Tech Insights: Transparent TVs

CES last month had LG and Samsung showing off their latest Transparent TVs to the world. Despite being in the limelight for a few years, the underlying mechanics never cease to amaze. How do these TVs achieve transparency? Where are all the wires?

LG relies on Organic LED (OLED) tech for its transparent displays. OLED displays have organic layers responsible for light emission between two transparent electrodes. Electrodes made of metals are usually opaque. Enter the star of the show: Indium Tin Oxide (ITO), a transparent Conductive Oxide. What makes something opaque is the fact that the incoming light is reflected or absorbed completely. The light we see has photon energies between 1.6 & 3.2eV. ITO has a large bandgap of more than 3.5eV, which means it just allows light to pass through. With high Tin doping in Indium Oxide, it becomes conductive with low resistivity, making ITO an ideal choice for electrodes balancing optical transparency and electrical conductivity.

Constructing the display involves taking glass as a substrate, depositing a layer of ITO, and adding multiple organic materials (hydrocarbon-based) on top. These organic layers emit light when an electric current passes through them. The structure is completed with another electrode made of an ultra-thin Magnesium and Silver layer, achieving an impressive 70% transparency. To preserve organic layers from water/oxygen, they are sealed with an encapsulant.

Now we need transparent display drivers below them. For that, we use Thin Film Transistors (TFT) made of a material called Transparent Amorphous Oxide Semiconductors (TAOS). Deposited on a glass substrate (not silicon), all layers of a transparent TFT consist of transparent oxides. These transistors regulate brightness beneath each pixel by turning ON and OFF. Optical filters for RGB on each sub-pixel complete the one full pixel. Multiply this process over a rectangular space, and voila, you have a Transparent TV.

PS: Hopefully, this hasn’t offended any material engg. aficionados. I might have made a few broad statements. ????

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Back to Basics: Battery Strap Fuses

Battery strap PTCs, or Positive Temperature Coefficient devices(a fancy way of saying its resistance increases with an increase in temperature), are components designed to protect batteries and associated circuits from overcurrent conditions. These are relatively unknown to folks outside the battery industry. Please don’t mistake them for the SMD fuses we fuse while designing PCBs for protection. Although they work on similar principles, the strap-ones are uniquely thin-shaped and attached directly to the batteries.

Battery strap PTCs act as self-resetting fuses. When an overcurrent condition occurs, the PTC heats up and its resistance rises dramatically, reducing the current flowing through it. This effectively limits the current to a safe level. Once the fault or overcurrent condition is removed, the PTC cools down, and its resistance decreases, allowing normal current flow to resume. Like all fuses, it will have a specific hold current(current allowed to pass in normal operation) and a trip current(current at which the device goes into high resistance mode). One major thing to keep in mind is that these hold/trip currents are temp-specific; as temperature increases, these values fall, and they will trip faster. In a way, it’s a nice thing because when the battery shorts, the temp shoots up and it reaches the trip point faster, but make sure it doesn’t trip in normal ambient or when the battery is getting charged(as the temp increases)

These fuses are usually spot welded or crimped on the battery terminals directly. It forms a great first line of protection in large battery packs and plays a crucial role in reliable and long-lasting battery solutions.

Have you ever used them in a project? How was your experience with it?

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