RAMPS 1.4 voltage question

 RAMPS 1.4 voltage question January 29, 2022 12:48PM Registered: 2 years ago Posts: 1
I recently got a RAMPS 1.4 board, and I've converted it for use with 24 volt. I've followed the multitude of videos and articles on how to do this, which all of them are practically doing the same thing. I tend to double check why they do it, and looking at the schematics I wonder if something has been overlooked by all of the articles/videos out there.

Now, my knowledge in electronics is very basic, so I could very much be wrong. But it would be nice to know why if so.

It's basically the "HEATER & FANS" part of the schematic. In there, on the "+12v" side, it has a LED for each output (LED2, LED3 & LED4), each with a resistor in series (R23, R24 & R25). If the voltage goes up to 24V, does that not mean the resistors will have to be recalculated and replaced? I suspect that this might not matter as much and the board will work regardless, but it might "leak" more leccy than necessary and not be as efficient. The lifetime of the LEDs might also be shorter? Is this true?

I've been thinking about potentially increasing the voltage even more, so wondering if without replacing the resistors the LEDs will fry at some point. I guess the easiest is to just remove the resistors and/or LEDs as they are for visual feedback only (I guess).

Many thanks
 Re: RAMPS 1.4 voltage question March 06, 2022 10:56AM Registered: 14 years ago Posts: 140
According to the schematics for RAMPS 1.4, the resistors for the Mosfet LEDs are supposed to be 1.8 K ohm (1K8)

If the voltage drop over the diode is about 2 volt, then the rest of the voltage is handled by the resistor.

At 12V this will leave 10V for the resistor. 10V / 1K8 ohm = 5.5mA

At 24V the resistor has about 22V to deal with. 22V / 1K8 ohm = 12mA

A LED will typically work just fine, all the way up to 20mA, so even though you use 24V, then the LED is not beyond its specified limits.

The resistors, however, they might get quite hot.

0805 sizes are typically rated for 1/8th watt (0.125W = 125mW), but recommended for only 1/10th watt (0,1W = 100mW)
0603 sizes are typically rated for 1/10th watt (0.1W = 100mW), but recommended for only about 60% of that (0,063W = 63mW)

At 12V the resistor deals with 10V x 5.5mA = 55mW - so that is okay.
At 24V the resistor deals with 22 x 12mA = 264mW - this is way over the recommended limit

If there is enough copper connected to the resistors, it will act as a heat sink, and will keep the temperatures lower.

But otherwise the heat from the resistors, will likely cause the LEDs to heat up, and the LEDs will no longer be able to sustain 20mA through them.

As the worst case (24V operation) will still only subject the LEDs to 12mA, everything may be okay. For a very long time. LEDs typically have a 20,000 to 50,000 hours lifespan. But longer if they are not subjected to the full 20mA.

I will typically use 3K3 resistors when I have a LED + resistor, that is subjected to 24V

With a 3K3 resistor (with 22V over the resistor) is 22V / 3K3 Ohm = 6mA

6mA x 22V = 146mW - still very high, but with enough copper around the resistor, it will survive just fine.

I hope these numbers are helpful for you, to determine if you want to worry further about the LEDs

Edited 2 time(s). Last edit at 03/06/2022 11:04AM by MrAlvin.
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