 1, 1970
0
Well, I did end up putting a voltage regulator in there, though I went for
a 7812 1 amp unit because of the fan that I want to power too. It seems to
make the relay very jumpy when it's switching off. Something about the
addition of the 7812 seems to sensitize the circuit more. I can tune it out
a bit with more feedback, but this widens my hysteresis zone beyond 1
degree C, which I don't really want. So I'll ditch it and go for resistors
on the relay coil and fan circuits, which is neat enough anyway. By my
measurment, the 7812 draws 3.7mA just to power itself, so I'm not overly in
love with it (speaking in perfectionistic terms, for the sake of it).

Measurements are as follows:

------------------

AC from transformer: 14.75V

DC with no load from fan: 17.9V

DC with fan drawing: 16.0V

Current of entire circuit, quiescent: 2.3mA AC, 1.9mA DC

Current of entire circuit, relay coil on: 66mA AC, 57mA DC

Current of entire circuit, relay & fan: 236mA AC, 220mA DC

Casualties: One 0.5A multimeter fuse, 6km round-trip

-----------------

Voltage at all times: 11.9V DC

Current of entire circuit, quiescent: 5.0mA DC

Current of circuit minus 7812, quiescent: 1.3mA DC

Current of entire circuit, relay coil on: 9.5mA DC (strange!)

Current of entire circuit, relay & fan: 171mA DC

Casualties: Another 0.5A fuse, another 6km round-trip tomorrow.

---
Buy a box of 'em and save yourself a lot of trips!^)

Offhand, I'd say that if you measured 9.5ma with the relay on, you
either measured the current incorrectly or you didn't install the 7812
properly. Moreover, you should check to see whether the fan can run
of on filtered DC) since if it will that'll save you an electrolytic
cap and take an additional load off of the transformer, since it won't
have to charge that cap if it isn't there.

Assuming that your fan can run on rectified, unsmoothed DC (I just
checked three different brushless DC fans and they all ran just fine,
so my guess is that it can) and that the fan will run when the relay's
on, your circuit (my LM311 version, I don't even want to mess around
with the 741 stuff) should look like this:

12VAC>-----------------------+
|
12VAC>--------------------+ |
| |
+-[1N4002>|]--+--|--[1N4002>|]-+
| | |
+-[1N4002>|]-----+--[1N4002>|]-+
| |
GND>--+-----+---[C1+]--------+--[|<1N4002]-+
| | |
| IN +------+
+-------------------[78L12] | |
| OUT | |
| | [R3] [R4]
| | | |
| | | |
| +--------+--------+ +-------+ |
| | | | |K | O---> |
| [R1] [10.7K] | [1N4001] [COIL]- - - - -|
| | | | | | O
| | +-[634K]-------+-------+ |
| | | | | |
| | | +------+ | |
| | +----|+ C|----------+ |
| | | |LM311 | | [FAN]
| [1000]<-----|----|- E|--+ | |
| | | +------+ | |+ |
| | | | | [10µF] |
| [R2] [RT1] | | | |
| | | | | | |
+----+--------+--------+-----+-------+------------+

If you're worried about burning up the fan or the relay, measure the
DC current into them (separately) and then adjust R3 and R4 until you
get their rated currents into them, then that should be it.

C

#### CF

Jan 1, 1970
0
Buy a box of 'em and save yourself a lot of trips!^)

Good idea!

Offhand, I'd say that if you measured 9.5ma with the relay on, you
either measured the current incorrectly or you didn't install the 7812
properly.

Some strange thing with the multimeter I think. Cosmic rays maybe.
Moreover, you should check to see whether the fan can run
of on filtered DC) since if it will that'll save you an electrolytic
cap and take an additional load off of the transformer, since it won't
have to charge that cap if it isn't there.

Assuming that your fan can run on rectified, unsmoothed DC (I just
checked three different brushless DC fans and they all ran just fine,
so my guess is that it can) and that the fan will run when the relay's
on, your circuit (my LM311 version, I don't even want to mess around
with the 741 stuff) should look like this:

12VAC>-----------------------+
|
12VAC>--------------------+ |
| |
+-[1N4002>|]--+--|--[1N4002>|]-+
| | |
+-[1N4002>|]-----+--[1N4002>|]-+
| |
GND>--+-----+---[C1+]--------+--[|<1N4002]-+
| | |
| IN +------+
+-------------------[78L12] | |
| OUT | |
| | [R3] [R4]
| | | |
| | | |
| +--------+--------+ +-------+ |
| | | | |K | O---> |
| [R1] [10.7K] | [1N4001] [COIL]- - - - -|
| | | | | | O
| | +-[634K]-------+-------+ |
| | | | | |
| | | +------+ | |
| | +----|+ C|----------+ |
| | | |LM311 | | [FAN]
| [1000]<-----|----|- E|--+ | |
| | | +------+ | |+ |
| | | | | [10µF] |
| [R2] [RT1] | | | |
| | | | | | |
+----+--------+--------+-----+-------+------------+

If you're worried about burning up the fan or the relay, measure the
DC current into them (separately) and then adjust R3 and R4 until you
get their rated currents into them, then that should be it.

Excellent John, thanks for this. I didn't realize I had all those options
of feeding different DC here and there. For the sake of simplicity at the
moment, I've just used the smoothed, rectified DC everywhere, but may
fiddle with it later, depending on whether I switch transformers, etc.

Today I've taken the plunge and wired the 311 in. It works! My circuit is
slightly different to the above, but at least it's going. I can refine it
later. I've got resistors in line with the relay and fan, so things are
nice there.

Milli_watts_? Surely you meant milli_amps_ of secondary current,
which would make the transformer rated for something like 12V * 150mA
= 1.8VA. Plus, there's the cost of running the capacitive input
filter, which can run up to about 60% of the load, which means you
should be using about a 3VA transformer.

Yes, typo. The total AC in now is around 180mA with the fan running. What
are the likely consequences of using a 150mA transformer (assuming that's
what it is)?

I've quickly posted my circuit as wired at the following page, less the
resistors mentioned above that are peculiar to my situation:

http://www.copperleife.com/craig/tech/thermo/final.htm

Kind regards,

CF

J

#### John Fields

Jan 1, 1970
0
I've quickly posted my circuit as wired at the following page, less the
resistors mentioned above that are peculiar to my situation:

http://www.copperleife.com/craig/tech/thermo/final.htm

---
The 1N4148 you've shown connected from the bottom of the relay coil to
ground isn't doing anything useful and could allow the 311's output
stage to be damaged/destroyed by the relay's turn-off transient. To
suppress the transient, that diode should be a Zener rated at a
voltage higher than the supply voltage, but less than the the maximum
voltage allowed across the 311's output. 25V should be fine. The
other (possibly more convenient) way to do it is to connect the 1N4148
across the coil with the diode's cathode connected to the supply.

C

#### CF

Jan 1, 1970
0
The 1N4148 you've shown connected from the bottom of the relay coil to
ground isn't doing anything useful and could allow the 311's output
stage to be damaged/destroyed by the relay's turn-off transient. To
suppress the transient, that diode should be a Zener rated at a
voltage higher than the supply voltage, but less than the the maximum
voltage allowed across the 311's output. 25V should be fine. The
other (possibly more convenient) way to do it is to connect the 1N4148
across the coil with the diode's cathode connected to the supply.

Ah yes, how silly of me. I was scratching my head about that one trying to
understand how it could possibly function in the same way as a diode across
the coil. I see now that it was the zener side of things that was doing the
work in that configuration. Thanks for putting me straight. I've fixed both
the diagram and real-life circuit.

Thanks for the capacitor info too.

CF

C

#### CF

Jan 1, 1970
0
Well, I want to follow-up for the record...

I've committed heresy and reverted back to the LM741 op amp version of the
circuit! I found that for my application, with a remotely-mounted
thermistor and temperature dial, perfboard construction, and wires going
everywhere (including mains), the 311 was just too prone to stray
oscillations. A fine comparator it is, but it's just too good for this job.

I was not able to get less than about 1 degree celsius of hysteresis
without the 311 going into convulsions. Even at that point, switching
things on and off and moving things would sometimes be enough to set it
twitching. I tried to cure this, but in the end decided it wasn't worth
messing around with when the 741 worked so nicely.

With the 741 and a 1 Meg feedback trimpot I can get down to about 1/3
degree hysteresis at 25C, which is great. It's stable. It draws a good
milliamp less, so why not use it? Horses for courses.

This interesting diversion and happy outcome would not have been possible
without the great input I've had here -- so many thanks to everyone.

CF

J
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