Multiplexing large 8Volt 7-Segment Common Cathode

I am trying to figure out how to control my several 7 segments. But right now I just want to get one working and working out the bugs first. Scaling later should be just a matter of determining if I have enough amps.

The 7 segment I am working with is a common cathode with a 8 Forward Voltage. I have it so it is what I am working with. I understand it is supposed to be easier to work with a common anode.

I am working with an Uno R3.

I worked with a smaller 7 segment display and got it to work with a MAX7219 but I realized later that it was a common anode display. I found I couldn't get the MAX7219 to work with the larger display due to lack of knowledge.

So I just wanted to start with a new setup for the larger display. From what I understand I need a current source driver. I am looking for one that can handle up to 3 7-segment displays. I was thinking of running this with a 9 volt wall wart unless you guys think I should go with a 12.

Found this going to look more into the TLC5940.

Based on rough calculations. I am assuming that the voltage for the display is 50mA for each LED. Each segment has 4 LED's so that would mean minimum power for the display required is around 4.2A. Then I would probably need another 500mA for the arduino and misc. If I use their spec of 30mA per LED for steady current then I would only need 3.15A but I am worried of the display being under-powered and won't light up.

Overall project is going to be a skeeball machine for fun.

Found this going to look more into the TLC5940.

That chip is only for common anode devices.

I am assuming that the voltage for the display is 50mA for each LED.

Voltage is measured in volts, current is measured in amps, so that sentence is nonsense.

Each segment has 4 LED's so that would mean minimum power for the display required is around 4.2A.

No the LEDs in a segment are wired in series, so they need only the current for one LED but they need more voltage than a single LED hence the 8V or greater you need.

The data sheet says that 30mA is the maximum current you must allow to flow, so each segment should take 30mA. Therefore the maximum any one display will take is going to be 7 * 30mA = 210mA.

You need what is called a "high side driver", one such chip is the Allegro 2981 chip

...or some pnp transistors.

Hi. You can use 8 pnp transistors like bc327 for the anodes. However, you can't drive these directly with Arduino outputs because they would be exposed the 9/12V and could be damaged. So you need to drive the pnps with either 8 npn transistors like bc337, or uln2803 chip, or tpic6c595 or tpic6d595...

For the anodes, you can just use npn like bc337, or other as long as they are rated for the max current the display consumes. These will only be needed if you are multiplexing the digits.

I think 12V rather than 9V. The pnp transistors on the anodes and the npn on the cathodes will drop around 0.3V each, and you need current limiting resistors for the segments, which also need at least 2~3V drop across them, so 9V is cutting it too fine with your 8V segments.

Do you plan to multiplex the 3 digits? It would reduce the component count and current requirements.


Note that as the decimal point has only two diodes in it you need a different value of resistor for that than for the other segments.

So in the thread someone said they got the TLC5940 to work with common cathode. But if it is for common anode it was probably used with common cathode in a round about way. Yeah, I had a brainfart with saying volts and giving you the current value. So yeah I just learned something, I was thinking the LED's would be treated the same as resistors. The current would drop as it passed through each LED. (Dumb assumption on my part). Mwahahaha. I am gaining knowledge :D.

I think I am going to multiplex to reduce parts and chances of error hopefully which would increase debugging time.

I have to look back at how transistors work (been a while since I have worked with them). Will also look into the high side driver that Grumpy Mike mentioned the, Allegro 2981 chip.

Thanks. I have to do more research on my part and I will come back with more questions when I come across them. This seems like a good start. Thanks for the responses Grumpy Mike and PaulRB.