LED Current Limiting Resistor Value

R

ROYMARNEWICK

New Member
Hopefully someone can help with what should be ultra-basic.
A 330R current limiting resistor is always indicated in the user manuals for driving an LED. This seems very high.
My understand for a general purpose 5mm Red led is that there is a 2v drop across the LED and the operating current is typically 20mA. So for a 5V power supply, we would need to drop 3.0V at 20mA. This equates to a 150R resistor. I.e. less than 50% of the recommended 330R.
Please shed some light on this.
 
S

steliosm

Senior Member
Yes, you are right, 330 Ohm resistor might be a bit high, but it makes sense to protect the LED and not drive it at it's limits. This will extend the LED's life. Also, you need to consider that you need to protect the Picaxe's pin as well, which has a sink/source capability of 20mA, if you are driving the LED directly from a pin.
 
Technoman

Technoman

Senior Member
Hi,
You are right.
For a reduced brightness that may suit your need, a lower current e.g. 10 mA and Vf=1,8V, then the resistor value would be : 320R. The nearest standard resistor value in the E24 serie (5%) is 330R.
Calculator : Digi-key's calculator
An interesting discussion here : Led brightness
 
R

ROYMARNEWICK

New Member
steliosm said:
Yes, you are right, 330 Ohm resistor might be a bit high, but it makes sense to protect the LED and not drive it at it's limits. This will extend the LED's life. Also, you need to consider that you need to protect the Picaxe's pin as well, which has a sink/source capability of 20mA, if you are driving the LED directly from a pin.
Click to expand...
It is good to hear that my understanding is correct. The data sheet states that they are rated for a continuous 20mA at 2V so I have based my design on this. Understanding most data sheets is another matter! I never considered that driving the pin at 20mA may be pushing it as I got smokescreened by the need to use a Darlington driver for 8 LED’s due to these requiring a total of 160mA. If driving individual pins directly, I will now remember to drive the pin at a lower current.
Thanks for your input.
 
R

ROYMARNEWICK

New Member
Technoman said:
Hi,
You are right.
For a reduced brightness that may suit your need, a lower current e.g. 10 mA and Vf=1,8V, then the resistor value would be : 320R. The nearest standard resistor value in the E24 serie (5%) is 330R.
Calculator : Digi-key's calculator
An interesting discussion here : Led brightness
Click to expand...
It is good to hear that my understanding is correct. I did consider driving the LED’s at a lower current of say 15mA but also thought that 10mA would reduce the brightness too much ( but have no evidence to back it up - just gut feel). I have furthermore always understood that redcLED’scshould be driven at 2V - again, probably from my interpretation of datashhets.
Thanks for your help.
 
Aries

Aries

New Member
I use different coloured indicator LEDs on most of my projects. Some time ago, I did a quick study to see how brightness of the different colours varied with resistance, so that I could match the levels reasonably well. The set-up was an ambient light sensor (ALS-5FLAMP, now discontinued) which has a sensitivity similar to the human eye, placed next to the appropriate LED, all in an enclosed box, with a Picaxe reporting the ADC reading.

These were the results:

KohmRedYellowGreenBlueWhite
0.33​
53​
188​
68​
186​
189​
0.68​
28​
173​
35​
138​
189​
1​
19​
129​
23​
106​
189​
2.2​
8​
62​
8​
57​
189​
4.7​
2​
27​
2​
28​
184​
10​
0​
10​
0​
12​
102​
22​
0​
3​
0​
4​
47​
OFF
0​
0​
0​
0​
0​

From this, it is possible to choose appropriate resistors to give a reasonably consistent brightness across the different colours. White is always a problem, because it is so bright.

PS these LEDs were a pack of 100 mixed (20 of each colour) from China, so I don't have any specs for them.
 
G

Goeytex

Senior Member
Personally I would only use the maximum current ( minimum resistance) if I wanted to blind myself (and others) with an LED. I size the resistor based upon the brightness level desired for the particular color and specific type of LED. As a general rule I use the minimum current ( max resistance) that provides adequate brightness. It is easy to overload a micro-controller with bunch of LEDs set to wide open.
 
R

ROYMARNEWICK

New Member
Goeytex said:
Personally I would only use the maximum current ( minimum resistance) if I wanted to blind myself (and others) with an LED. I size the resistor based upon the brightness level desired for the particular color and specific type of LED. As a general rule I use the minimum current ( max resistance) that provides adequate brightness. It is easy to overload a micro-controller with bunch of LEDs set to wide open.
Click to expand...
Goeytex said:
Personally I would only use the maximum current ( minimum resistance) if I wanted to blind myself (and others) with an LED. I size the resistor based upon the brightness level desired for the particular color and specific type of LED. As a general rule I use the minimum current ( max resistance) that provides adequate brightness. It is easy to overload a micro-controller with bunch of LEDs set to wide open.
Click to expand...
Thanks for your input.
I need the LED’s to be bright. As I will be driving about 8 of them, I will be using a 7 channel Darlington chip so there is no need to consider overloading of the Picaxe chip.
 
hippy

hippy

Ex-Staff (retired)
Staff member
I recall 330R, which at one time everyone used, was based on having a 5V supply, a LED with a forward drop of 1V6 and needing a 10mA drive.

Since then, LED technology has moved on, drive drive current for full brightness has increased and they are usually much brighter than they used to be, but that 330R 'default' has stuck around as it is usually suitable in most cases.

I personally find some LED's are far too bright and have either replaced the resistors in products or resorted to applying layers of sellotape to dim their brightness. I have long used 1K8 even with older, less efficient LED's. The unusual 1K8 value because I picked up a reel of 1,000+ cheap.

The advantage of driving LED's with a high resistor value, giving low ~2mA current, is one can attach multiple LED's to a PICAXE without exceeding its current capability, without needing a Darlington driver. That's easier, saves board space, is cheaper, and, these days can be considered "greener", more environmentally friendly.

The disadvantage is they won't be as bright as they could be which may be desired in some applications.
 
R

ROYMARNEWICK

New Member
hippy said:
I recall 330R, which at one time everyone used, was based on having a 5V supply, a LED with a forward drop of 1V6 and needing a 10mA drive.

Since then, LED technology has moved on, drive drive current for full brightness has increased and they are usually much brighter than they used to be, but that 330R 'default' has stuck around as it is usually suitable in most cases.

I personally find some LED's are far too bright and have either replaced the resistors in products or resorted to applying layers of sellotape to dim their brightness. I have long used 1K8 even with older, less efficient LED's. The unusual 1K8 value because I picked up a reel of 1,000+ cheap.

The advantage of driving LED's with a high resistor value, giving low ~2mA current, is one can attach multiple LED's to a PICAXE without exceeding its current capability, without needing a Darlington driver. That's easier, saves board space, is cheaper, and, these days can be considered "greener", more environmentally friendly.

The disadvantage is they won't be as bright as they could be which may be desired in some applications.
Click to expand...
Thanks for your input. All valid arguments. However, in this case I am purposefully seeking a bright light output.
 
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