Re: schottky tempco

gearhead wrote:

On Dec 12, 7:34 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 12 Dec 2007 16:55:34 -0800 (PST), gearhead





<nos...@xxxxxxxxxxxx> wrote:

On Dec 12, 1:23 pm, gearhead <nos...@xxxxxxxxxxxx> wrote:

Is it 2.2 mV/C?

I need a diode with a low voltage drop and a known temperature
coefficient.
I'm going to put it in series with the adjust terminal of a lm317 in
constant-current
configuration, so the voltage with respect to ground at the output pin
of the 317 will be 1.2 volts plus the voltage drop across the schottky
(which will have a constant current through it).
This way I can tune the circuit to give me the exact dV/C I need. It
will be for temperature compensation in a lead-acid battery charging
circuit. Since I need about 2000 ppm/C, or maybe a little more, a
"regular" silicon diode won't work. It would have 2.2 mV/C out of
nearly 2 volts (1.2 + Vf), giving me in the 1000 ppm range. So I need
to consider using a schottky.

Apparently temperature coefficient varies with the log of the current
according to the shockley equation. If I can determine the parameters
(like the ideality factor) for a particular schottky I can get the
math worked out and fiddle with the current setting to get the
temperature characteristic of the circuit right.

I'm going to set the current at 5 or 10 mA to make sure the 317 works
right.

Schottkies tend to run less, -1.5 maybe, except that the very small
signal-level diodes have decreasing tc's as the current increases.
Some go to zero tc at 10-20 mA. I think the exact tempco depends on
the metal used.

How about an LM35? It outputs 10 mV per degree C, so you could scale
that as needed. But it won't sink much current, so you'd have to
buffer it with an opamp or something.

Don't run an LM35 from over +5 volts! And don't pull the output below
ground!

John- Hide quoted text -

- Show quoted text -


Whatever kind of temperature sensing element used, be it a diode, LM35
or whatever, needs to be screwed on to the battery terminal. The LM35
probably comes in some fragile package like a TO-92. On the other
hand, I can solder a ring terminal right onto the lead of a diode and
it will be rugged enough to screw to the battery terminal, and the
diode's lead will provide a good thermal path from the battery into
the diode.
Ambient temperature sensing really isn't feasible here. The battery
enclosure's temperature will undergo temperature variations to
extremes, from around freezing to at least 85 C (outdoor machinery).
The temp sensing must be through a direct connection to the battery
terminal. I think I'm kind of stuck with a diode just for physical
reasons.
Thanks for the advice, John.
why not NTC or PTC resistors?
you can get those with face mountable tabs.


--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5

.

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    (sci.electronics.basics)