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I have a simple question, but cannot find the answer anywhere and the datasheets are not very clear. Each 7-segment display have maximum current per segment. If this current is 40mA, single digit display can draw 40mA * 7segments = 280mA. OK...

Here is the question - if a datasheet of a 4 digit display states maximum current per segment 40mA, does a 4 digit display draw 280mA * 4digits = 1.112A (a lot!) ? Or does the value mean "maximum current per segment of all digits shining simultaneously" ?

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    \$\begingroup\$ Share the datasheet link in the question \$\endgroup\$
    – User323693
    Aug 20 '19 at 22:42
  • \$\begingroup\$ These may be of interest: LED rated current and Multiplexed display. (My musings.) \$\endgroup\$
    – Transistor
    Aug 20 '19 at 22:45
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    \$\begingroup\$ 40mA will be very bright. Do you really need it that bright? \$\endgroup\$
    – user253751
    Aug 20 '19 at 22:54
  • \$\begingroup\$ There's been no added thoughts from you, j.kaspar. Is this a hobbyist question? Or something more of a professional project nature? There are many additional facets to consider when making a good 4-digit display and not all of them are electrical ones. For example, there is a whole field on the topic of optical enhancement methods to improve the readability of displays that has little (though some) to do with anything electronic. Could you discuss more about what you are attempting to achieve? You've given us only a narrow tunnel view. If you'd talk more, you might get better overall advice. \$\endgroup\$
    – jonk
    Aug 21 '19 at 17:04
  • \$\begingroup\$ Thank you for comments and answers. I am aware of multiplexing, maximum vs recommended parameters etc. I just wasn't sure, if I understand the term "segment" correctly. I.e. that 1 segment = 1 LED... @jonk - the methods to improve the readability sounds very interesting. My goal is to create a device, that will display data from various sensors in an industrial enviroment. The primary concern regarding the display is readability from a great distance (like 10 meters or maybe more) \$\endgroup\$
    – j.kaspar
    Aug 22 '19 at 10:40
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You kind of answered your own question: the datasheet states max per segment.

Is your 4-digit display direct-drive or multiplexed? If it is multplexed the max total current will be for one digit only. If it isn't, then it's max current per segment * #segments * #digits, which can be a lot for a big display as you can see.

Multiplexing the digits helps reduce the current since you light up only one digit at a time. You can use this technique even for a non-multiplexed display.

You can also use PWM techniques to manage the LED drive to achieve good brightness with less power.

The tradeoff of multiplexing and PWM is in increased electromagnetic noise due to the switching activity, which you need to manage at the system level.

Finally, you need to consider not driving at max current. You can get very good brightness at even 1/4 the max or so, especially if the LEDs are high-efficiency.

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Yes, your calculation is correct.

However, if the 40 mA/segment rating is listed in the "Absolute Maximum Rating" table in the datasheet, you should not operate the display at that current. Look for a "test conditions" or "Electrical Characteristics" table to see normal or recommended operating current - this will be much less than the Abs Max rating. Also, LEDs are not fussy about current - lower current will result in dimmer displays, but modern LEDs will produce useful light at quite low currents.

As @hacktastical says, multi-digit displays are often multiplexed, so only one digit is on at a time, reducing the maiximum current required.

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