eVOLVER Forum

Clarification on Current Power Supply Set-Up

Our lab has 12V blue LEDs currently linked to the 5V and I am thinking of putting the PWM that controls them on the 24V lead so we have access to the full range of brightness, but I am a little confused on what I would need to do for this.

According to the supplement, the PWM can amplify a 3.3 V signal from the Arduino between 5V and 24V. It also states that for both the fan and temperature, the signal is amplified to 12V. I have a little confusion on what this means with respect to the fan and temperature receiving power from the 12V supply through the labeled 24V circuit as I do not see any resistors on the PCB for these components (to those unfamiliar, the SA is either under the 5V or 12V power supply; each SA on the motherboard has golden leads above it that can be put to either of these).

If I put it on the 24V power supply with the Arduino set to initialize the PWM at 4095, will this propagate a 12V or 24V signal to the LED? Or is this actually handled in a different part of the Arduino code?

My intention with asking is to know whether or not putting the prongs for the SA slot to 24V would necessitate putting 600-ohm current-limiting resistor in series with our 12V LEDs to prevent shorting. It’s probably still a good idea to have a current-limiting resistor of some sort regardless, but it’d be good to be able to start testing more with lights while I wait for parts to arrive if possible.

The power supply is actually 12V, not 24V. The boards were rated up to 24V but we decided to use 12V but the annotation on the boards were never updated. There is no 24 V in your system. The fans and heaters both use 12V and are specified by the jumpers on the system (“the metal prongs”).

I do not see any resistors on the PCB for these components

These parts do not need resistors as they don’t need to be current limited and will be OK with direct DC voltage. LEDs are different.

If I put it on the 24V power supply with the Arduino set to initialize the PWM at 4095, will this propagate a 12V or 24V signal to the LED? Or is this actually handled in a different part of the Arduino code?

Due to the architecture of the system, 4095 actually means that the transistor is turned OFF, whereas 0 is ON. The part that controls the PWM board is this line of code. You can find that line of code anywhere theres a PWM board controlled.

Tlc.set(LEFT_PWM,i, 4095);

Tlc.update();

What current limiting resistor depends on your part. Can you link the part number here?

Okay, that is what I had suspected the case was but wanted to make sure.

The LEDs were left behind from previous work so I didn’t have the exact part number, but I was able to track down what I am 99.9% sure are them , which apparently have built-in resistors for operating at 12V.

Yup, you should be able to plug these in directly to PWM.

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