Help ! I am stuck on stupid today..

[dacflyer]

i cannot rember how to figgure resistor needed.
i got a led display that uses 20ma, and my supply is 9 volts. how do i figgure the resistor needed
( in plain math please )

i think its 2.2k but not sure .
usually i use a 1k for 12 volt supplys
but why would 20ma require a 2.2k if the voltage is lower?

if i do not make sence here,,,then you know why i am stuck on stupid....i know its a simple problem,,but i cannot think today,,must be the new meds i am on.

[MrAl]

Hello there,

We need to know the nominal voltage of the
LED (at 20ma).

For examples:

[1]
A 2v LED would require a 350 ohm resistor

[2]
A 3.5v LED would require a 275 ohm resistor.


Take care,
Al

[positronicle]

--Edited by Positronicle--

[Dave Dixon]

Here's a cool site that someone contributed awhile back...
LED Resistor Calculator Site

Hope it helps,
Dave

[dacflyer]

the display i have is a LEDTECH 10
LA2311-41
cannot find no info on it,,but it lights up decently around 6 volts. but its brightness is very touchy..if i go before or beyond the 6 volts the brightness dimminishes.. i know the dispplay is not a bright one,,but i am close..

[dacflyer]

positronicle >> thanks, i got it now. i think...

so if 9 volts is the supply to the display
(each segment) and any segment draws 20ma.
a 450ohm resistor will be needed for each segment
right?

[positronicle]

--Edited by Positronicle--

[dacflyer]

charactor height .57"? this display i have measures 2-1/2".
it is nice now,, not as bright as i'd like to think it should be,, no wonder they was so cheap on e-bay...each segment is drawing about
.20 - .22ma so i am satisfied..
i have since got that problem figgured out as far as what size resistor i will use.. i noticed the display has 2 leds per segment instead of one.

i have another question.. reguarding the sn7407n
this chip is a hex buffer , is it normal for it to draw about 30ma with no inputs or outputs?
this ic X4 sure makes a 7805 heat up all 4 7404's
draw about 107ma. sure seems like a lot of wasted current for doing nothing yet.

<small>[ March 10, 2006, 10:49 PM: Message edited by: dacflyer ]</small>

[rshayes]

According to the SN7407 data sheet, the supply current with outputs high is typically 29 mA, but can be as high as 41 mA. With outputs low, the current is typically 21 mA, but can be as high as 30 mA. This does not include the current through whatever load is being controlled by the open collector outputs, which can sink as much as 40 mA each.

It sounds like the devices that you have are very close to typical devices.

Now you know why there was a market for 5 volt, 50 amp power suppllies.

[Gorgon]

Hi Dacflyer,
Have you received the display yet?

TOK

PS: 57mm = 2.24"

<small>[ March 11, 2006, 02:31 AM: Message edited by: Gorgon ]</small>

[dacflyer]

Gorgone >>> nope not yet..
Ahhh 57MM oops, i thought it was .57" lol..

gorgone...wasn;t that a action figgure in a 80's movie? gorgone had his guys,,,and the enemy was some messed up army action figgures?
cannot rember the name of the movie,,but it was cool..

[dacflyer]

is there a more power efficient substitute for a 7407?? with little or no mods.. thanks..

[MrAl]

Hi there,

What are you doing with it? Perhaps there is
an alternate design that would fit the bill
just as well...

Al

[dacflyer]

MrAl >> i have a Lm7107 and its output is 8ma per segment, i have a common anode led display that uses 20ma per seg.
i need a buffer to give more current to drive the led display
the led display is a common anode
the Lm7107 output drive is NEG output
so the buffer device i'd like to have must be little parts as possible and have a active low input and output. the 4707 just draws too much current. but it does work.. so i'd like somehting simmelar if possible,,thanks.

[Gorgon]

Hi Dacflyer,
I've checked for cmos versions of the 7407, and there are several types you could have used if you use 5-6V as supply voltage. I seem to remember that you use 9V, and that is too high for these parts. If you could reduce the LED voltage to 5V, ther is no problem to get cmos replacements.

TOK

<small>[ March 22, 2006, 03:39 PM: Message edited by: Gorgon ]</small>