Nico Coesel said:
Jim Thompson said:
On Thu, 11 Apr 2013 18:13:50 +0000 (UTC),
[email protected] wrote:
I need a Voltage rail to track another rail with a fixed 5V drop
between the two. [...] I'd also like the tracked output to have
some current protection in case the output gets shorted.
Main Rail o---+---------------------------------o Main Rail
|
+--->|--->|--->|--->|--->|---~~---o Tracking Rail
D1 D2 D3 D4 D5 F1
Ground o-------------------------------------o Ground
D1-D5: 1N4001
F1: about 1 A; fast or slow blow to taste
Ooooooh! That is SO accurate!
It meets all the stated requirements of the original poster.
It doesn't.
Key word: stated.
Quick version: I know the voltage drop from the main rail to the
tracking rail is going to vary. The original poster didn't say how much
variance was acceptable.
Long version:
Look at the datasheet of the 1N4001 and especially the graph forward
current versus forward voltage.
I actually did look at one data sheet (Diodes Inc) before I posted the
first time, because I had one memory of the nominal forward voltage
being 1.0 V and another of the nominal forward voltage being 0.6 V.
That data sheet quotes a maximum forward voltage of 1.0 V at 1.0 A
forward current, 25 C ambient, single phase, half wave, 60 Hz,
resistive or inductive load. (Yes, this application does not meet
those conditions.) So I went ahead with 5 of them in series.
That data sheet also has a graph that shows forward voltages of about
0.7 to 0.9 V over currents from 0.01 to 1.0 A at a _junction_
temperature of 25 C, but this graph also has the test conditions of
"pulse width 300 us, 2% duty cycle". I'm sure there is a reason for
this test condition, but I don't know what the reason is, so it seems
a little weird to me. The application that the original poster asked
about is probably much closer to continuous than pulsed.
I just looked at some other data sheets. Vishay's has a similar If vs
Vf chart, with about the same curve, except their test condition is a
1% duty cycle. Fairchild has a similar chart and curve and uses the
2% duty cycle. ON has kind of a different chart; it doesn't have the
pulsed test conditions, and has curves for three different Tc (case?).
For 25 C, Vf varies from about 0.79 to 0.91 V over If 0.1 to 1 A.
And then look at what the temperature does to the forward voltage.
The Diodes Inc, Vishay, and Fairchild data sheets don't have any
temperature vs forward voltage data. The ON one sort of does, in the
three curves on the same graph mentioned above. At a constant If of
0.5 A, Vf varies from about 0.72 to 0.86 V as Tc (case?) varies from
150 C to 25 C.
So yeah, Vf will vary over If and temperature. The original post did
state a current of "up to" 0.5 A, which implies that it could be less,
so If will probably be changing. The original post did not state a
temperature requirement; I realize the diodes will heat up some from the
power dissipated in them, so temperature will probably be changing.
This means that in this circuit, Vf will change, so the total voltage
drop over D1-D5 will change, roughly in the range of 3.5 to 4.5 V.
There will be a small additional voltage drop from the fuse (Bussmann
says an AGC-1/2 is 0.59 V typical at rated current), so the total
voltage drop from the main rail to the tracking rail will vary over
the range of roughly 4.1 to 5.1 V.
The original post did not state a tolerance on the 'fixed 5 V drop'
requirement. The claim could be made that "4.1 to 5.1 V doesn't meet
the 'fixed 5 V drop' requirement", but that would mean that an
alternative solution that provides a 4.9999 to 5.0001 V voltage drop
also doesn't meet the 'fixed 5 V drop' requirement.
The fanciest DMM I can find in a little bit of Googling is an 8.5-
digit one (Agilent 3458A), and its resolution on the 10 V range is
100 nV. So, I guess if there is a solution that provides better than
4.9999999 to 5.0000001 V voltage drop, it would be hard to prove that
it didn't meet the requirement.
Data sheets referred to:
Diodes Inc
http://www.diodes.com/datasheets/ds28002.pdf
Vishay
http://www.vishay.com/docs/88503/1n4001.pdf
Fairchild
http://www.fairchildsemi.com/ds/1N/1N4001.pdf
ON
http://www.onsemi.com/pub/Collateral/1N4001-D.PDF
Standard disclaimers apply; I don't get money or other consideration
from any companies mentioned.
Matt Roberds
