Diode resistance?

[beakie]

Hi all,

Basic question about something that should be simple but isn't working as expected

Why does this diode not work as I imagined it would? When I connect the switch it only lights pin 6 (segment a), but doesn't light the segments connect via this diode. Does the diode have too high a resistance to allow the current to go down all connected pins at the same time? If I flick the switch, these pins light as expected.

 

[crutschow]

Your imagination is a little off. The problem is that the added voltage drop of D1 is causing pin 6 to hog all the current. If you use a separate resistor for the diode circuit and the non-diode circuit, then it will work.

In general you should not operate LEDs in parallel off the same resistor. Each LED should have a dedicated resistor to obtain uniform brightness between segments.

 

[beakie]

If one diode works and the other doesn't, then I expect it's a bad diode.

It can't be, I have tried different components!

 

[crutschow]

You caught me while I was editing my incorrect reply.

 

[MrAl]

Hi,

I have to agree with crutschow (his corrected post) in that for driving the segments you get better results using individual resistors.

In addition, the diodes may be superfluous in that if your switch does not have a center off position then you can do this without any diodes by running pins 7,2,4,9,10 through resistors directly to the 13.8v supply, then run pin 6 through a resistor to the switch, and you can get away with a SPSP switch then too. If your switch does have a center off position then you can eliminate D2 unless you see too much of a brightness change (which i doubt you will).

 

[smanches]

(nvm, I got it. Edited out stupid question.)

And crutschow is right. Segment 'a' is hogging all the current due to the extra voltage drop on the diode. Diodes act like switches, not resistors; for the most part they are either on or off.