Arduino and Transistors Question - RE: School Musical Production

Question

One of the directors has asked me to create a special backdrop for our college musical. This particular backdrop has a particular design (see below) that includes LED strip lighting to give the feel of a 1920s casino sign – similar to what is seen today in Las Vegas.

I have done up a detailed design for this sign, based off the rough sketch given to me by the director and the dimensions of the backdrop. On this diagram, the arrangement of the LED strip lighting can be seen.

The LED Strip lighting that will be bought for this project will be sourced from Jaycar – “Low Cost 5m Flexible Adhesive LED Strip Light – Warm White” (ZD0577). According to the specifications off Jaycar’s website, these LED lights need 1.1 Amps per metre @ 12 volts. This means that the longest segments of lights need 1.1 * 1.5 = 1.65 Amps of power (rounded up by 20% for safety: 2.0 Amps @ 12 volts).

This would not be an issue if I was just turning these LED Strips on and off with mains power BUT I want to control these LEDs via an Arduino, so that they can do fancy things like flash and chase. That means that I would need a circuit to control these lights with an Arduino so that everything remains safe and does not blow up.

I have come up with this circuit (below) using a couple of NPN transistors and two power supplies. In the diagram below, I am aware that the transistor pinouts are EBC. The ones I am going to buy have a pinout of BCE.

The only problem with this is that I have very little understanding of transistor circuits. What this circuit has to be able to do is:

Using the smallest amount of current from the Arduino (at 5 volts) or a Raspberry Pi (3v3 volts), turn on the circuit to allow the 12 volts that powers the LED strip lighting to flow.

I was thinking of using either TIP41C NPN Transistors (in a darlington Array) or TIP122 NPN transistors.

This circuit will be replicated 8 times over to accommodate for the number of LEDs I am controlling.

Can anyone help me:

Design a circuit that will work 100% and will be safe (ie: low heat)
1. This includes base resistance

Confirm which transistors I should be using for this project.

Any help will be much appreciated.

Problemchild · Accepted Answer

I think Douglas means like this:

dougw · Answer

You can avoid using resistors if you use a logic level FET instead of bipolar transistors. The NTD4906N-35G is a low cost power FET that will not get hot at this current. This transistor will generate less than .025 watts of heat when driving 2 Amps: If you want to limit the transient current to the transistor gate you could insert a resistor between the arduino and the transistor - anywhere from 1K to 10K ohms should be fine.

dougw · Answer

1. The FET suggested by Peter (or the one I suggested) should not get hot or damage anything during low frequency operations. (the solution suggested by John can work as well, but this explanation doesn't cover that solution or associated risks)

2. A resistor on the gate protects the arduino, but can cause the FET to get hot during high frequency operations (like PWM) so best design depends on how often the FET will get switched. Either resistor in or out should be safe for this application - a low value resistor like 470 ohms is a reasonably even compromise slightly favouring the more expensive arduino component.

3. These FETs only require about 100 nanoamps to the gate in static states, even with a 2 Amp load. During switching the gate capacitance must be charged and discharged causing transient currents to flow. Atmel seems to use average current to define danger levels (calculated by Voltage x Gate Capacitance x Frequency) Where frequency here would be PWM frequency. This should be under 1 mA for these FETs in this application - well within the arduino output limit.

Arduino and Transistors Question - RE: School Musical Production

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