7 segment display brightness low

hi guys!

I have put together a little circuit that has two 4 digit CA displays one small and one big.
the problem with the big display that it isnt bright enough. i have checked its individual segment current and found it to be 15mA. normally a display of this size, 4inch height, is rated for 40-50mAs current per segment. so i conclude that the display is not getting sufficient current.
i am using a 12v supply for the big display and 100 Ohms current limiting resistors for each segment.
i dont think there is any issue with the code/multiplexing as otherwise my small display too would have been dim.
attached are the display connections and driver connections for your reference. i havent connectd my 2003 com port to 12v supply but i guess thats ok.
thanks in advance.

cheers,
marrc

display1.pdf (16.1 KB)

TD_1.PDF (65.2 KB)

Can you post some images of the hardware? You may have made a wiring error. Why do the diagrams above not match? Which one should I believe? What do you mean, "i havent connectd my 2003 com port to 12v supply but i guess thats ok"?

Also, you say nothing about software. Are you taking measurements with a fully functional sketch with all the bells and whistles, or are you running a display test program? Have you measured the current in a static state (no scanning)?

I think the ULN2003A's are current sinks. For the common anode (digit) drivers you need current sources. The only current going into the common anodes of the big displays come from the 10K pull-up resistors which you have fighting a set of 10K pull-down resistors!

To get more brightness, at least take out the pull-down resistors. Then reduce the values of the pull-up resistors to allow at least 280 mA to get to the Common Anode to be shared among the segments. I think this might be a problem for the ULN2003A which is rated to sink 250 mA.

I think the proper solution is a driver that can source 280 mA to the Common Anode (more if you use the Decimal Point?)

johnwasser:
I think the ULN2003A's are current sinks. For the common anode (digit) drivers you need current sources. The only current going into the common anodes of the big displays come from the 10K pull-up resistors which you have fighting a set of 10K pull-down resistors!

I thought so too, but it's not the case. It's a case of poorly chosen signal names, DIG1_BIG and DIGIT1_BIG that blur together and read the same in the tiny print. The digit anodes are connected to the collectors of Q2-Q5.

But since you mention it, consider the current when all segments are on - about 10V/100ohms * 7 = 700mA. Now I have doubts about the ability of the transistor circuits to supply that much and stay saturated. It would be a good idea to measure the voltage at the collector with all the segments on.

aarg:
I thought so too, but it's not the case. It's a case of poorly chosen signal names, DIG1_BIG and DIGIT1_BIG that blur together and read the same in the tiny print. The digit anodes are connected to the collectors of Q2-Q5.

You're right. Nevermind. I confused "DIGx_BIG" and "DIGITx_BIG".

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Thanks in advance for your cooperation.

I will assume the 15mA is for a single segment, and that the same segment is illuminated in all 4 digits when testing (so as to eliminate the multiplexing).

Otherwise 15mA through a single segment with only that segment illuminated and the other 3 off is approx 15 x 4 = 60mA.

15mA through 100R = 1.5V

That leaves 12V - 1.5V = 10.5V unaccounted for.

What is the quoted forward voltage of the DIS1 display segments?

Is that entire 10.5V dropped across the display segment?
Or is voltage also being dropped across the LN2003A or the BC327s?

Measure and find out. Then you will have your answer as to if decreasing the current limiting resistors will actually increase brightness.

Oh, yeah, I forgot about multiple LEDs in series per segment... but with a 3 inch display, there shouldn't be too many. Still, with 100 ohm limiting resistors, 15mA sounds very small.

aarg:
Can you post some images of the hardware? You may have made a wiring error. Why do the diagrams above not match? Which one should I believe? What do you mean, "i havent connectd my 2003 com port to 12v supply but i guess thats ok"?

Also, you say nothing about software. Are you taking measurements with a fully functional sketch with all the bells and whistles, or are you running a display test program? Have you measured the current in a static state (no scanning)?

disp1 shows how digit1-4 are connected to the rest of the ckt. i have not conected com pin of my 2003 it is open. i will try measuring current without multiplexing.

johnwasser:
You're right. Nevermind. I confused "DIGx_BIG" and "DIGITx_BIG".

DIGX_BIG is connected to DIGITx_BIG. RN1-9 do not exist in my ckt. they are there only for simulation purpose. current flows from 12v supply to the emitter and through the collector of pnp transistor and into the respective cathode segment. each of the cathode is connected to 2003 op while the common anode is connected to the collector of pnp transistor. so to turn on a digit only the base current needs to be supplied by 2003.

Looking at your display.sch. All i can say i don't think i would do it like this, because you can never have more than a 25% duty-cycle per 7-seg. I drive my 7-segments using CMOS 4015-bitshifters instead of the (one for each 7-seg) and use the (in your case PNP) transistor to actually control the brightness and momentarily dim the segments while shifting out the data. (though this is not actually required, i can not see them light up when i update) This means i only need 3 pins for the whole thing, Data, Clock & Brightness, and my CPU is only occupied for a moment. Constant updating does cause flicker, but there is no need for constant updating.
Works well for me at 5v, at higher voltages you may need open collectors to dissipate more than 200mW the 4015 can dissipate.

Have you tried connecting up the displays on a breadboard so you can have them lit constantly and compared the brightness of the different type displays? You may need to experiment a bit to get what you would perceive as equal brightness, because of the differences in the actual LEDs, the material the display is made of, area that the LED has to light, etc, I'd expect the smaller display to appear brighter just because the light from the LED is concentrated into a smaller segment area.

Hopefully you are scanning the displays as two groups of four instead of a single group of eight, scanning through eight digits would keep each digit on for a shorter duration and require higher current for a given brightness.