Al wrote:
In article N7AUg.2324$6S2.1177@trndny02,
ehsjr wrote:

Al wrote:
In article , jasen
wrote:


On 2006-10-01, Al wrote:


So, if the LED is specified to give a certain light output at 10mA at
4.5V and if the source voltage is a constant 6V, I would use two diodes
whose forward voltages are specified as 0.75V at 10mA to give me a 1.5V
drop.
Typically leds are specified with a voltage range for their limit current.
get the curren right and the voltage will be somewhere in that range, it
depends on the device and the environment.

If that particulasr led is 4.4V with 10mA flowing through it it could with
4.5V the current could be 30mA or more.


The forward voltage drops of a typical 1N914 diode are shown as:

Vf If
volt ma

0.6 3.0
0.7 10.0
0.8 30.0
And when they get warm, it drops, this can lead to thermal runaway.

ordinary resistors consistently out-perform ordinary diodes as a LED
current source.

Bye.
Jasen

Sigh! Yes, that's true. But I routinely use one or two diodes, depending
upon current, to drop my 6Vdc battery down to a nominal 5Vdc that my
circuits use. It works. What can I say? It's a cheap and dirty way of
doing it for home projects.

For example, I have a circuit which nominally draws 50ma during
operation. Upon the needs of the project, a 200mA pulse (100mS) drives a
latching relay. If I used a 20 ohm resistor to drop the voltage to 5Vdc
during norminal operation, it would really screw up the circuit during
the pulse as the series resistor would drop the voltage by 4V!

So, tell me, how many components would you use to drop your 6Vdc to 5Vdc?

Al
What someone else would use is irrelevant. You use
whatever you want to meet the circuit requirements.
Here's a possibility for your pulsed 200 mA.
Substitute whatever you're using in place of the switch.

/
+6 ----+-----o o--------+
| |
[20R] [5R]
| |
[50 mA Load] [200 mA Load]
| |
Gnd ---+----------------+

For the LED circuit that was discussed a resistor
is fine. For something that requires regulated 5,
you can use an LDO like
http://www.national.com/ds/LP/LP38690.pdf

For something like a 4.5V toy motor, your diodes
may be fine.

Ed

You are correct!

I was just using the 200mA load situation just to show how a resistor
would not do the job. And, one should use what works in their particular
situation.

I've always wondered why the industry settled on 5 volts as the standard
for TTL esp. since 6 volt batteries were so common. In the 60's I
actually worked on a logic system which did use +6 and -6 volts for the
logic. I can't remember if +6 was a one and -6 was a zero, or was it
visa versa? For experimentation with this logic type, we actually used 6
volt lead/acid batteries for power. We quickly learned never to short
anything here!

Al

When I did my electronics training in mid-70's, I can recall a cct that
was part positive logic (1=-5V, 0=+5V) and part negative logic (1=+5V,
0=0V).

Just a little confusing.

Daniel

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