Servo Capacitor charging

parts-man73

I have a Propeller Protoboard, and I noticed a 1uH inductor in between the main power supply and the servo connectors and thier associated 1000uF Cap.

Am I right to assume this is to limit the current drawn to power the servos, possibly preventing a brown-out condition?

I want to incorporate something similar into one of my own circuits. After much Googling, I have yet to find any guidelines or examples of an inductor used in this fashion.

Any help/pointers are appreciated!

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

Mike Green

This is not intended to limit the current for the servos. It is intended to filter power surges from the abrupt demands of motors like servos when they first begin to move. The capacitor and the inductor can both supply energy for brief periods of time (to compensate for the surges) and the inductor resists any abrupt change in the current going through it, thus isolating the Prop from transients. Look up power supply filters on the Wikipedia. This is sometimes described as a choke-input filter.

parts-man73

Thank you for better explaining that Mike. I had thought I heard a discussion once about "limiting the inrush current" I think that was in reference to a USB powered device drawing too much current when first connected to a USB port. I guess I was applying that logic to the capacitor/servo circuit on the Protoboard.

It still sounds like a good idea to include in my circuit, just for a different reason. I'll look up the wiki on power supply filters so that I'll understand it better.

One last question. What is involved in the selection of a inductor, how do I know what size/type to use?

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Brian Meade

"They who dream by day are cognizant of many things which escape those who dream only by night" - Edgar Poe

Paul Baker

parts-man73 said...

One last question. What is involved in the selection of a inductor, how do I know what size/type to use?

The critical value when choosing the·inductor is not the value, but that it has a very low series resistance. The value can vary but we used a 1uH part. I did some modeling of the frequency response and step response to make sure it blocked a good portion of what we wanted to reject.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Paul Baker
Propeller Applications Engineer

Parallax, Inc.