Spring 2016 RoFi: Power Trade-Off Study
Christopher Andelin (Project Manager)
Mario Ramirez (Systems Engineer)
Qui Du (Manufacturing Engineer)
Andrew Laqui (Electronics and Controls Engineer)
Henry Ruff (Electronics and Controls Engineer)
PTC Trade-off Study
Henry Ruff (Electronics & Control)
In the schematic design for RoFi, the PCB mainly entails connections to the main four components: bluetooth, accelerometer/gyroscope, ultrasonic sensor, and servos. For the twelve servos in particular, each one needs a polymeric positive temperature coefficient (PPTC) fuse, in order to protect them from drawing too much current at stall conditions. These PTC fuses can be implemented on the PCB itself, and for this purpose, several surface mount varieties were compared. The company Littelfuse provided the following guide for comparing potential components, and only the surface mount series were considered in this study. Datasheets for specific components within each series were found in the second link.
http://www.littelfuse.com/products/resettable-ptcs/surface-mount.aspx
The following table is a basic trade-off between the available series of PTC fuses, such that each series can be further looked into afterwards for a more specific trade-off series.
The hold current (IHOLD) is the normal operating current of the servo, which for the PowerHD 1501-MG is 500mA at 6V, although ideally the servos will be supplied with 5V. The trip current (ITRIP) is where the PTC fuse will begin to impede excess current, desirably chosen to be less than the servos’ stall current of 2500mA at 6V. After looking at the cons, the 1206L, 1210L, 1812L, 2016L, and 2920L series were looked at for comparison, using components that would be the closest match for desired purposes. IHOLD would be up to around 1A to allow for variation due to operation and load, while ITRIP would be under 2500mA. For the following table, each component had functionally the same pros, therefore only cons and price were compared.
Mainly due to price comparisons, the Littelfuse 1206L110TH was then chosen from this study to be used on the PCB board. However, higher numbered series had trip times up to 0.5 seconds as opposed to the 1206L110TH which had a trip time of 0.1 seconds, and was better because of which.