Large 7 segment display multiplexing

I have only been learning electronics for about 1 month or so and am stuck on what i believe to be a simple problem for anyone experienced.
I am trying to multiplex 4x 4inch 7 segment displays from a PIC 16F627/8 microcontroller.
It all works fine on my small test circuit with a 0.5 inch common cathode 7 segment display. What i can figure out is how to provide enough current to drive the larger display. From 12v, i need to use a 800 Ohm current limiting resistor and a transistor (BC550), but how do i configure the multiplexing to switch on each display one by one?
I have included a sketch of what i expect, but can someone please help?

Thanks
Wayne

Hi Wayne,

Your diagram shows the right principle for switching the multiplexed segments. Just a couple things to note.

Are you using the BC550 for both the top and bottom transistor? The BC550 is an NPN, so if you use it at the top, you provide 5V to the base, and the emitter will be about 0.7V less than that, so around 4.3V. It sounds like you were expecting 12V and sized your resistor accordingly, so maybe that's why you aren't seeing enough current. Try something like a 330 ohm resistor.

Note also that the top transistor will have a drop of almost 8V and have something like 10 or 15 mA flowing through it. That leads to a power dissipation of about 100mW. In this case that's okay since you're working with a relatively small current. Normally though you would use a PNP transistor and turn it completely on (saturation) to keep it from getting hot. PNP also prevents the problem that your base voltage has to stay above the emitter voltage, which varies based on the load. Again, though, it shouldn't be a problem in this application.

Make sure you put a resistor of around 1k in series with the base and the microcontroller to prevent large currents from flowing.

Watch out at the bottom because if all seven sements are on, the transistor will have a total of about 100mA, which is its maximum rating. You might want to first switch on half the segments and then the other half in order to keep the total current down. Of course this rating is mainly due to thermal effects, so it might be alright since you are multiplexing.

Regards,
Chris

Thanks Chris,
My problem probably stems from the fact that i am porting this over from driving the display from a CMOS 4033 decade counter, and the whole circuit was run on 12v. I was not aware that the emitter voltage would be the same as the base. I also made a mistake in that my resistor was only about 200 Ohm not 800).
So, i need to look at using a PNP transistor (with current limiter to the base)? Would the BC550 be OK for the bottom, or should i look to change it as well?

In general, the circuit is fine though? There is some good news then

Thanks for your help
Wayne

p.s. This is the display for a game that i am building to be used for charity fundraising

Hi Wayne,

You should be able to just switch everything over to 5V without any problems, as long as you adjust the resistors accordingly.

I was not aware that the emitter voltage would be the same as the base.

Click to expand...

The emitter is about 0.7V less than the base.

So, i need to look at using a PNP transistor (with current limiter to the base)?

Click to expand...

You definitely need a current limiter, at least for the bottom, but switching to PNP isn't really necessary here if you already have a bunch of NPN transistors.

Would the BC550 be OK for the bottom

Click to expand...

Yes.

Regards,
Chris

Chris,
Sorry to keep coming back on this....but.
I believe that the voltage drop across a segment of the display is 8v, that is why i decided to change to the current setup.
I am needing the 5v Micor to drive a 12v circuit at high speeds (around 1Mhz on the switching for multiplexing).
What type of transistor is now best at top and bottom?

Thanks for your help
Wayne

wikkid said:

I believe that the voltage drop across a segment of the display is 8v

Click to expand...

Oh really? So this is some kind of high power display? Normal LEDs are something like 1.7V. Hm, then you might have to change the transistors or the timing of the multiplexing to handle the current. Do you have a part number for the display?

1 MHz is awfully fast for display multiplexing. I guess you can do it though if your controller can keep up.

Regards,
Chris

RS part number 235-9376
there are 8 led's in each segment.

Ok, thanks for the info. Sorry, I should have guessed that a big 4 inch display would needed a higher voltage.

Yes, you'll have to switch to PNP at the top to get enough voltage to the LED. We're only talking about 20 mA, so basically any PNP is fine.

It'd be good to find an NPN transistor with a higher current rating for the bottom. You can keep the BC550 if you only turn on four of the eight segments at once. Otherwise the rating will be exceeded.

I've attached a schematic that might help you.

Regards,
Chris

Most helpful, Thanks!
Sometimes i wish i was an engineer rather than coming up with ideas and having to have a go on my own, only to get stuck before the end.

I will get the appropriate parts ordered.

Thanks again Chris
Wayne

hey Wikkid,
means you interfaced 20 big displays without using latch or driver ic.Great!
I also want to run 25 displays(common anode) but I couldnot found any driver ic will you please me.
If possible please post ckt dia.