Week 20 (Feb. 27, 2019): Power Electronics – Battery & Amplifier

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Since last week, the team has been working on the current amplifier for the induction charger.

We decided to abandon using the TPS53313 step down voltage regulator-current amplifier IC (rated at a maximum of 6A continuous output current) because we found that supplying +5V to a 3.7V LiPo battery was possible and safe. Additionally, charging a LiPo battery at 6A was found to be dangerous; the battery should be charged at 25% of the capacity rating (.25 x C), or .25 x mAh. In the case of the 6600mAh battery, it should be recharged at a rate of 1.65A or less. The last reason is that dealing with a surface mount device (SMT) is a bit difficult.

I started out using a BJT NPN transistor (model: 2N3904), something we were most familiar with. However, its datasheet states that it has a 200 mA maximum continuous collector current rating. This is a significantly low current. Most, if not all, fast charging adapters today have a 2 A charging rate. While we don’t want to charge the LiPo batteries at 2 A, since it could be dangerous (it’s recommended to charge at 0.25C, or 25% of the battery’s capacity.

Instead, we are planning to use another type of BJT NPN transistor, specifically the MJE270G Darlington Pair transistor, which has a maximum continuous collector current rating of 2A (https://www.mouser.com/datasheet/2/308/MJE270-D-110810.pdf).

Additionally, instead of using the 7.4V 2600mAh LiPo battery that the Small-Scale Controls Team last semester used, we are aiming to use a 3.7V 6600mAh LiPo battery instead. The rationale behind this possible change is because the 6600mAh battery has a higher capacity (6600mAh), meaning the more charge it will hold and the longer it will take to discharge, being able to hold out much longer before being charged again. So, for the 6600mAh battery, a safe charging rate is 0.25 x 6.6Ah = 1.65A. But just to be even safer, 1.5A would probably safer.  

After some testing last week, 3.7V can indeed power the Arduino MEGA (which is normally powered at 5V) and a 12V DC motor, albeit it provides less power to the Arduino and the motor. However, there is still more than enough power to drive the gimbal motor at our target speed of at least 1.084 m/s. As a side note: at 3.7V, the clock speed is approximately 16 MHz, while at 5V, the clock speed is approximately 20 MHz (http://www.gammon.com.au/forum/?id=11497).

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