I'm probably stupid, let's summerize:
pin CA : is common anode from my 7-segment LED
Keith said :
/-->---[LED|>]-- RESISTOR --->--- pin /a
+5V -- pin CA --+-->---[LED|>]-- RESISTIR --->--- pin /b
\-->---[LED|>]-- RESISTIR --->--- pin /c
Mike said :
/-->---[LED|>]--->--- pin /a
+5V -- RESISTOR -- pin CA --+-->---[LED|>]--->--- pin /b
\-->---[LED|>]--->--- pin /c
That's true, I usually see limiting current resistor placed before a LED
and not after. Two much current entering the LED with burn it.
For LTS-546AP:
- Foward Voltage Per Segment : typ=2.1V max=2.6V (test cond Ir=20mA)
- Reverse Voltage Per Segment : 5V <=== what's that ?
* Who is right ?
* Can I connect the common anode to 3.3V or it must be fixed 5V ?
Mike Dodd wrote:
>>Concerning resistors (outputs of 74LS47), I don't understand:
>>
>>The output pin (/a, ..., /g) are sinking current, so the current go from
>>the LED to an output pin :
>>
>>5V -->---[LED|>]---->--- pin /a
>>
>>So this resistor (located after the LED) won't affect the lighting power
>>of the LED, it's just for limiting the current entering inside 74LS47,
>>am I right ?
>>
>>
>
>No, you are wrong.
>
>An LED is a light-emitting DIODE, which means it will pass as much current
>as you try to push through it (until it is destroyed). The purpose of the
>resistor is to limit the current to a safe level.
>
>If you look at the spec sheet for the LED display, you will probably see a
>specification that describes "forward voltage drop." This is the voltage
>you can measure across the diode when current is flowing through it. I'm
>not sure what this is, but it's probably about 2V to 3V. This voltage is
>approximately constant, regardless of the current flowing through the LED.
>So to limit the current, you need a resistor in series. The circuit is:
>
>5V --> RESISTOR --> LED --> pin /a
>
>To calculate the value of the resistor do this:
>
>1. Determine the supply voltage (let's say 5V).
>2. Subtract the "forward voltage drop" from the spec sheet ((let's say it
>is 2V).
>3. The result from #2 (3V) is the voltage that is dropped across the resistor.
>4. Use Ohm's law R = E/I. E is the voltage from #2. Choose a current for I;
>let's pick 10mA (0.010A), which is a safe value. Therefore R = 3/0.010 =
>300 ohms.
>
>A resistor in the range of 270 ohms to 330 ohms should be good, if our
>assumptions are correct. But the important thing is to READ THE SPEC SHEET
>TO DETERMINE THE ACTUAL FORWARD VOLTAGE DROP ACROSS THE LED. That will
>determine the voltage in step 2, and thus will determine the value of the
>current-limiting resistor.
>
>
>
>>About voltage, how to know how many volts have we in pin /a; is 15V enough ?
>>
>>
>
>The minimum required voltage is the forward voltage drop across the diode,
>plus a bit more. So if the diode drops 2V, you need at least 2.5V (through
>the appropriate current-limiting resistor). You could use 15V, but your
>resistor value would be much larger, and you'd be dissipating a lot of
>power in the resistor, so I suggest you go with 5V.
>
>
>
>>V Off-State Output Voltage /a to /g : 15V
>> O (off)
>>I On-State Output Current /a to /g : 24mA
>> O (on)
>>
>>
>
>You are misinterpreting the specifications. These probably are MAXIMUM
>values, not what you want to use in normal operation.
>
>If you have a display and a 5V power supply, get some resistors in the 200
>ohm to 470 ohm range, and connect one segment as I showed above, using
>different values for the resistors. Measure the voltage across the segment
>while it's lighted; this is the actual forward voltage drop. Calculate the
>resistor value, then try the display with the new resistor. You can reduce
>the value to make the display brighter, or increase it to make it dimmer. I
>suggest keeping the current below 15mA (0.015A).
>
>Hope this helps.
>
>Mike
>
>
>------
>Mike Dodd - Montpelier, VA
>http://www.mdodd.com
>
>
>
>
>
>Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
------------------------ Yahoo! Groups Sponsor --------------------~-->
Get Bzzzy! (real tools to help you find a job). Welcome to the Sweet Life.
http://us.click.yahoo.com/A77XvD/vlQLAA/TtwFAA/CFFolB/TM
--------------------------------------------------------------------~->
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/ts-7000/
<*> To unsubscribe from this group, send an email to:
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
|