C
Chris
- Jan 1, 1970
- 0
frogfot said:
Nope. The voltage divider between the 47K resistor and the thermistor
would be buffered without adding more than a couple of millivolts of
offset by the LM358. The gain is 1, period, and the offset won't
change more than a millivolt or two with reasonable changes in
temperature. Whereas the initial circuit's NPN transistor buffer will
change all over the place, for even a 15 degree change in ambient from
25C to 40C.
I'm assuming your op amp input is basically "floating in the breeze"
with a remote thermistor. I don't like that, because a lot of things
can go wrong there. The cap, diodes, and series resistor at the input
of the op amp may help prevent any accidental excursions (like
electrostatic discharge) above/below the supply rails from damaging the
op amp input.
from a small transformer, so that I don't have to change any batteries
in the final device
Either way, a 7805 5V post-regulator after your 7812 can't hurt. I'm
making some assumptions about the susceptibility of the circuit to
oscillate at switching point, and the 5V reference is one of those
things that might not be necessary depending on what you do and how you
construct your circuit, but it can't hurt and it just might pervent
serious problems.
Yup. I'm a little biased against FETs because of bad experiences inGot it, though if I understood the graphs for LM358 (op that Chris
suggested) correctly, at +10 mA the output should be at maximum 0,2 V.
And that current should be more than enough for my "coil driver".
Lol, the reason I try to avoid MOSFETs is because they don't like me..
First time I bought 2 of them, they died before I could test them.. (I
heard they're very sensitive to static electricity..)
And besides, I get most of my transys from my dead tv, there's no
MOSFETs for some reason
ancient history. In days of yore, they used to ship each early FET
encircled by its own anti-static brass ringlet which you only removed
after the device was safely on the circuit board, and needless to say,
mortality rates were very high. You would look at these things wrong,
or even just open the box, and they'd die in droves. Things are a lot
better now for many reasons, but I still won't use a MOSFET if a
transistor will do the job. I guess I just feel more comfortable with
them, and transistors generally are cheaper, which is why you see many
more of them as discretes in scavenged consumer electronics (although
you seem to have the prices under control, as long as you don't cost
your time).
Yp, exactly, I just wanned it to have somewhat wider "regulation window"
or how to call it..
Heh, now I really don't know which circuit to use..
the main differences in this circuit and that 2 op circuit that Chris
posted? Soz, I'm not that much into electronics to see the difference
Well, why don't you try 'em all, and see which one works best for you.
Do Mr. Fields' circuit first -- his is a more elegant, lower parts
count and lower cost solution. Read up on both of 'em, look at the
datasheets, post questions, learn like heck. Who knows, your final
circuit might be any one of them, a combination of all of them, or
something you thunk up yourself. It's a wonderful world, isn't it?
Good luck
Chris
