# Is this a good circuit that I have designed?

1. ## Is this a good circuit that I have designed?

Is this a good circuit that I have designed?

I am only concerned about RN8 (24R). It allow a max common anode current of 500 mA per digit and also a max 500mA can sink through each channel of the ULC2823.

ULN2823 can sink max 500mA per channel.

0.5A * 0.5A * 24R = 6W and so I should choose 10W resistors for 24R?

Actually my chosen 4 inch 7 Segment display (CA) draws 50 mA per segment and total 8 segments including dp.

50 mA * 8 = 400mA.

12V/400mA = 30R (ULN2823 pull up)

0.4 A * 0.4 A * 30R = 4.8W per resistor.

How can I avoid this large current sinking from ULN2823 common anode drive channels? If I use a 1k or 10l resistor between 24R and ULN channles then display doesn't turn On.

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Issue solved. I replaced the common anode driver to UDN2982A and added a hex inverter.

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Here is the final one (TD-2.pdf, TD-2.png)

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2. ## Re: Is this a good circuit that I have designed?

Hi,

In case this is a high volume project and you want to safe cost, you may optimize:

Since the MAX7219 is able to drive 8 digits x 7 (8) segments .... it should be possible to drive all displays (small and big) from one MAX7219.
I see the displays need different supply voltages and you solved it with external driver ICs.

If the voltage at the low side driver of the big display is less than 5V at the low side driver when the segment is OFF ....you even may omit extra low side drivers and can be driven directly by the MAX.

Klaus

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3. ## Re: Is this a good circuit that I have designed?

Yes, I had thought about that, that is driving both CA big and CC small displays from the top MAX7219 by connecting the small display between RN1 & RN2 resistor network and MAX7219 and sending data 123 like 123123 but later thought about extending the system as an 8 digit (big) and 8 digit (small) counter.

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Made the single MAX7219 based circuit and code. I still have to clean up the code. It is working but I have to remove leading zeros.

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4. ## Re: Is this a good circuit that I have designed?

Okay, All is well now. Finished the project using one MAX7219 and tested.

Big display is this one. http://www.kingbrightusa.com/images/...SA40-19GWA.pdf

Any changes needs to be done to my circuit?

5. ## Re: Is this a good circuit that I have designed?

I don't see how you can achieve a stable segment current with the driver circuit. The driver ICs have relative large voltage drop, you would need a higher supply voltage, e.g. 15 V to get a certain current regulation effect by the segment series resistors.

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6. ## Re: Is this a good circuit that I have designed?

Okay, I will increase the 12V to 15V for the big display driver. I have not yet built the hardware. I was just testing it in Proteus.

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