Shopping for LEDs - Forward Voltage, etc.

Aloha!

I'm currently in the process of ordering components for a project that was designed by the manufacturer. The project calls for LEDs, although it doesn't tell me exactly the specs on the LEDs.

Looking at the schematic diagram, the anode is powered by a 5VDC source, and also has a 4.7uF capacitor attached to the same trace. On my tests, the overall voltage reaching the anode is 4.66VDC.

On the cathode end, it is attached to a 150-ohm resistor. On the other end of the resistor is a connection to an FTDI chip, specifically TXLED# or RXLED#. According to the datasheet for the chip, these are receive/transmit data line drivers specifically for LEDs, and they drive from tri-state to low.. If anyone wants to follow along, the FTDI chip a FT232RL (USB UART,) and I'm tapping into pins 22 and 23.

So in preparation for finding LEDs for this project, things like "forward voltage" keeps popping up. The forward voltage on virtually any LED I find ranges from 2.0VDC to 3.4VDC, depending on the color of the LED. I want to use red and blue LEDs. The blue LED I found has a "rated voltage" of 3.2 to 3.4VDC, while the red is 2.0 to 2.2VDC.

I'm trying to understand forward voltage, and it is slowly clicking, but not quite. So based on what I've written, would these LEDs be okay to use?

Thanks!

 

[rickman]

groink1@gmail.com wrote on 9/16/2017 5:35 PM:

Aloha!

I'm currently in the process of ordering components for a project that was designed by the manufacturer. The project calls for LEDs, although it doesn't tell me exactly the specs on the LEDs.

Looking at the schematic diagram, the anode is powered by a 5VDC source, and also has a 4.7uF capacitor attached to the same trace. On my tests, the overall voltage reaching the anode is 4.66VDC.

On the cathode end, it is attached to a 150-ohm resistor. On the other end of the resistor is a connection to an FTDI chip, specifically TXLED# or RXLED#. According to the datasheet for the chip, these are receive/transmit data line drivers specifically for LEDs, and they drive from tri-state to low. If anyone wants to follow along, the FTDI chip a FT232RL (USB UART,) and I'm tapping into pins 22 and 23.

So in preparation for finding LEDs for this project, things like "forward voltage" keeps popping up. The forward voltage on virtually any LED I find ranges from 2.0VDC to 3.4VDC, depending on the color of the LED. I want to use red and blue LEDs. The blue LED I found has a "rated voltage" of 3.2 to 3.4VDC, while the red is 2.0 to 2.2VDC.

I'm trying to understand forward voltage, and it is slowly clicking, but not quite. So based on what I've written, would these LEDs be okay to use?

The voltage drop on a resistor is related linearly to the current, ohms law
which I'm sure you know. The forward voltage drop on a diode is related to
the log of the current and so appears to be relatively constant at currents
above a few hundred uA. So at the operating current (specified in the data
sheet) there will be a small range of forward voltage on the LED depending
on the color. This is because the production of light happens by emitting
quanta which require different energy levels which means different voltages.
Green, Blue and ultraviolet require the highest voltages and Red, Orange
or Yellow require the lowest voltages. White LEDs are either Blue or
ultraviolet with added phosphors to produce other colors resulting in white.

To set the current in the LED subtract the LED voltage from the power supply
voltage and divide by the required current. This will give you the resistor
needed. 5 volts supply minus 2 volts Red LED leaves 3 volts across the 150
ohm resistor giving 20 mA. The Blue LED will draw about 14 mA with that
same value resistor.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998

 

[John Larkin]

On Sat, 16 Sep 2017 14:35:50 -0700 (PDT), groink1@gmail.com wrote:

Aloha!

I'm currently in the process of ordering components for a project that was designed by the manufacturer. The project calls for LEDs, although it doesn't tell me exactly the specs on the LEDs.

Looking at the schematic diagram, the anode is powered by a 5VDC source, and also has a 4.7uF capacitor attached to the same trace. On my tests, the overall voltage reaching the anode is 4.66VDC.

On the cathode end, it is attached to a 150-ohm resistor. On the other end of the resistor is a connection to an FTDI chip, specifically TXLED# or RXLED#. According to the datasheet for the chip, these are receive/transmit data line drivers specifically for LEDs, and they drive from tri-state to low. If anyone wants to follow along, the FTDI chip a FT232RL (USB UART,) and I'm tapping into pins 22 and 23.

So in preparation for finding LEDs for this project, things like "forward voltage" keeps popping up. The forward voltage on virtually any LED I find ranges from 2.0VDC to 3.4VDC, depending on the color of the LED. I want to use red and blue LEDs. The blue LED I found has a "rated voltage" of 3.2 to 3.4VDC, while the red is 2.0 to 2.2VDC.

I'm trying to understand forward voltage, and it is slowly clicking, but not quite. So based on what I've written, would these LEDs be okay to use?

Thanks!

Sure. The brightness will depend on the LED and on the series
resistor, and on your subjective needs, so the best thing to do is
experiment with resistor values until you like what you see. 150 ohms
might be a tad low.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

On Saturday, September 16, 2017 at 1:48:35 PM UTC-10, John Larkin wrote:

Sure. The brightness will depend on the LED and on the series
resistor, and on your subjective needs, so the best thing to do is
experiment with resistor values until you like what you see. 150 ohms
might be a tad low.

I've also seen 270 ohms tossed around. Brightness is more of a visibility matter, so is it possible that the amount of resistance depends on the type of LED, for example thru-hole vs SMD? Since SMD is much smaller in physical size, would it be safe to say that the designer chose 150 ohms because he also assumes SMD, in order to overcome the smaller size to achieve better visibility?

 

[rickman]

groink1@gmail.com wrote on 9/16/2017 10:22 PM:

On Saturday, September 16, 2017 at 1:48:35 PM UTC-10, John Larkin wrote:
Sure. The brightness will depend on the LED and on the series
resistor, and on your subjective needs, so the best thing to do is
experiment with resistor values until you like what you see. 150 ohms
might be a tad low.

I've also seen 270 ohms tossed around. Brightness is more of a visibility matter, so is it possible that the amount of resistance depends on the type of LED, for example thru-hole vs SMD? Since SMD is much smaller in physical size, would it be safe to say that the designer chose 150 ohms because he also assumes SMD, in order to overcome the smaller size to achieve better visibility?

I don't think the mounting type impacts the visibility much, but I guess if
you have an LED buried in the innards of a unit with a long light pipe it
could impact visibility a bit. I used 20 mA on an SMD LED and found it was
way brighter than needed. I ended up pulsing the LED with a 4:1 duty cycle
(or maybe it was 8:1) rather than add another line item to the BOM.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998

 

[Jack]

<groink1@gmail.com> wrote:

Aloha!

I'm currently in the process of ordering components for a project that was
designed by the manufacturer. The project calls for LEDs, although it
doesn't tell me exactly the specs on the LEDs.

call customer, ask which LED they want.

Bye Jack
--
Yoda of Borg am I! Assimilated shall you be! Futile resistance is, hmm?

 

[default]

On Sat, 16 Sep 2017 19:22:58 -0700 (PDT), groink1@gmail.com wrote:

On Saturday, September 16, 2017 at 1:48:35 PM UTC-10, John Larkin wrote:
Sure. The brightness will depend on the LED and on the series
resistor, and on your subjective needs, so the best thing to do is
experiment with resistor values until you like what you see. 150 ohms
might be a tad low.

I've also seen 270 ohms tossed around. Brightness is more of a visibility matter, so is it possible that the amount of resistance depends on the type of LED, for example thru-hole vs SMD? Since SMD is much smaller in physical size, would it be safe to say that the designer chose 150 ohms because he also assumes SMD, in order to overcome the smaller size to achieve better visibility?

Like Rickman has suggested. This is on a data line(s)? If so, they
are already going to be pulsed on and off so the low value may be to
control the average current. (and heat)

I find the best course is to experiment. There's nothing more
irritating to me than finding the LED I intend as a pilot light is so
blinding I can't read the displays. Some color leds produce more
output for the same current, and the angle the typical 5mm leds can be
anywhere from 40 degrees to 6 degrees so the apparent brightness
depends on where you might be standing.

Flat SMD leds have ~120 degrees of beam spread. (to the half-power
points off-axis)

 

[John Larkin]

On Sat, 16 Sep 2017 19:22:58 -0700 (PDT), groink1@gmail.com wrote:

On Saturday, September 16, 2017 at 1:48:35 PM UTC-10, John Larkin wrote:
Sure. The brightness will depend on the LED and on the series
resistor, and on your subjective needs, so the best thing to do is
experiment with resistor values until you like what you see. 150 ohms
might be a tad low.

I've also seen 270 ohms tossed around. Brightness is more of a visibility matter, so is it possible that the amount of resistance depends on the type of LED, for example thru-hole vs SMD? Since SMD is much smaller in physical size, would it be safe to say that the designer chose 150 ohms because he also assumes SMD, in order to overcome the smaller size to achieve better visibility?

A surface-mount LED may have a bigger or a smaller, or a more or less
efficient, chip inside as compared to a leaded part. There is a huge
range of variation in LED brightnesses amd efficiency.

If we put an LED on a circuit board to indicate power up or maybe FPGA
configured, we might run 100 uA into it, so as to not blind people
probing nearby. If an LED shoots light through a hole in a PCB, then
through a silicone pushbutton layer, then through a hole in a metal
front panel, then through a frosty sticker (all of which I just did)
different colors might look good at 3 to 12 mA.

Buy some LEDs and try it.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics