about 7 segment led displays A few questions

[zoeylucy]

Looking for a little information about 7 segment displays, I found 2 diferent ways to conect it:



And:



As I understand, the A circuit is the right one, but I seen a lot of B. Mi reasoning:



Is equivalent to the first circuit.

And:



Is equivalent to the second.

I alwais understood the first option is the correct one for conecting leds in paralel, and the second one is wrong.

In the case of the 7 segment display, also, using the B option, the current throroug any individual segment will change a lot depending how many segments are lit. A nice way to burn some segments prety fast. With the A option, the current through every segment is controlled by its own resistor, so how many segments are lit makes no diference.

Is my reasoning correct?

Second question: I'm to use 74HC595 to drive the leds. (Serial shift register) It's convenient because I can control as many displays as I want with only 3 microcontroller pins (And an aditional one for PWM dimming).

I seen a lot of circuits using it to drive directly the displays, but, in the datasheets I see 35mA per output pin, that seems OK, but "DC supply current, VCC and GND pins = 75mA" (MAX). As I understand, this means that an individual pin can supply up to 35mA, but the total output can't exceed 75mA. Is this correct? If yes, this means it can't drive directly the display, and I will need to use one transistor for every segment, or find a more suitable driver.

Thanks in advance.

[altenbach]

This does not sound like a LabVIEW question.

[BauerPower]

Your reasoning does sound correct.