Comments (6)
carneeki
commented on June 6, 2024
Maybe it's meant to be 270 ohms? We can swap the resistance and I_led in
the equation to get I_led:
(5-1.8) / 270 = 0.012 = 12ma
(5-3.6) / 270 = 0.005 = 5ma
and don't seem unreasonable values, though 5ma might be too dim for some
use cases.
…On Mon, 12 Feb 2024 at 20:51, Marco ***@***.***> wrote:
Hello Terje,
Sorry to bother you.
I made some layout changes to your MPG DRO board, to make it a little more
versatile (in my opinion), but the circuit is the same.
Among the various small modifications (I'll tell you about them in detail
in another context), I added a double Pad to the LED resistors.
In this way, the predefined SMD resistors (on bottom) can be easily
replaced with Custom resistors on Top
(1/4W, standard tht), simply cutting the track of the corresponding SMD
resistor (the precise point is indicated on Bottom).
Finally, the calculation is needed for the correct resistance value, for a
given LED Vf (based on the color chosen) and for a given current (say 10mA,
as an average value).
I imagined it was:
R(led) = (Vcc - Vf) / Iled
So giving two examples:
(5 - 1.8) / 0.010 = 320 ohm (red LED, Vf=1.8V)
(5 - 3.6) / 0.010 = 140 ohm (white/blue LED, Vf=3.6V)
But in this post:
#3 (comment)
<#3 (comment)>
you said you used 2.7K resistors for standard 3 mm LEDs, obtaining
different light intensities, I suppose depending on the color of the LEDs.
If the formula above was correct, with a res. from 2.7K, I would get about
1 mA for a red LED, 0.5 mA for a white one.
Something doesn't add up.
Where am I doing wrong?
Regards,
Marco
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Overlights
commented on June 6, 2024
Hi Adam,
Exactly.
12 mA would be an absolutely reasonable value, while 5 mA, a bit low.
Having to choose a conservative standard value (for pre-soldered SMD resistors), it would be the one for an average LED current of 10 - 15 mA, for the lowest Vf value (red LED, approximately 1.8V).
So:
(5 - 1.8) / 0.010 = 320 ohms
(5 - 1.8) / 0.015 = 213 ohms
Or as you were saying, 270 ohms, for 12 mA.
All LEDs with higher Vf will have a proportionally lower brightness.
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carneeki
commented on June 6, 2024
Hi Marco,
I'm assuming you're wanting to optimise either the number of resistors you
keep on the shelf (or maybe how many reels a pick'n'place machine will need
loaded). To that end, I'd suggest using the same value resistor either in
series or parallel, and the E24 resistor values are really well optimised
for both cases -
For red (1.8V):
(5-1.8) / 300 = 10.7mA
For white / blue (3.6V), 2x 300 in parallel:
(5-3.6) / 150 = 9.3mA
This comes at the compromise of needing to use more components, but they're
the same value. It also permits (with a solder jumper and the parallel
pair) the option to have this as something the end user can choose if they
are plugging LEDs into a header.
…On Mon, 12 Feb 2024 at 21:37, Marco ***@***.***> wrote:
Hi Adam,
Exactly.
12 mA would be an absolutely reasonable value, while 5 mA, a bit low.
Having to choose a conservative standard value (for pre-soldered SMD
resistors), it would be the one for an average LED current of 10 - 15 mA,
for the lowest Vf value (red LED, approximately 1.8V).
So:
(5 - 1.8) / 0.010 = 320 ohms
(5 - 1.8) / 0.015 = 213 ohms
Or as you were saying, 270 ohms, for 12 mA.
All LEDs with higher Vf will have a proportionally lower brightness.
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Overlights
commented on June 6, 2024
No Adam,
I'm not going to change the default value of the SMD LED resistors, just make sure it was correct.
I guess Terje can clarify this.
As written above, I modified the layout of the Board with a double SMD - THT pad, for all the LED resistors.
The SMD resistors will all have the same value, suitable for the worst condition described above (for example, 220 ohm, for a red LED with a Vf of 1.8V for approximately 15 mA, or 270 ohm for approximately 12 mA)
But anyone will be able to implement LEDs of other colors, therefore with a specific Vf, replacing the default SMD resistor with a 1/4W THT resistor of the most suitable value.
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terjeio
commented on June 6, 2024
Not easy this as modern LEDs seems to vary quite a bit in brightness depending on the color and perhaps the manufacturer as well?. E.g. a test board I made with SMD LEDs uses resistors ranging from 680E to 4K7. So having the option of adding a second resistor is IMO a good idea.
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Overlights
commented on June 6, 2024
Thank you Terje,
I mainly wanted to make sure that the calculation for the LED resistors was correct, so that I could choose a standard value for all SMD resistors.
As described above, I would use 270 ohm, a suitable value for a standard Red LED (3 or 5 mm.). So a Vf of around 1.8V for 12 mA.
LEDs with different characteristics can still be implemented, using the second THT Pad for custom resistors.
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