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Richard 19th September 2020 08:35 AM

LED Indicator Resistor Calculator
LEDs are a current device and are often supplied by an over-voltage supply which is current-limited by a series resistor.

The most important value we use to set the brightness and not over-run the LED is the amount of current we want the LED to pass.

There are newer LED versions on the market such Super Bright and Ultra Bright but of the ones I've tried these just allow greater current, and hence brightness, and behave quite similarly to Standard Bright at lower currents.

For an indicator, a Standard Bright Diffused is more evenly lit at low levels. A Super Bright can be run low and tends to emit a point souce light at lowest levels which can look good depending on what you want. I avoid Clear LEDs for this job as they can dazzle and really only look good at higher light levels.

Of the popular colours Green and Blue appear brighter than Red at lower current levels.

The simplest thing I can say is you'll find a setting of 3mA - 5mA will be good for a panel indicator using a 3mm or 5mm Standard Diffused Green or Blue LED. For a Red indicator you may need double that current for a similar visual level.

The general Max for these LEDs is around 20mA so start there if you like and wind it down but the eye adjusts for low light levels so don't be surprised how low you end up setting it.

There's a formula for working out the current manually but this calculator is vastly quicker and lets you input options and adjustments instantly,

I've left the default setting at the one I've just installed which is a Standard Green 5mm power indicator run off the amp's 50V rail. Have fun :)

colin.hepburn 19th September 2020 12:56 PM

Re: LED Indicator Resistor Calculator
Hi Richard /All
Yes that looks like a nice easy the use and clear to see schematic, as well as a visual Diagram, I also like the resistor calc makes it a lot easier for me to follow another good calc. i use is electronics 2000
Here’s the link for the Electronic 2000 Calcs also handy to have

Richard 7th December 2021 10:17 AM

Re: LED Indicator Resistor Calculator
2 Attachment(s)
<<I've left the default setting at the one I've just installed which is a Standard Green 5mm power indicator run off the amp's 50V rail.>>

Just to update this thread with a couple of pics,

first, the Quad 405 amp running that 50V supply voltage. By 1990 when this one was made the long running 405 had grey paintwork to match with the updated 34 pre but it still had the bright red power LED. So I swapped this one to green in line with its sibling 306 and 606 power amps which were grey with a green LED,

secondly, (pre-calculator) a pic of a set up on a bench power supply to choose a brightness for the blue LEDs for WD3. The supply can be set to limit voltage or current. Here voltage is set to 6.3V and a 1K resistor is in series with the LED and the 3mA current drawn is showing on the right. Once you find a setting which doesn’t blow the LED(!) you can adjust the resistor and choose the brightness. The bench supply is only being used for convenience and a similar empirical method for choosing brightness could be done with dry cells and a DMM monitoring current.

Phil Y 7th December 2021 07:17 PM

Re: LED Indicator Resistor Calculator
As an addition, LEDs can also be run from an AC supply but if this is done, a small conventional diode like a 1N4148 or similar must be placed in inverse parallel with it. Otherwise the generally low peak inverse voltage of the LED can be exceed.


Richard 8th December 2021 08:18 AM

Re: LED Indicator Resistor Calculator
That's interesting Phil. A quick look around also suggested 2 LEDs in inverse parallel fed by a capacitor at one end. It would be interesting to have a play with these ideas fed by a small transformer on the bench :thumbsup:

bob orbell 8th December 2021 08:57 AM

Re: LED Indicator Resistor Calculator
Is this acting as a Clipper diode ? Bob

A Stuart 8th December 2021 11:54 PM

Re: LED Indicator Resistor Calculator

Originally Posted by bob orbell (Post 93347)
Is this acting as a Clipper diode ? Bob

Having the two diodes paralleled (Hope this is the correct number of L's?) facing opposite ways means current will pass either direction with a drop of a couple of volts, the conducting one protecting the non-conducting one at that moment, from the reversed supply voltage, the bulk of the reverse voltage being dropped across the capacitor/resistor.

Phil Y 11th December 2021 06:50 PM

Re: LED Indicator Resistor Calculator
Not seen the capacitor addition, just the normal current limit resistor.(perhaps calculated using 0.707 of the peak AC voltage?). Maybe the cap would reduce the inevitable 50Hz "flashing" of the LED when running from AC.


Richard 12th December 2021 09:54 AM

Re: LED Indicator Resistor Calculator
Hi, I didn’t think about it too much after a quick google and agree with what you both say.

Given the rising parts count my thoughts now are that the LED needs a resistor, then by the time we’ve added another diode or LED, and a cap, we may as well configure those parts instead into a simple diode/cap half wave rectifier (giving around 1.4x rms AC) and treat the LED and resistor as usual.

Flicker would be down to the size of the cap I think but it wouldn’t need a big cap for a few mA load. I’ve not tried it though and don’t know how manufacturers would do it.

Perhaps the single diode in parallel as Phil originally said would be good enough as an indicator if the flicker isn’t obvious.

Phil Y 12th December 2021 11:03 AM

Re: LED Indicator Resistor Calculator
Hi Richard,

From the couple of times I've done this over the years as a simple "power on" indicator or whatever, the flicker is not especially obvious unless you quickly move your head, then you kind of see it in the corner of your eye.


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