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I have sourced some LEDs from everyone's favorite high-quality electronics marketplace, eBay.

I have a simplistic "datasheet" for the 7-segment LEDs that I purchased, which show a Vf of 3.8v across an array of LEDs. Does this give me enough information to know what current to drive these LEDs at or what a "safe" value would be? If not (and I am fairly certain it does not) what approach could I take to ensure I don't burn out my LEDs?

LED Datasheet

mark
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  • It varies highly. [LEDs are current-driven devices](http://electronics.stackexchange.com/questions/151627/why-does-an-led-have-a-maximum-voltage). – Shamtam Nov 26 '15 at 04:10
  • If you have doubts then a series resistor will avoid tears .The internal or bulk resistance of the leds is generaly low and poorly specified ,hence it shouldnt be relied apon to keep current safe.Many chinese torches connect a white led directly to a lithium cell and they all die sooner or later. – Autistic Nov 26 '15 at 04:48
  • @Autistic Agreed. I plan on driving these with special LED driver ICs. Thanks for the note. – mark Nov 26 '15 at 05:07
  • Golden rule. DO NOT BUY ANYTHING THAT DOESN'T HAVE A DATA SHEET. – Andy aka Nov 26 '15 at 08:44
  • The [datasheets](http://www.arkch.com/jszc_list/&newsCategoryId=6&comp_stats=comp-FrontNewsCategory_tree01-1332378552312.html) are in Chinese. – CL. Nov 26 '15 at 10:02

2 Answers2

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It looks to me that the "3,8" shown on the drawing is the pin numbers of the common anode or common cathode connection, not the recommended voltage.

If the drawing is correct, there are six LEDs in series for each segment. A typical red LED has a forward voltage of about 1.8 volts, so you will require somewhat more that 10.8 volts to light a segment. 20 mA seems to be a safe current for most LEDs, but they will work (but be dimmer) at lower currents - I'd aim for 10 mA for a start.

If you have a 12 volt supply, you want a series resistor that will drop 1.2 volts (12 - 10.8) at 10 mA, which would be 120 ohms.

For the decimal point, with only two LEDs, the resistor should be abou 820 ohms.

To be certain of the actual voltage and permissible current, you should demand a proper datasheet from the vendor.

Peter Bennett
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  • Excellent point about the "3,8" pin numbers. I did not realize that. These LEDs are actually white but do come in red variants. I have requested the additional information from the seller. – mark Nov 26 '15 at 05:04
  • White LEDs are generally a bit over 3 volts, so you'd need a ~20 volt power supply. If you have a variable power supply, you can easily find the required voltage of a segment by connecting a 200 ohm, or so, resistor in series with a segment, across the supply, and turning the voltage up til the segment produces a suitable brightness. – Peter Bennett Nov 26 '15 at 05:14
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No it doesn't, the specified forward voltage and the safe forward current are largely unrelated. Also the diagram hows many LEDs in series, 3.8V seems a rather high voltage for an individual LED but rather low for a chain of that many LEDs.

10ma is safe for most LEDs but the only way to be absoloutely positive is to get in touch with the manufacturer.

Peter Green
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