Electronics > Beginners

How can I determine the resistor value for an unknown type of LED?

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xenium:
I'm working on a little project to build a modem emulator for my 8-bit retro computers. Basically, a Raspberry Pi in an old Hayes modem case, which is running a modified version of Jim Brain's tcpser modem emulator software. This will allow me to make Telnet (and other TCP) connections from the retro computer's serial port, using an old-school terminal emulator. Fun for "dialing in" to Internet-connected BBSes and such. The modifications I'm making to the tcpser software will allow the Raspberry Pi to blink the modem status LEDs, which will be attached to the Pi's GPIO lines.

I want it to look just like a real Hayes modem, so I want to re-use the LEDs from the original donor modem. The problem is, I haven't been able to find *any* specs for these LEDs, so I don't know what size current-limiting resistors to use. I have both a 1200 and 9600 baud Hayes modem which I can take the LEDs from. I checked both of them, and found that one of them is using 200 ohm resistors, while the other is using 400 ohm resistors. I have no idea what voltage they are running the LEDs at though, so that info does me no good (and I can't measure it, because I don't have the power supplies for the modems, and I don't think they are in operable condition anyway.) They use 13.5 volt power supplies, so it could be any voltage up to that I suppose.

The LEDs are square, with the anode and cathode coming out of the side of them, so they emit light at 90 degrees to the circuit board they are mounted on. They emit red light, but are in a clear package. There is a dome on the side of the LED that emits the light. I haven't even been able to find a photo of these LEDs online, and the modems are from the 1980's, so it's entirely possible they aren't manufactured anymore.

I've found a number of formulas online for calculating current limiting resistor values, which say to measure the voltage drop across the diode and then use the formula to select a value based on the desired current, but I don't even know what current these should be run at. I've experimented a little, and have gone down as far as 50 ohms @ 5V. The LEDs seem to operate well at that value, and I've left a test one running like that for a few days and it did not burn out or get warm, but that seems like a really low resistor value to me, so I'm afraid I'm overdriving the LED and will cause it to die prematurely. If I use a more typical 200 ohm, or even 100 ohm resistor the LED seems a bit dim though.

Any ideas on what I should use? Does anyone know where I might find specs for these LEDs?

Brumby:
Don't worry too much.  You have done enough practical work to get all the information you need.

I suggest you take this experiment:

--- Quote from: xenium on January 29, 2020, 03:54:48 am ---I've experimented a little, and have gone down as far as 50 ohms @ 5V. The LEDs seem to operate well at that value, and I've left a test one running like that for a few days and it did not burn out or get warm,

--- End quote ---
... and measure the voltages across the LED and across the resistor.

Using Ohms Law for the resistor will give you the current running through the circuit.  Take note of this value.  Also, take note of the voltage across the LED.

Using these two figures, you will then be able to calculate the resistance required for any given supply voltage.

In relation to your concern:

--- Quote ---...but that seems like a really low resistor value to me, so I'm afraid I'm overdriving the LED and will cause it to die prematurely. If I use a more typical 200 ohm, or even 100 ohm resistor the LED seems a bit dim though.

--- End quote ---
... you could try other resistor values between 50 and 100 ohms - choosing the highest value that gives you acceptable brightness and re-do the calculations.

tooki:

--- Quote from: xenium on January 29, 2020, 03:54:48 am ---The LEDs are square, with the anode and cathode coming out of the side of them, so they emit light at 90 degrees to the circuit board they are mounted on. They emit red light, but are in a clear package. There is a dome on the side of the LED that emits the light. I haven't even been able to find a photo of these LEDs online, and the modems are from the 1980's, so it's entirely possible they aren't manufactured anymore.

I've found a number of formulas online for calculating current limiting resistor values, which say to measure the voltage drop across the diode and then use the formula to select a value based on the desired current, but I don't even know what current these should be run at. I've experimented a little, and have gone down as far as 50 ohms @ 5V. The LEDs seem to operate well at that value, and I've left a test one running like that for a few days and it did not burn out or get warm, but that seems like a really low resistor value to me, so I'm afraid I'm overdriving the LED and will cause it to die prematurely. If I use a more typical 200 ohm, or even 100 ohm resistor the LED seems a bit dim though.


--- End quote ---
50 ohms sounds risky to me. Indicator LEDs are typically rated for 10-20mA drive current, while 50 ohms on 5V is about 70mA for a standard 1.8V red LED.

Remember that LEDs from that era weren’t nearly as bright as modern ones, so if you drive them to modern brightness levels, you’re overdriving them significantly and will reduce their lifespan.*

Now, some research found a source that says that the very first red indicator LEDs (1970s) were specced for drive currents of 50mA, while late 70s to mid-80s-onward high efficiency (“low current”, “super high brightness”, and “high efficiency” types) red LEDs were typically rated for 20mA, often with 30mA absolute max.

I’d either measure the actual current across them in their original circuits (just measure the voltage across the dropper resistor and use ohm’s law to calculate the current), or just stick with the usual 220 ohms for 5V, which will drive them at a conservative 15mA, or 180 ohms which is around 20mA. If you need more brightness, find a modern substitute. There’s a good chance something is still available in the same package. Heck, I’ll even help you look, since I’ve spent unhealthy amounts of time looking at LEDs on eBay, AliExpress, and other suppliers and so have a good grip on the right keywords and vendors. :p


*When I resumed my electronics hobby after a decade+ hiatus, I got various modern LEDs, and also dug out my bin of late-80s radio shack and mail-order grab bag LEDs. When testing, I was surprised at just how dim those LEDs are. Like, I knew they’d be dimmer, but I’d forgotten just how big the difference was! (And that’s not even considering the new-type ultra-efficiency green and blue LEDs, whose intensity is just insane.)

P.S. #1 It’s almost guaranteed that the modems’ internal circuitry used 5V for the logic that drove the LEDs, so using the same resistance values they used is a safe bet. In the unlikely chance they used a higher voltage, then you’d merely be running them at lower current.

P.S. #2 Another potential solution is to use modern light pipes and modern LEDs, or even to saw apart the old LEDs to turn them into light pipes for use with modern LEDs.

atmfjstc:
I couldn't find any schematics for Hayes modems, but if you could take a hires pic of the top and bottom of the PCB, we might be able to figure out what voltages the original LEDs would have been driven at. Possibly 9V for the 400Ohm resistor ones.

Jwillis:
This is how I get an approximation of the forward current of a LED.
I take a  resistor and measure the exact resistance. Record that resistance
Set up a the led and resistor in series on a breadboard. Apply a voltage until  you Just achieve the brightest LED.I do this in the dark because it helps me  to see small changes to the brightness.  You want the transition where the difference in brightness is very noticeable .Adding more voltage will only make very small changes. Measure the actual Voltage of the source. I do this because the readout on my supply is a little out. Record that voltage.  Measure across the LED. and record that voltage.

Plug the numbers into  R=(Vs-Vled)/Iled

Ohms law I= E/R

For example I have a resistor that measure 470.5 \$\Omega\$   The voltage applied to circuit is 10 volts for the brightest LED. I kind of play with the voltage up and down.
and the voltage across the LED is 2.070

470.5= (10 - 2.070)/I is is to say I= 7.93/470.5 = 16.8mA 
Close enough to 20mA rating .

I find this works even for some high power LEDs I have . Some with Vf as high as 60V and If as high as 1A

The brightness of LEDs and the If is non linear so you kind of have to play with the source voltage and make the best judgment you can.

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