controlling 9v through tip120 - only hitting 5v max

Other Hardware Motors, Mechanics, Power and CNC
1

Hi all. I am building out a slot racer controller using a tip120 to control the voltage to the track but I seem to only be able to supply up to 5v max. My setup is:

  1. 9v dc supply +ve running to the collector on the tip120
  2. tip120 emitter running to the +ve line on the track
  3. -ve line on the track running to ground on the arduino
  4. pwm pin to the base on the tip120

So the pwm pin is being sent a value between 0 and 255 from a processing sketch powered by a mindwave mobile headset, and the voltage value is changing correctly as per a graphing representation in the processing sketch vs the voltage value read through a multimeter across the two track lines, however the total voltage only seems to change between 0 and 5v, not 0 and 9v.

Any thoughts as to what may cause this?

thanks!

2

TIP120 is NPN and can only switch on the low side. Emitter to ground, collector to negative side of load.
You are using it as an emitter follower, which is not a switching configuration at all.

To switch high-side you need a PNP device and something to level shift 5V to 9V to drive it.

3

ahhh, that would explain it! thanks for your help, time for a trip to my friendly neighbourhood component supplier :slight_smile:

4

You can't just switch on the low side?

5

yes, sorry - wasn't caffeinated enough - looks like I have the correct voltage running through now, just need to figure out why the cars aren't running