update ups post

Author: RCmags

collections/blog/Computers/_posts/2024-06-16-laptop-ups.md

@@ -5,6 +5,15 @@ title: "DIY Laptop powerbank / Battery power supply"
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 Sometimes its cool to try out different projects just to see if you can pull them off. One of them happened to be making a battery backup for a laptop. Now, at first this sort of doesn't make sense. Don't laptops already have batteries? Yes, they _generally_ do, but there are times when either the capacity is insufficient, or the battery doesn't work or is too much work to replace. In those cases, what can we do to run the laptop without AC power? Simple! Just "mimick" the power provided by an AC adapter with a battery! 
@@ -26,4 +35,15 @@ It's possible to make something very similar with a few inexpesive components. T
 
 - Some communation module or a relay: The ups needs to switch between charging and discharging the battery, so some kind of device needs to be used to redirect the AC current into either case, depending on whether AC is available or not. To this end, a very simple solution is a relay switch and that activates with the AC current, and connects the battery to the step-down converter/charger.
 
+With this combination of components we can make a system that automatically switches between AC and DC power. In the case of AC power failure, the relay will quickly switch to battery power to maintain power. However, since the relay is a mechanical device it will take a finite ammount of time to change state. This delay will cause power to the laptop to be dissabled momentarily, meaning the laptop will have to be powered by its internal battery for a short ammount of time while the UPS transitions. As a consequence, this design means we have __interrupted power supply__. Despite this drawback of a relay, it comes with some advantages:
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+- A relay has very little contact resistance since its just a mechanical switch. Either when its coil is powered or unpowered, the voltage drop across it is small. 
+- There is little heat loss by the relay in its unpowered state. This is also helped by the low contact resistance since it the very low loss means even less heat.
+
+By comparison, other switching methods like transistors or diodes will generate more heat when current flows through them. On the upside, being solid-state devices their switching time is instant compared to a mechanical switch. 
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+Putting all of this information together, we can assemble the components into the diagram below:
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 ![image](/img/laptop-ups/laptop-ups.png)
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+Note how overall system is fairly simple. It's essentially a pass-through for the AC-to-DC converter, and an automatic charger for the battery. Moreover, the __input voltage__ from the AC-DC converter just has to be __higher__ than the __battery voltage__, but it does NOT have to match the voltage required by the laptop. As a consequence, the we can use a ~12v converter up to a ~20v converter with this design. It's input voltage is very flexible.