Running LEDs & voltage question

[William At MyBlueRoom]

I've attached a schematic of a project under development.

The question is since I'm running the LED anodes on a string of 10 green LEDs (a 5" seven segment display) with a voltage drop across the LEDs of 21v (I've yet to measure the drop in a 74LS47 display driver 0.7v?)

I'm using an LM317 regulator; if I adjust the regulator to 21.7 volts can I omit the current limiting diodes? Or shall I use a small current limiting resistors 10ohms.

I'm not trying to cut the resistors out of the circuit, I'd simply like to keep the voltage required down. (I'm at 24V for this circuit)

I'd like to build displays similar this type.
**broken link removed**
some of their displays use 12 LEDs per segment.

 

[Russlk]

I have operated series LEDs without a series resistor and it worked, but for safety I recommend a current limit circuit on the LM317 output. The data sheet should show how to do that.

 

[hyedenny]

Hi,
It looks like each segment of the display you're using has 10 LED's. The 74ls47 can only sink 24mA per segment. This will result in a very dim display (2.4mA per LED), or a burnt out BCD driver.

Furthermore, I dont know how many digits you have, but even if you have only ONE digit with all segments decently lighted, that will require about 1.4 amps, which is going to turn your LM317 into a space heater. [20mA x 10LEDs/seg x 7seg = 1.4A]

How about using transistors between the chip and the display?

 

[William At MyBlueRoom]

Hi,
It looks like each segment of the display you're using has 10 LED's. The 74ls47 can only sink 24mA per segment. This will result in a very dim display (2.4mA per LED), or a burnt out BCD driver.

Furthermore, I dont know how many digits you have, but even if you have only ONE digit with all segments decently lighted, that will require about 1.4 amps, which is going to turn your LM317 into a space heater. [20mA x 10LEDs/seg x 7seg = 1.4A]

How about using transistors between the chip and the display?

Well it's a series LED circuit. The total draw per segment can be set for 24ma (thats all 10 LEDs get 24ma each at 21v) if the LEDs were in parallel then yes you'd need lots more current.

Heres a link on series LEDs
http://www.kpsec.freeuk.com/components/led.htm

For proof try wiring 5 red (1.6v) leds in series with a 50ohm resistor on the end. Attach 9v and measure the current. The current in a series circuit will be the same 20ma throughout the entire circuit.

The entire display when fully lit assuming 24ma per segment (the 74LS47 limit) would be 24x7 or 168ma well under the LM317 limits.

 

[Oznog]

1. "current limiting diodes"? Diodes do not limit current.
2. Every series LED string must have its own current limiting resistor. If you have 7 segments which are lit in parallel, each segment must have current limiting. LEDs must always have some form of current limiting, without exception, or they will burn out.

 

[Mike - K8LH]

Hi Bill,

I would keep the segment current limiting resistors...

I would also recommend a minor circuit change to provide more capability while maintaining a simple 2 chip design by swapping out the 'LS47 with a Micrel MIC5821 or Allegro A6821 serial-to-parallel 8-bit driver chip to eliminate the decimal point jumper and provide programmable PWM brightness control... This driver would also provide a reasonably simple method for expanding the display to multiple digits, all driven by that little 8-pin PIC... And yes, I know, the 'LS47 has that nice BCD input and the alternatives don't, but the tiny bit of software 'overhead' is worth the gain in capability...

Regards, Mike

 

[William At MyBlueRoom]

Micrel MIC5821 or Allegro A6821 serial-to-parallel 8-bit driver.

Looks like a teriffic chip, not easy to find here in Toronto. I've started working on a updated desing using a 16F628A and a ULN2803

Cheap & common. I'm only looking to drive 1 display at a time. They would be on a simple network rx only required. I might use a 16C54 and put the DIP on the 8 bit driver port and scan it real fast (too fast for a motor and it wouldn't hurt an LED display.