Unexpected behavior from vintage 7-SEG display

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
I found a big bag of old 7-SEG display, turns out these are FND-503 type, common anode (originally manufactured by Fairchild I think).

Anyway I was playing with some of these (they are surplus I bought many years ago and promptly forgot about) and found that at 1.6V they begin to faintly glow with around 16mA flowing, then at 1.7V they go brighter and consume 48mA, but then as I increase the voltage further the current is rock steady at 48mA and the brightness shows no perceptible change.

A forward biased diode would be expected to obey ohms law-ish once its started to conduct, so what gives?

(Also no datasheet can be found, I was able to deduce connections from experiments and there are datasheets for FND-500 etc which are likely close)
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
How did you connect the display to power?
Are you using individual resistors on the segment pins?
All eight pins (seven segments and dot) are connected to -Ve and the anode to +Ve and I have a precision multimeter measuring the DC current as I manually increase the applied voltage (which has 0.01 V granularity), so this is effectively eight LEDs in parallel.
 

dl324

Joined Mar 30, 2015
14,321
I was playing with some of these (they are surplus I bought many years ago and promptly forgot about) and found that at 1.6V they begin to faintly glow with around 16mA flowing, then at 1.7V they go brighter and consume 48mA, but then as I increase the voltage further the current is rock steady at 48mA and the brightness shows no perceptible change.
Old LEDs, and many new ones that aren't high power, are spec'ed to operate at a maximum continuous current of about 20mA. They can operate at higher currents when pulsed.
1640539938084.png
1640539975471.png
Note that even though forward voltage is specified at 60mA, the maximum continuous or average current is 20mA:
1640540125618.png
The IV characteristic you observed is somewhat expected:
1640540180280.png
Brightness isn't linear with current and the human eye is logarithmic, so you can't differentiate a brightness change of less than about a factor of 2.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
Old LEDs, and many new ones that aren't high power, are spec'ed to operate at a maximum continuous current of about 20mA. They can operate at higher currents when pulsed.
View attachment 256103
View attachment 256104
Note that even though forward voltage is specified at 60mA, the maximum continuous or average current is 20mA:
View attachment 256105
The IV characteristic you observed is somewhat expected:
View attachment 256106
Brightness isn't linear with current and the human eye is logarithmic, so you can't differentiate a brightness change of less than about a factor of 2.
Yes I understand that, what puzzles me is the contancy of the current as voltage is increased to 2, 3, 4 and more volts, like the current is rock steady at just over 48mA.
 

MrChips

Joined Oct 2, 2009
25,930
This has been stated repeatedly. Don't do what you are doing.
Put a 470Ω resistor in series with the LED otherwise you are at risk of burning out the LED.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
This has been stated repeatedly. Don't do what you are doing.
Put a 470Ω resistor in series with the LED otherwise you are at risk of burning out the LED.
Hi, let me explain, I don't care if the device gets damaged, I have forty of them in a junk box. I am simply experimenting. I will add a resistor though as it will help make these adjustments more smoothly.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
This has been stated repeatedly. Don't do what you are doing.
Put a 470Ω resistor in series with the LED otherwise you are at risk of burning out the LED.
Only had 390 Ohm, but anyway, at around 10V now the device glows weakly and pulls around 20 mA, as I adjust voltage the current varies, but does eventually reach 43mA again and never rises thereafter.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
Hi A,
An LED is a Diode, and diodes do NOT follow Ohms Law.
E
I know, that's why I wrote "obey ohms law-ish once its started to conduct" because once conduction reaches a certain point we expect the current to rise rapidly for small increases in voltage.

1640544913444.png
This is what puzzles me, what am I failing to take into account here?
 

sghioto

Joined Dec 31, 2017
3,061
the FND500) is designed to pass no more than 25mA continuously give or take.
That's the max current per segment. The specs say low current requirement of only 5 ma per segment.
In your setup you are showing 25 ma for the entire display. Divide by 8 and its only 3 ma per segment
 

ericgibbs

Joined Jan 29, 2010
15,353
I know, that's why I wrote "obey ohms law-ish once its started to conduct" because once conduction reaches a certain point we expect the current to rise rapidly for small increases in voltage
hi A,
They do not even obey Ohms Law-ishly .
Forget the Ohms Law and focus on the published V/I curves for your particular LED.
E
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
hi A,
They do not even obey Ohms Law-ishly .
Forget the Ohms Law and focus on the published V/I curves for your particular LED.
E
Look at the example chart I included above, between 1V and 4V the line is approx straight, that is consistent with Ohms law Eric.
 

Thread Starter

ApacheKid

Joined Jan 12, 2015
618
That's the max current per segment. The specs say low current requirement of only 5 ma per segment.
In your setup you are showing 25 ma for the entire display. Divide by 8 and its only 3 ma per segment
Yes, you're right, I was hasty it is 25mA per-segment! I just adjusted everything again and increased the current limit.

OK 1.78V causes almost exactly 200mA and it looks like I'd expect it to look, pretty bright but not as bright as today's efficient one.

Thanks, this was all due to me being a bozo!
 
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