Sorry of this info is available in another thread - I've read through as much as I can but can't find a simple description of how to select an appropriate LED.
My Spud amp has an 82ohm resistor with 470uF cap for cathode bias. There is about 2.5V over the resistor so there's about 30mA.
How do I use this info to pick an appropriate LED?
Thanks in advance - and Merry Christmas!
My Spud amp has an 82ohm resistor with 470uF cap for cathode bias. There is about 2.5V over the resistor so there's about 30mA.
How do I use this info to pick an appropriate LED?
Thanks in advance - and Merry Christmas!
Type Colour IF
max. VF
typ. VF
max. VR
max. Luminous
intensity Viewing
angle Wavelength
Standard Red 30mA 1.7V 2.1V 5V 5mcd @ 10mA 60° 660nm
Standard Bright red 30mA 2.0V 2.5V 5V 80mcd @ 10mA 60° 625nm
Standard Yellow 30mA 2.1V 2.5V 5V 32mcd @ 10mA 60° 590nm
Standard Green 25mA 2.2V 2.5V 5V 32mcd @ 10mA 60° 565nm
High intensity Blue 30mA 4.5V 5.5V 5V 60mcd @ 20mA 50° 430nm
Super bright Red 30mA 1.85V 2.5V 5V 500mcd @ 20mA 60° 660nm
Low current Red 30mA 1.7V 2.0V 5V 5mcd @ 2mA 60° 625nm
http://www.kpsec.freeuk.com/components/led.htm
max. VF
typ. VF
max. VR
max. Luminous
intensity Viewing
angle Wavelength
Standard Red 30mA 1.7V 2.1V 5V 5mcd @ 10mA 60° 660nm
Standard Bright red 30mA 2.0V 2.5V 5V 80mcd @ 10mA 60° 625nm
Standard Yellow 30mA 2.1V 2.5V 5V 32mcd @ 10mA 60° 590nm
Standard Green 25mA 2.2V 2.5V 5V 32mcd @ 10mA 60° 565nm
High intensity Blue 30mA 4.5V 5.5V 5V 60mcd @ 20mA 50° 430nm
Super bright Red 30mA 1.85V 2.5V 5V 500mcd @ 20mA 60° 660nm
Low current Red 30mA 1.7V 2.0V 5V 5mcd @ 2mA 60° 625nm
http://www.kpsec.freeuk.com/components/led.htm
What? There isn't some magically optimal choice?
Does no one care to specify a dynamic resistance?
Obviously they have enough resistance to drop
the specified volts, but then what happens? How
well do they keep steady voltage as current rises?
Even the shape of the knee might be important.
They can't all be the same. Are two reds better
than one blue? The question has to be asked...
Does no one care to specify a dynamic resistance?
Obviously they have enough resistance to drop
the specified volts, but then what happens? How
well do they keep steady voltage as current rises?
Even the shape of the knee might be important.
They can't all be the same. Are two reds better
than one blue? The question has to be asked...
Hello,
I suggest you read this very good article:
http://syclotron.com/?page_id=3
Kind Regards
Andrew
I suggest you read this very good article:
http://syclotron.com/?page_id=3
Kind Regards
Andrew
I've ordered a bunch of these red LEDs. Page 5 of the data sheet shows that I should get about 2.35V drop with 30mA current.
LED data sheet
The graph looks pretty linear from 20-80mA, so hopefully this will be fine.
I'm assuming that dropping the bias from 2.5V to 2.35V won't be an issue. I actually ordered 15x LEDs so I can test them and use the ones with the highest V drop.
LED data sheet
The graph looks pretty linear from 20-80mA, so hopefully this will be fine.
I'm assuming that dropping the bias from 2.5V to 2.35V won't be an issue. I actually ordered 15x LEDs so I can test them and use the ones with the highest V drop.
From the datasheet graphs, the amber look to have much lower dynamic resistance. The red are about 12 ohms where you want to run them. Amber look to be about 2 ohm, but only a 2V drop. You might consider using two IR LEDs in series which will give you 2.4-2.5V, and the better ones are in the 2 ohm range.
Don't be afraid to mix and match technologies either. If you achieve most of the drop you need with a *good* LED - low dynamic resistance, or maybe even more important - linear resistance within the expected current range, you can use a series resistor for the rest of the voltage drop. This assumes that you have a little gain to burn and that the output impedance still be low enough with the added series resistance.
Sheldon
Sheldon
What about using the amber LED with a couple of schottky diodes?
That would get the voltage with the lowest Rdyn short of a battery.
As the OP said it's a spud amp, so I'm assuming it's an output stage
and low dynamic cathode resistance is king.
If it's a driver stage the Rdyn may be less important.
If it's a high-u spud amp using a 6C45pi for example, then you really
want low low cathode resistance as every ohm in the cathode gives
you ~50 ohms at the anode.
What tube is it?
Cheers,
Michael
That would get the voltage with the lowest Rdyn short of a battery.
As the OP said it's a spud amp, so I'm assuming it's an output stage
and low dynamic cathode resistance is king.
If it's a driver stage the Rdyn may be less important.
If it's a high-u spud amp using a 6C45pi for example, then you really
want low low cathode resistance as every ohm in the cathode gives
you ~50 ohms at the anode.
What tube is it?
Cheers,
Michael
dave slagle said:
I've measured conventional silicon diodes and high speed versions (1N4007/UF4007) and saw numbers in the 4-5 ohm range. Putting enough of these in series to get the desired drop brings the impedance higher than the corresponding LEDs. I haven't tried Schottkys yet, but their whole raison d'etre is the low drop, so even more are needed in series.
If the Scottkys measure low, then Michael's suggestion or combining them with the amber LED is a good one. Otherwise, you can get a 4 ohm impedance with a couple of IR diodes in series.
SY,
Would this be a good choice for an IR LED (2 in series required)?
OPE5594A
The forward voltage increases from 1.3V to 1.4V over 7.5mA to 90mA forward current.
My amp is the Hawthorne Spud kit so this is to bias the 6CL6 tube. Apparently the bias should be 2.5-3V so two of these IR LEDs puts me right in the middle of the recommended range.
Thanks for all the help so far.
Would this be a good choice for an IR LED (2 in series required)?
OPE5594A
The forward voltage increases from 1.3V to 1.4V over 7.5mA to 90mA forward current.
My amp is the Hawthorne Spud kit so this is to bias the 6CL6 tube. Apparently the bias should be 2.5-3V so two of these IR LEDs puts me right in the middle of the recommended range.
Thanks for all the help so far.
sharp, it looks superb at currents over 10mA. Resistance is deltaV/deltaI or about 1.1-1.2 ohms, if my squinting at the chart is correct. Two in series is 2.2-2.4 ohms which is as good as you'd ever want.
The other nice thing about this bias scheme is that, unlike capacitor bypasses, they don't go inductive at high frequencies.
The other nice thing about this bias scheme is that, unlike capacitor bypasses, they don't go inductive at high frequencies.
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