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Classic LED displays - Printable Version

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Classic LED displays - John Robinson - 10-01-2009

Hi, can anyone tell me what is the normal forward voltage and current for a segment in the classic LED displays ?


Re: Classic LED displays - Joel Setton (France) - 10-02-2009

Hello John,

The average current in a segment typically ranges from 1 to 3 mA. However, the Classic display is multiplexed, so the peak current is approximately 15 times this value. You'll find a detailed description of the Classic display multiplex mechanism in one of the HP-35 patents, available on Eric's website: http://www.brouhaha.com/~eric/hpcalc/patents/3755697.pdf.

I hope this helps!

Joel Setton

Re: Classic LED displays - John Robinson - 10-02-2009

Thanks Joel, I'll check out the patent. I just wanted the information for constructing a tester for the classic displays. I won't be multi-plexing, just driving each segment with it's own current limiting resistor.


Re: Classic LED displays - Jacques Laporte - 10-05-2009

Good question John.

You put the 3.6 V DC '+' polarity on pin 14 of a Classic LED cluster (Anode a) through a 100 ohms resistor (here I took a 82 ohms handy) and the negative polarity on pin 7 (cathode 1) ; you light segment 'a' on digit 1.
If you use a 300-400 ohms resistor the light will be dimmer.
These LED voltage drop is 1.7V .

A cheap Classic Led tester!

See the Led Cluster pin out on my site here (to test other segments and other cathodes):

HP-35 Display system

Edited: 10 Oct 2009, 4:44 p.m. after one or more responses were posted

Re: Classic LED displays - Andrés C. Rodríguez (Argentina) - 10-06-2009

Red LEDs forward drop is 1.6 Volt (Plank's constant and the electron charge are related to such voltage drop and the wavelenght of the red light obtained; but that is another story), so you can select your resistor as per

R = [[VCC - 1.6 V]*1000] / I


R is the series resistor, in ohms

VCC is the power supply voltage, in volts

I is the desired current, in milliamperes

For VCC = 5 Volt, and I = 10 mA, a 330 Ohm resistor is adequate. Please check with the display spec sheet about a safe current value before starting your tests. Good luck!!

All this assumes a permanent condition, no multiplexing or pulsing of any kind.

Re: Classic LED displays - Joel Setton (France) - 10-10-2009

Hi John,

As I was sorting old stuff I found the datasheet for the Classic displays (5082-7405). If you'd like a copy, please let me know.

Joel Setton