simple 7 segment driver, but higher power

[ex99125b]

I have a need to build a circuit which will drive a 7 segment display, Since its a static display a simple 4511 with DIP switches would suffice. The problem is that I need to drive a larger (CC) display that draws, lets say up to 150ma per segment. The 4511 will do 25. How would I go about increasing the power output? I know that adding transistors sounds fine, but I just dont know how thats done. I cannot multiplex the segements also, due to synch issues on camera.

[pebe]

A simple way would be to use NPN transistors configured as emitter followers. But their collectors need to be fed from a voltage higher than that supplied to the 4511. Can that be done?

Are you using common cathode displays?

[ex99125b]

if I go with a 4511, yes the displays will have to be CC. Using a single voltage would make it simpler for me since I really dont know what Im doing. I am open to any kind of circuit which allows me to set a dip switch and turn on segments. Its a simple on-camera display, static , never changing unless I reset the switches. I am going to build it from scratch so any thing is possible limited by my knowledge and the help I get here.

[pebe]

You mentioned 150mA per segment, so you must have some idea of the type of displays you have in mind. Also, the 4511 has a binary input so you will need to change four dip switches to change the reading.

Can you be more specific about the display LEDs, because their current and forward voltage must be known before a circuit can be given?

[ex99125b]

So in the shower last night the brilliant "Doh!!" struck. I dont need anything but the dip switches and a resistor for each segment. I can drive each segment directly, once I have chosen the display. No drivers necessary. Its really that simple, i think.

[ex99125b]

specifically, if I use a seven segment ( 1") display with a max current of 105 ma per segment, and drive each segment with, say 100 ma, using a 5 v supply, I can throw 50 ohm resistors on the outputs of a dip switch and get my result....right?

But then the display Im looking at HDSP-3900, while spec'ed at 105 ma Avg. Pwr per Seg. and 40 ma. Max DC forward Current per segment. I am confused.

Which do I use? since I will be driving it at DC (no pulse) am I correct in assuming that the Max forward current takes into consideration the extra heat generated at DC and therefore I should go with 40?

[pebe]

You have misread the data sheet. The figure of 105 that is given is the maximum power in milliwatts per segment, not the current in milliamps.

The maximum current per segment is given as 40mA. At the maximum forward voltage drop of 2V that would give a dissipation of 80mW. The reason that is below the 105mW stated maximum is probably to allow for the extra heat when all seven segments are lit together.

But the test current they used for measuring forward voltage, is 20mA, so that is probably its preferred current.

[ex99125b]

ahhh...thank you very much