I am buying some resistors and some of them have axial-leaded in their descriptions.

What does this actually mean? I have had a look around and its something to do with the construction process?

Does it mean anything in terms of performance? I was looking at some 1% tolerance resistors (http://uk.farnell.com/welwyn/mfr4-100rfi/resistor-metal-film-100r-0-5w/dp/1099867) but then found some 0.1% (http://uk.farnell.com/welwyn/rc55y-100rbi/resistor-metal-film-100r-0-25w/dp/9499865).

Both say axial-leaded. The 0.1% tolerance one is also small and black - does that mean anything or just how it looks? Since 0.1% tolerance is obviously better as I'm powering LEDs that need to be accurate in their light output.

Haven't bought resistors before so I'm a newbie sorry!

 

I believe that is just the package type. Here is some more information
https://en.wikipedia.org/wiki/Through-hole_technology#Axial_vs._radial_leads
and by the way, I highly doubt you need .1% resistors, are you looking for a specific number of lumens?

 

I am buying some resistors and some of them have axial-leaded in their descriptions.

What does this actually mean? I have had a look around and its something to do with the construction process?

Most through hole resistors will have axial leads; which means the leads come out of the "ends" of the body.

Other form factors are SIP/DIP for resistor networks. You can sometimes buy axial resistors with the leads bent so they can be mounted vertically.

Does it mean anything in terms of performance?

No.

I was looking at some 1% tolerance resistors (http://uk.farnell.com/welwyn/mfr4-100rfi/resistor-metal-film-100r-0-5w/dp/1099867) but then found some 0.1% (http://uk.farnell.com/welwyn/rc55y-100rbi/resistor-metal-film-100r-0-25w/dp/9499865).

Both say axial-leaded. The 0.1% tolerance one is also small and black - does that mean anything or just how it looks? Since 0.1% tolerance is obviously better as I'm powering LEDs that need to be accurate in their light output.

Don't waste your money on tight resistor tolerance for LEDs. The luminous intensity varies exponentially with forward voltage and variation within the same lot will be more than a percent.

What type of sensor will be used to determine relative brightness of the LEDs? The human eye response is logarithmic and variations of 2:1 are seldom noticeable.

 

It has little to do with performance and nothing to do with tolerance. It is simply the physical packaging of the component.

https://www.google.com/search?q=axi...=5tgDV7igIMm0jwOesaPADw#imgrc=LPJBYnP9DRMSfM:

A separate issue is your notion that you need LEDs that are "accurate" in their light output. What do you mean by that? If you make your circuit and simply swap out one LED for another the light output will change by far more than what would happen if you changed the resistor value by 0.1%, so why waste money buying overly precise resistors that get you nothing?

 

I am creating an LED array for optogenetic stimulation (like they do with neurons). So we need to be able to control the PWM brightness of the LEDs precisely etc which we're doing with some LED driver chips, as well as how long they are on/off for. So it will be tested with some sort of photodiode circuit to make sure they are behaving as expected.

Probably be tested with the photodiodes to create a scale from 1-100% brightness and what output intensity that gives on the 'subject' surface. So then the user can select say 40% brightness (giving 36.4mW/mm2 or whatever, (arbitrary numbers for the example)).

So sadly I can't go 'by eye'!

I figured going for the smallest tolerance possible is just better as it is reducing the error from that part of the circuit? (Assuming the cost isn't too high, I'm on a budget!)

 

1% 300Ω Resistor should be fine.

 

If you are sensing the actual output, then it doesn't matter what resistor you use (within reason) since your driver is going to adapt (or be calibrated) based on the actual LED characteristics. You have other considerations to consider such as the output as a function of temperature.

The first thing you need to determine is how accurate and precise (those to two different things) you NEED the system to be.

 

You might want to use a constant current source versus a resistor for current limiting..
Even between "identical" LEDs their forward voltage may be slightly different thus changing the current the LED will see..
A constant current source will not have that issue as it will maintain a fixed current through differing voltage requirements..

 

Okay!

Is there any difference between the different series of resistors? MF12/MFR/RC55 etc?
I am powering two colours of LED (both 20mA, amber at 2.1V and blue at 3.2V, supply is 5V).
So for amber I need 150ohm and for blue I need 100ohm?

As you say, it doesn't really matter. Just as long as I have one there that is of a realistic value. But since I am buying them today, is it worth grabbing some of the calculated values or just grab a bulk load of say 220ohm resistors instead?

I currently have these two selected:
http://uk.farnell.com/multicomp/mf12-100r/resistor-100r-0-125w-1/dp/9342397
http://uk.farnell.com/multicomp/mf12-150r/resistor-0-125w-1-150r/dp/9342613

Or there is this 'series':
http://uk.farnell.com/welwyn/mfr4-100rfi/resistor-metal-film-100r-0-5w/dp/1099867
Has a higher voltage rating and power rating .. but neither of those matter as I'm not anywhere near them. It will be a maximum of 0.064Watts so well below the 0.125 of the first two resistors anyway.

(Or just buy a load of these 220ohm ones! http://uk.farnell.com/multicomp/mf12-220r/resistor-220r-0-125w-1/dp/9342826 )

Thanks for the help.

Temperature is something I am also looking into at the moment - but as far as the resistors go, they should be fine (0.00005 variation per degree etc).

edit: I do have a constant current chip so that's a possibility. I'm just looking into all options as I'm learning as I go (also a reason why I may repeat things .. sometimes takes a while for stuff to be absorbed)

 

But since I am buying them today, is it worth grabbing some of the calculated values or just grab a bulk load of say 220ohm resistors instead?

Brightness will match better if you use current sources and LEDs from the same manufacturing lot.