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LED current limiting resistors

There doesn't seem to be a wiring diagram for V2 but on the V1 diagram i'm seeing four LEDs in series with just a 5ohm resistor running off 12V. This will run the LEDs at ~240mA which is far, far too much!

It also leaves very little margin for error with regard to supply voltage. Assuming about 2.7V drop across each of the four LEDs that leaves only 12V - 4*2.7V = 1.2V across the resistor. A small fluctuation in supply voltage will dramatically change the current through the LEDs.

With a good stable 12V something like 120ohms (for 10mA) would be better for 4 LEDs. Or three LEDs in series with around 390ohms. I'm not sure what the built-in LEDs on the recommended switches are rated at so i'd be conservative. (I have not tested this for sufficient brightness, will do when i get to that stage in my build)

I think the current design is a recipe for blown LEDs or at least shortened lifespan of the LEDs.

Disclaimer: I'm not an electronics expert but this is to the best of my knowledge regarding driving LEDs.

4 comments

4 Comments

Karl Clarke

Mar 21

Hello,

Each white led tactile switch requires a voltage of 3.2V at 20mA. 4 Leds in series requires no resistor at all, but originally a 1 Ohm resistor was installed to prevent current surge on application of power. As you stated, they are too bright, so I switched to a 5 ohm resitor which took off the bright penentrating glare.

I also highly recommend putting a diode in circuit to prevent cross connection and that would blow all leds!

Kind Regards, Karl

Trevor F

Mar 21

Replying to

Hi Karl,

LEDs must have the current flow limited or they will be damaged. The 3.2V (I measured my actual switch LEDs at 2.76V) is just the voltage drop across the LED. What's left will be seen across the resistor and you need to choose a value that sets the current for the whole circuit.

For example: If we have a 5V supply with one LED and one resistor in series there will be 3.2V across the LED and 1.8V across the resistor. V=IR (ohms law). Therefore 1.8V = 0.020A x R. Solving for R = 90ohms. A 90ohm resistor will give 20mA through the resistor and also through the whole circuit/LED.

Cheers,

Trevor

Karl Clarke

Mar 21

Replying to

Hi Trevor, you are correct for the 5v but we will have to disagree on the 12v. Let's just say each led is 3v, 4*3 is 12v. Zero volts through the resistor. I too have measured the voltage across my leds at 20mA which for my tactile leds is 3.2v. You can check out my video showing this using a bench power supply.

My FMC has now been working for over 2 years and not had an issue yet. That said, if you have measured your forward voltage at 2.7v please use the resistor you have calculated.

Kind regards, karl

Trevor F

Mar 22

Replying to

Hi Karl,

I stand corrected, and I apologise if I've created any confusion.

You prompted me to do some reading on LEDs. For the benefit of anyone else following along: you can indeed power an LED at its forward voltage (no resistor) without overdriving it. The datasheet for the particular LED will give the Forward Voltage vs. Forward Current curve. You do have to be careful though because a small change in voltage can have a large effect on the current through the device.

My eBay switches don't have any such datasheet AFAIK and I don't have a good bench power supply handy to test them myself, so in my case I think I'll just be overly conservative and go with a higher valued resistor that I know 100% will never over-drive them. I would hate to have to replace them all after a mishap!

Appreciate your time!

Cheers,

Trevor

LED current limiting resistors

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