PWM with a TRANSISTOR on flexible LED filament

Question

I would like to create a custom lighting, with PWM capability, powered from battery. I would like to use flexible led filaments, and since those consumes more than 40mA, I can't use them directly on a board. I found a link explaining to use a MOSFET transistor, to dim my leds. So the transistor that I need would be one with a 5v power supply, and a 5v Voltage input.

The problem is that I can't find any transistor matching those specs. I also noticed that they have a heatsink, which mean that they need to dissipate heat, and are therefore not power efficient (which i do not like). Am I missing something ?

Any help will be apreciated, thanks !

Answer 1

You cannot find a MOSFET with that exact rating, because transistors can be used with all voltages inside of their specific limits. Most MOSFETs can be used with way higher voltages than 5V.

What you need is a logic level MOSFET, which means, that it will be in saturation (aka being fully ON) at the typical logic level of 5V on its gate. In the datasheet of a MOSFET you will find a diagram which shows the drain current depending on the gate-source voltage. At your logic level (5V in this case) you want the curve approaching a plateau.

Another important value is R_DS(on). That is the resistance of the MOSFET when its on. The lower the resistance, the lower will the power lost to heat.

I also noticed that they have a heatsink, which mean that they need to dissipate heat, and are therefore not power efficient (which i do not like).

Every MOSFET dissipates some energy as heat, since no MOSFET can have R_DS(on) of zero. The typical heat sink flap is meant to dissipate the heat for keeping the MOSFETs temperature in the specified range, even when drawing the maximum specified current. The datasheet of a MOSFET has a section with maximum ratings. There you should see the maximum drain current I_D with its corresponding needed temperatures. For example have a look at this datasheet of a random logic level MOSFET from mouser. There is no specification of how hot the case gets with a specific current, since this depends on the suroundings, but the datasheet states, that it will support up to 80A, when you keep the case to a temperature of 25°C.

It also states what electrical power it can dissipate with its package: P_D. You should look at how much current you need for your LED filament. You can calculate the dissipating power with P = R * I² (P being power, R the resistance R_DS(on) and I the current through the MOSFET).

Answer 2

Welcome! You are missing some information. A transistor will work but you will lose about 0.7 Volts when it is on. That voltage times the current will tell you how many watts it needs to dissipate. The data sheet will give you all this information. The transistor is a current controlled device so there will not be a Base voltage rating. You can determine the current for the base by dividing the load current by the HFE then adding a bit.

By connecting more than 20 mA to the port pins especially more than 1 may destroy the Arduino. Surprise the LED is also a current device so you need to limit the current into it. This can easily be done with a resistor.

The MOSFET would be the easiest solution. Put a resistor maybe 10K from the port pin to ground. The reason is the port pins are floating from power on until your software defines and sets there level. Then connect the port of the Arduino to the gate of the MOSFET which has its source grounded. Connect the LED(s) to the drain and place a resistor in series with the LED and + voltage. It can be 12 Volts or whatever you like. You will need to size the current limiting resistor to the LED(S) appropriately and be sure the voltage rating of the MOSFET is at least 10% higher then the voltage.

There is a great book available called the Arduino Cookbook, getting that and reading it will be a big help to you.