Re: op amp output


frogfot wrote:
> Thanks for all replies. I'm pretty sure that all parts are ok and
> connected correctly, I've looked it through several times.
>
> John Smith wrote:
> >Change the opamp for one which gives rail-to-rail outputs.
> >Or create your own rail-to-rail output using transistors.
>
> How stupid of me to forget mention that I used another op, TL082.
I've
> tested to change to LM358 and it gave almost 0 on low!
> Huh, I beginned to use an op instead of transistors for this
> application because I thoat this will give simpler circuit.
>
> Ban wrote:
> >The whole design on this web page doesn't have
> >any hysteresis, which should be there. Just solder a 1Meg resistor
> >from pin12 OP to pin4 +IN.
> >...
> >When the comparator is low, the voltage on pin 12 should be less
than
> >0.3V with the lowish current draw of the 5k6 pull-up resistor. Thr
> >relais is then off. The output should be able to drive the relais
> >directly, if the current is less than 25mA.
>
> But is 1M resistor really needed? I thoat it only decreased the
> precision.. Now I included it.
> The relay coil will drain quite alot. It will control a ~500W boiler.
>
> Chris wrote:
> >You're getting pretty close to the full battery voltage when the
> >comparator is off, and about 1.3V when on. Again, looking at the
> >data***, you can see that the comparator transistor "on" voltage
> >starts to rise pretty dramatically when it's trying to sink more
than
> >about 5mA.
>
> The high/low output doesn't change much if I remove all load from the

> op output (if my multimeter isn't counted as a load). But anyway,
I'll
> use different op amp.
>
> John Fields wrote:
> >You don't need Q1 and, assuming that your thermistor has a
resistance
> >of around 1000 ohms at your setpoint, this should work:
>
> Mine NTC is 10k at 25*C and 500 at 100*C. I don't understand why
you've
> used an MOSFET (I'm more used to transistors :P ).. and what's the
> purpose of Z15V, is it a zener diode?
>
> I was going to use this circuit to control temp in a water-bath at
> 0-100*C.
>
>
> --
> Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

Hello again. I was under the impression you were working with a
comparator and trying to get the circuit to work. Having switched over
to an LM358 (which will get within a couple hundred mV of GND while
sinking a couple of mils) should have handled your immediate problem of
getting the relay transuistor to turn off.

However, you've got a couple of serious problems that don't have
anything to do with that. First, the schematic transistor TR1 is a
current-driven device whose gain will be very dependent on temperature.
A small change in the ambient temperature around Q1 will cause a large
change in voltage impressed across the 2K load resistor. Bad news if
you want to maintain a steady setpoint.

Second, whether you use an op amp or a comparator to drive the relay
transistor, you're going to have problems with transistor oscillation
when the thermistor voltage gets close to the pot voltage. That can be
hidden a bit by the fact that the relay coil will act to average the
voltage impressed across it, and that relay coils have inherent
hysteresis (you usually have to get them up above 50% of rated voltage
for them to turn on, but once on, they will almost always stay on until
coil voltage goes down below 25% of rated voltage).

In short, I think the author of the web page knew more about web page
design (pretty good, actually) than electronics (inadequate, actually).
It's a common problem. But there's a pretty good basic circuit on the
web similar to yours that doesn't have the problems listed above:

http://www.uoguelph.ca/~antoon/circ/heatsens.html

This circuit will also have the issue about the turnoff voltage of the
op amp not being quite low enough to turn off the transistor. It also
lacks consideration for using a remote thermistor.

But, considering you've got an LM358 dual op amp to solve the turn-off
issue, you might want to try something like this (view in fixed font or
M$ Notepad):


VCC
+
|
| VCC
.-. +
| |47K |
| | .---o
'-' | |
| VCC | C|
| + - C|
o--. | 220K ^ C|
| | 1N914 | ___ ___ | |
| | - .-|___|-. .--|___|--. | |
.-. | ^ | 22K | | | | |
| | | | | |\ | | VCC | '---o
| | | | '-|-\ | ___ | |\| | |
'-' | 0.1uF | ___ | >--o-|___|-o--|-\ | ___ |/
| '------------o---o-|___|-|+/ 10K | >---o-|___|-o-|
'-----------. | | 22K |/ .--|+/ 2.2K | |>
Thermistor | --- | | |/| .-. |
<--o --- - | GND | | |
Shield | | ^ | 2.2K| | ===
=== === |1N914 | '-' GND
VCC GND GND | | |
+ ____ === | ===
| | | GND _V_ GND
o-|7805|-o----------------------------|___|--.
| |____| | 50K |
--- | --- ===
--- | --- GND
| === |
=== GND ===
GND GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

The first op amp is used as a voltage follower to buffer your signal
from the thermistor. Make sure to use the shield connection as in your
original art. The second op amp is set up as a comparator, but has a
220K feedback resistor to provide hysteresis. You can decrease the
value to 100K or so if you're still getting oscillations at the output.
However, the more you increase the hysteresis, the moreexcursion
you'll get on water temperature for this "bang-bang"-type controller.
Try to use the higher value resistor. The 7805 is a cheap way to get a
good reference voltage for the 50K pot, so your reference voltage will
be stable when the relay turns off and pumps current into the power
line.

When constructing the circuit, make sure to use a bypass capacitor on
the op amp, and make sure your power supply wires are close to the
board. If not, add a really good (1000uF) cap across the power supply
to stabilize the power supply. You should also try to avoid contact
arcing at the relay contacts by either putting a quencharc across the
resistive heater load, or at least physically moving the relay some
distance from the circuit.

The problem with using op amps as comparators is that they have
relatively slow transistions, and tend not to have logic level highs
and lows. Of course, the relay open/closure time (one to several ms.)
is far longer than any delay caused by op amp slew rate. And if you
use an op amp made for getting output close to the negative supply, you
won't have a problem with turning off an NPN transistor. The LM358
isn't a bad choice for this application.

I hope this has been of help.

Good luck
Chris

.