What resistor value to use?

[Lac]

When I connect a Led-Display to a BCD-Encoder chip (the 7447) I need to connect a 330 Ohm resistor in serie with the output pins at the 7447 pins to each pin at the Led-Display so the each of the leds in the display wil light up right (each of them draw 15mA). But what if I connect 4 Led-Displays to a single BCD-Encoder? Shall I use 330Ohm at each of the pins? or 80 Ohms? Or something else? I really don't know, even if this is basics.

Thanks!
Lac.

 

[TillEulenspiegel]

Most 7-seg displays use a common VCC resistor instead of 1 per line , as the 7447 is neg logic , I assume your display is as well , this is the most common configuration. Look for the pin-out on the 7-seg if the VCC is the common this is the way to go use a 1/2 watt 330 Ohm resistor.

 

[Lac]

There aint any VCC pin, at least not at the datasheet.

 

[Sebi]

I recommend to use resistor for all segments, for each display.
When You use only one resistor seriel with common anodes, the display segments changes the brite depnding from numbers.

 

[Lac]

True true.. But I have still not got a answer to my question.

 

[TillEulenspiegel]

Post the pinout of the 7 seg You have.
edit : use Ohms law and figure 15mA * 7 @ ( I assume ) 12V. There is no current degradation in these specs. Not for modern displays

 

[Russlk]

I assume you are multiplexing the displays (otherwise you need multiple 7447s), so you need 7 330 ohm resistor at the 7447 output (or 8 with the decimal point). The Pinewood Derby Timer schematic on my website shows how I did it.

 

[tavib]

:lol:
usual 7 seg displays have small LEDs inside, for these LEDs a 15mA current is a little too big, a 5mA current is enough to obtain a good bright, depending of display size.
For unmultiplexed display a single resistor between common anode of every cell in your case of 7447 decoder and +Vcc is a simple case and will work fine if you accept the bright change with number displayed. These common resistors will carry out a current bigger than a single segment current, but be aware if you set up a 7 time bigger current 7*15 in your case, when have 1 on display only 2 segments will share the entire current, so will have 50mA/segment and this is too much. You must make a compromise, and use 7*5mA for example at 35-40mA total current, in this case you have 20mA/segment in max case, and 5mA/segment in min case.
In case of multiplexed display is recommended to use a resistor on every segment(7 resistors in total) and a single decoder. But in this case the segments will bright only a period of time and is off when others cells is on, in this case the medium current through segments of one cell is equal with current through segments when the cell is on, divided at number of cells that are multiplexed. For have a medium current at 5mA for example and if you have 4 cells, you need to calculate your resistors to carry out a 4 time bigger current than medium current, at 4*5mA. But in this case you are help by the fact that the LEDs used in pulsed wave develop a little higher light emission efficiency than the same LEDs used in continuous wave, but this characteristic is deppendent by LEDs type and you can or not to based on it.

 

[Lac]

soooo... then I have to connect 250R resistors to each of the seven pins? That would give 5mA at min, and 20mA at max.

Am I right?

Edit: remebered that 250R aint a normal resistor value. I assume I can use 220R instead?

 

[Lac]

anyone?

 

[ante]

You can use 270R or 220R, no problem.

Ante :roll: