Re: schottky tempco

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






.

Relevant Pages

  • Re: Paper: Solid-State Thermal Rectifier
    ... with the "temperature" of what you say. ... if a molecule was about to escape. ... |> The electrical equivalent might be thought of as a diode between two ... |> capacitors that are charged to some voltage. ...
    (sci.physics)
  • Re: Paper: Solid-State Thermal Rectifier
    ... If you placed such a thermal diode between two chambers that are in thermal ... Since heat flow is driven by the temperature *difference* ... capacitors that are charged to some voltage. ...
    (sci.physics)
  • diodes, ideal diode equation, and temperature coefficients
    ... The ideal diode equation seems to predict that for a given current ... there will be a fixed ratio between temperature and voltage. ... predicted for the temperature coefficient as it relates to current. ...
    (sci.electronics.basics)
  • Re: schottky tempco
    ... so the voltage with respect to ground at the output pin ... temperature characteristic of the circuit right. ... Whatever kind of temperature sensing element used, be it a diode, LM35 ... needs to be screwed on to the battery terminal. ...
    (sci.electronics.basics)
  • Re: schottky tempco
    ... according to the shockley equation. ... temperature characteristic of the circuit right. ... then you can take some measurements of the diode at ... schottky, also a pink led. ...
    (sci.electronics.basics)