Back to Basics: Inrush Current Limiters

Inrush Resistors
Inrush Resistors

When designing circuits, one of the key elements that are missed by many designers is the addition of inrush current limiters because they are mostly unaware of its use case. Inrush current as the name implies, is a large flow of current at the power on of a device. When PCBs are designed, we do scatter decoupling and bulk capacitors around the entire board to maintain the power supply stability for individual ICs. There can be massive bulk capacitors which are used specifically in large current applications like motors, audio amplifiers etc. When power is off, think of these as massive empty tanks. When the power is turned ON, there will be a massive surge to fill up all these empty tanks and hence you can expect an order of magnitude of instantaneous current flow in the circuit. So why is this large rush a problem? The large draw will immediately collapse your input power rail momentarily by a big factor, now if components of your circuit are not resilient to these power supply changes it can damage parts or may cause temporary glitches at the startup which may not initially be obvious. Even a load capacitance of 100uF can generate a 6.88A of inrush current which can cause a supply rail to drop 3.3V to 960mV. So it’s not something which can be ignored.

You can fix this by adding two things to your circuit, one is an integrated load switch with an adjustable slew rate which limits the rate of current draw for the downstream components. Adjusting the rate drastically reduces the inrush currents. The second method is the one which you see in most circuits is to use NTC thermistors as limiters. Connected in series, these have high resistance when starting off and as soon as current starts flowing, it heats up and the resistance drops to let more current through. So you effectively slow down the inrush requirement. Both have their pros and cons, so next time do check out the need for these in your circuits while building them.

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Temperature of Light and White Balancing

LED Temperature

I am sure most folks when they are picking out what LED bulbs, usually buy LEDs by colour and go via marketing jargons like Warm White, Cool Day, Warm Yellow etc. These terms usually represent the output colour of your LEDs. Some of you would know that there is a standard way to represent them in terms of colour temperature with units in Kelvin(These would be mentioned on your bulbs). Let’s explore what those are today.

LED Temperature White Balancing

The temperature of colour is a weird way to represent information. Colour has no “heat”. So why use temperature as a measurement unit for it? It all goes back to a concept known as a black body radiator. In simple terms, it’s a theoretical material which emits radiation(aka light) when heated to different high temperatures. For example, think of a metal(which is a somewhat close real-life black body), when it’s heated to say 1500K has a particular glow in the red/orange range. Now when you start increasing the temperature of this material, the colour slowly starts to change white(Bluish tinge). So hence temperature Kelvin is used in relation to the colour of light. The temperature range can go from 1500K – 10,000K+. 1700K range corresponds to candle-flame-orange light & 6500K is clear sunlight on a bright day. So you have a huge range of LEDs to choose from based on what you want.

Temperature and White Balancing

Now let’s come to photography of this light. Where it’s all relative to your settings. Open your mobile camera and go to the manual mode in your camera you will see there is something known as White Balance on the settings, This is also measured in Kelvin. This setting is telling your camera sensor what pure white light is. Let’s say you put the WB setting on the camera as 5000K and take a photo of an object. So any light source in your scene with a colour temp of 5000K will appear white on your image whereas any light source lesser than that will appear Yellow and anything above will appear bluish. It’s as simple as that. Now if slide up your white balance to say 6500K setting you will see more things looking yellow (if the scene lighting is at say 5000K). Knowing this concept will help you take killer pics and set the lighting mood for your camera. Take a look at the picture for seeing how an image will come out for the same lighting scene shot with different WB settings.

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