It has been many years since my electronics days, but I believe a diode
does have a fixed voltage drop. I think it's around .7 volts if I remember
correctly. So it does regulate the voltage in some respects.
Yes, that is a forward voltage drop across the junction. The voltage
depends on the materials used in the device. The regulation function of
a diode is usually a reverse voltage taking advantage of the breakdown
characteristic. IOW a reversed diode as a shunting device. The same can
be used as a reference diode to control a transistor regulator. Zener
diodes come to mind for this.
A "regular" silicon diode has *about* a 0.7V forward drop. Though the
actual drop is dependant on the current flowing through the junction (as
current increases, the drop increases -- but only slightly; not
proportionately as with a purely resistive device). There's a "knee"
in the V-I curve at that point (but, all knees are "rounded" to some
LED's, OTOH, have a much higher forward drop. Some of the high efficiency
ones are as LOW at ~1.7-1.9V. Most, however, tend to be higher -- ~2.1
for Red/Yellow/Orange and closer to 3.4 for the Blue/White devices.
(Even these are just ballpark numbers as it depends on the dopants used)
Germanium ("Is there life on Germanium?") diodes have forward drops in the 0.2V
range. Schottky diodes are in the 0.15-0.45V range.
And, of course, you can get zeners at all sorts of voltage ranges (but, these
are operating in reverse bias when zenering)
The (forward) voltage drop depends on the semiconductor material. .7V
for silicon diodes, around 2V for LEDs.
Reverse voltage drop can vary greatly according to design. I've see
about 3V (for a LED) to 1KV (1N4007 IIRC).
With either polarity, voltage exceeding that will cause current to flow.
This is called "breakdown" and is not harmful unless the diode's maximum
current is exceeded. A LED is normally operated in forward breakdown
(necessary for it to light).
A diode is a voltage regulator. If you connect a LED (and nothing else)
to a car battery, the LED will try to bring the battery down to 2V, and
try to handle nearly infinite current. I did this once and didn't even
see any light before there was a POP and half the LED disappeared. You
need a resistor that drops the excess voltage (10V here) without
exceeding the LED current (typically 30mA).
On Sat, 4 Jun 2016 15:01:11 -0000 (UTC), HerHusband
Galium Arsenide Red is around .1.4 to 2.6 Red, yellow, green and
blue have higher forward drops in that order, up to about 3.8 for
ultra blue. The white ones are closer to 4 volts - 3.7 to 3.8 rings a
bell. A silicone diode is around.0.7 volts, and a Shotky diode is
closer to 0.3 or 0.4 volts. A galena diode only drops about 0.2, and a
selenium rectifier diode drops about 1 volt per cell.
A very simple definition that is sort of correct but leaves out a lot of
A diode has a threshold voltage. It the supplied voltage is less than
that, there will be no current. Otherwise the diode will conduct. It
will conduct as much as needed to limit the voltage across it to the
threshold. This means a diode may conduct excessive current if nothing
is done to limit it. This is frequently a resistor in series.
An important thing about a diode is that the threshold is different for
each polarity. That simple definition is correct when the voltage is
between one threshold and the other.
When the diode is in series, it will have a voltage drop equal to the
threshold voltage (which threshold depends on polarity).
BTW, I know that semiconductor junctions are even more complex than that.
That's only half the definition. It has both forward and reverse
voltage drops, and forward current draw is determined by the voltage
drop and the supply voltage.
As long as the supply voltage does not excede the forward voltage
drop by ant significant amount, the current will remain within limits.
On 06/04/2016 07:59 PM, email@example.com wrote:
Supply voltage (voltage across the diode) WILL be equal to voltage drop.
That is the diode acting as a voltage regulator. That explains the need
for a resistor with a LED.
If the supply voltage exceeds the forward voltage drop by ANY amount,
the current will be infinite* (as the diode tries to regulate E by
conducting until supply voltage drops to the diode's forward voltage).
* - limited only by the power source's capacity.
Lithium is lower voltage than 3 AA or AAA alkalines so the current
will be less and the light output moderately less. Doesn't matter how
"stiff" the battery is as long as the output voltage is low enough
that the LED does not draw too much current.
3 aaa alkalines top out at about 4.5 volts. Lithiums are 3.7 nominal
- freshly charhes possibly as much as 4.2 for a very short time
I remember the last 3 (more than an hour or two) outages here and they
were about 8 years apart.
BTW, during the last outage (from the tornado) wired phone service was
out too but my cell phone (Verizon wireless) was OK.
That reminds me of when I was working behind a relative's console TV
(this was before LCD so it was really big). Light was dim back there but
She decided to turn on a nearby lamp and dim became BLACK.
Our utilities are below grade so seldom affected. Our last outage was
a distribution transformer for the neighborhood catching fire. Before
that, IIRC, a fire in a cable vault.
In 20+ years, I think we've lost power 3 or 4 times. (lost natural gas supply
once -- THAT was interesting!)
Most of my kit hides under things -- under dressers, tables, beds, etc.
And, most of the time, this is "just fine" (TmReg).
But, on those occasions when you need to plug/unplug a cable or needs
to survey the contents of a box (without moving it as it's tethered to
<whatever>), a light is essential. OTOH, you're only using it for a few
seconds so even the crappiest "batteries" will last a long time!
Our electric is on poles, the wires weave through miles of half-dead maple trees. Consequently, every summer we get a thunderstorm or two that
takes out power for 2-5 days. Before power company deregulation, we almost *never* had power outages...and when it did go out, the outage lasted
less than 6 hours. The idiots that passed the power deregulation laws ought to be electrocuted!
Similar situation. Ours is underground but before it goes to our street
it is above ground with lots of trees. That's why I bought a generator
and had a transfer panel put in several years ago.
I've complained to the power company many times about their not keeping
up the access trimming but even after they do it we could still lose it
in a bad storm. Last one blew a big tree down that not only took down
wires but took down a pole.
My system is too small to handle the whole house. I was mainly
interested in furnace, well and freezer. Newer systems can handle the
whole house by proper managing although not everything can be on at once.
I have to bring in gasoline but if we had natural gas I would use that
as fuel source.
I live in one of the few areas around here that don't have deregulation.
One difference is rates are much lower (I pay about half what people in
a nearby city pay).
BTW, when figuring rates, I skip all the misleading stuff and just
divide total bill by usage.
I put all of "my" lines underground, but the power company's cables are all
We typically lose power a few times each winter, usually due to wind storms
blowing trees onto power lines. Occasionally ice will take down a line, or
someone will take out a pole with their car.
Most outages only last a few hours, but once we had an outage that lasted a
day and a half.
When I lived in the midwest, everything was overhead. I could count on
an outage in my neighborhood every month or two. A drunk taking out a pole;
an ice-laden branch falling on the wires; high winds; etc.
We've had so few, here, that they come as a real "surprise"!
We occasionally get what I call "annoyance" outages. That's when the power
blips off for a minute or two before coming on again. Just long enough to
screw up all the clocks in the house, annoying. :)
Thankfully, most of our clocks are battery operated now, and the UPS for my
computer sails right through those.
On 6/4/2016 7:45 AM, HerHusband wrote:
I had an ISP thast must have been plagued with a similar problem
(or, a software bug) as I noticed that if I was pulling down an
ISO at certain times of the overnight that the connection would
I knew it wasn't anything on my end (as I'd be sitting there,
at the time -- doubtful they had staff on site at all hours of the
I've been trying to replace all of our clocks with designs of my
own (I like clocks!). Part of the design process is to include
"time services" so they automatically track a common reference
(that *they* elect, if the previous "reference" is unavailable).
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