# Convert 12-14 vdc to 9 vdc for a device only needing 0.3ma...

Discussion in 'Electronic Design' started by Default User, Dec 28, 2008.

1. ### MooseFETGuest

I learned it as 0.7V

It is the number I still use. I know that it is fairy good at 10mA on
a 1N914 or a 2N3904's collector.

2. ### MooseFETGuest

0.5V is about right for 1N914ish sized parts at a few uA. I'd expect
a chip designer to be working with a smaller part if they are working
with uA levels.

3. ### John DevereuxGuest

I had occasion to look into this recently, and the drop at low
currents was less than I expected.

For the 1N914 it seems to be [email protected]

<http://www.fairchildsemi.com/ds/1N/1N914.pdf>

4. ### kevin93Guest

....

Minor correction - kT/q is about 26mV.

So for every doubling of current the voltage rises by this amount -
each decade change will then result in the 60mV you mention (25mV*ln
(10)).

kevin

5. ### Jon KirwanGuest

Thanks, Kevin. I should have paid more attention to the d ln(I) in
the divisor when writing. I knew this and if I'd bothered to actually
calculate kT/q for a moment, I'd have remembered. My laziness.
Appreciated.

Jon

6. ### Jasen BettsGuest

the knee in the curve is in the 0.6-0.7 range, at very low currents
0.5V or less can be measured, and near the max working currents it can
be more than 1V

bye.

7. ### krwGuest

There is no "knee" in a diode curve (until avalanche or failure). If
you plot the diode curve on semi-log paper it's pretty straight.

8. ### Jon KirwanGuest

I seem to remember closer to .6 than .5... maybe .57 or .58 at 1mA for
the 1N4148. I rounded it up in my mind a while back when I was
looking before. I suppose I could go to the bench and check, though.
I agree about the 100mV per decade current change. For the bipolars
I'm most familiar with as a hobbyist (2n2222 and 2n3904 and 2n3906),
liking as I do one penny parts I can hold in my hand rather than
tapping them out of a salt shaker by the dozens, is more like .65V at
100uA or so and going up by the usual 60-70mV per decade. So I'd
estimate more like .71-.72V at 1mA for them. At a guess. If I
weren't snowed in, I'd trapse over to the bench and check. But that's
untrustworthy memory, for now.

Basically, our thoughts on the thrust of it are the same. Just that I
don't lump the 2n2222 BE junction quantities at specific points to be
the same quantities and quantified variations with current with the
1N4148. Mostly because it makes enough of a difference at times when
figuring out how much remaining voltage I may have somewhere else to
work with, I suppose.

Jon

9. ### GreegorGuest

So despite this curve, diode forward bias can still be
a good voltage reference as long as the current is fixed/known right?

Is there a lot of variation between production runs or
even within production runs?

I'm thinking now that even my concepts on zeners
might need some fine tuning as well.
Are zeners as rigidly plateu as I think?
Do designers commonly get dripped up by
the part of the curve inside the designated
current range? Are zeners really as immune
to current variation (within their range) as
some of us get in the habit of thinking?

Is the plateu for high current zeners
flatter or less flat than the plateu for
lower current parts?

10. ### krwGuest

A resistor makes a pretty good voltage reference as long as the
current is known/fixed.
There is variation, sure. Resistors can be easily had that are far
more precise.
Depends. Low voltage (under, IIRC 6V, "zeners" are pretty bad. Above
that they're actually avalance diodes and can have a pretty low
dynamic impedance. If you want even lower, band-gap reference diodes
are even flatter, though have limited voltage selections. It depends
Commonly? Designers shouldn't. The curves are published. Some even
have reasonable Spice models. ;-)
High voltage "zeners" tend to be flatter than low voltage. I've never
looked at the threshold voltage vs. current for high vs. low current
diodes. I've never used a zener at more than a few tens of milliamps.
I tend to use them as references rather than shunt regulators, though
I have one (albeit very low current) shunt regulator in my current
design.

11. ### James ArthurGuest

Zeners have pretty significant series resistances.

The crispness of their 'knees' varies with voltage
rating--low voltage types are notoriously weak-kneed.

Cheers,
James Arthur

12. ### Jasen BettsGuest

I stand corrected.

I noticed the graphs on the IN914 datasheet agree completely with that,
a nice long straight line from nanoamperes to the current limit.

If you go low enough in current will the drop go negative

13. ### Jasen BettsGuest

'fair' perhaps I wouldn't go as far as 'good'

the main thing is you get a fair voltage reference with
a lousy current source say you give a 1N914 12ma with 5% ripple
(so it's varing from about 10mA to 14mA)

across the 1N914 you measure a voltage that varies from 716 to 723 mv
so 719mv with 0.7% ripple
not a much as you get from things like temperature changes.
they vary with changing current too, but not to the same degree.
If the low power parts are built as scaled-down versions of the high
power parts then performance is comparable, often they aren't and the
lowe power zeners perfrom much worse.

14. ### krwGuest

Yes. A negative current. ;-)

1N914

16. ### JamieGuest

About time you got something right you old
geezer!

BTW,
Happy fringing New Year, and may the
USA bite you in the ass!

http://webpages.charter.net/jamie_5"