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abishur
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Figuring out resistor for 7 segment display

Wed Jul 01, 2015 4:30 pm

I'm trying to figure out something here. I've purchased some 1" 7 segment displays, that I'll be powering them off 5v. Looking up the datasheet, it shows that each LED segment is driven by 2 LEDs for a max current PER SEGMENT of 15 mA.

I can easily understand that when the specs say there is 3v4 total voltage drop per segment that it equates to 1v7 voltage drop per red LED. But saying that it wants a 15 mA per segment is really throwing me off here, what are they looking for?
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rpdom
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Re: Figuring out resistor for 7 segment display

Wed Jul 01, 2015 4:47 pm

That sounds straightforward enough. Two LEDs in series per segment. For the normal calculations the LED forward current would be 0.015A and the LED forward voltage would be 3.4V.

For a 5V supply that gives (5 - 3.4) / 0.015 = 106 Ohms. Something like a 110 or 120 Ohm resistor for each segment should do the trick

drgeoff
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Re: Figuring out resistor for 7 segment display

Wed Jul 01, 2015 8:48 pm

1. " Looking up the datasheet, it shows that each LED segment is driven by 2 LEDs for a max current PER SEGMENT of 15 mA." That doesn't mean that they need 15 mA. You can run them with less if the resulting brightness of them is acceptable.

2. Unless you are hard wiring them to show fixed segments, you will have some drivers to turn segments on or off and possibly other drivers to mutiplex the individual digits. If present they will drop some voltage too, perhaps 0.2 volts each. That also comes out of the 5 volt total.

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abishur
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Re: Figuring out resistor for 7 segment display

Thu Jul 16, 2015 6:43 pm

rpdom wrote:That sounds straightforward enough. Two LEDs in series per segment. For the normal calculations the LED forward current would be 0.015A and the LED forward voltage would be 3.4V.

For a 5V supply that gives (5 - 3.4) / 0.015 = 106 Ohms. Something like a 110 or 120 Ohm resistor for each segment should do the trick
Okay, that's what I was leaning towards but sometimes you just read something and it doesn't click right.
drgeoff wrote:1. " Looking up the datasheet, it shows that each LED segment is driven by 2 LEDs for a max current PER SEGMENT of 15 mA." That doesn't mean that they need 15 mA. You can run them with less if the resulting brightness of them is acceptable.

2. Unless you are hard wiring them to show fixed segments, you will have some drivers to turn segments on or off and possibly other drivers to mutiplex the individual digits. If present they will drop some voltage too, perhaps 0.2 volts each. That also comes out of the 5 volt total.
Oh they'll need the full 15mA, I'm slightly disappointed with the brightness being put out by the module. I'm using a MIC2981/82YN to provide the necessary voltages
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