In my last blog we talked about how noise can creep into and destroy your seemingly clean system when you have a switching device in the vicinity. I thought as we were already discussing the topic of noise let me also give you another example of a noisy source.
Now a days every one is familiar with dc power adapter which comes with various electronic equipment including mobile phone chargers, microcontroller development board and other stuff. Pretty handy little device light weight directly plug into wall socket and provides +12V, +5V, +9V voltage (depending on the rating) to power or charge your device.
What is inside these devices are called switching regulator circuitry rather than linear regulator. Most advantage of switching regulator is the power dissipation within its series pass transistor is minimum giving it ultra high efficiency of more than 90% some times. However the circuitry is complex and suffers from noise being injected in the output dc rail. The noise is a result of the high frequency switching which is used internally within the regulator.
In most of the case when you are designing your circuit you will have the output of the adapter to go into a linear regulator say 7805 to get a low noise regulated dc output provided you decouple the output properly. The scheme should look something like this.
The output in this case is properly decoupled to reduce noise content in the 5V rail, however the 12V rail remains to be decoupled. I have probed the two Rails with ac coupled channel in my scope to find out the noise content within each of them.
Noise @ +12V Rail ~ 96.8mV
Noise @ +5V Rail ~ 11.2mV
I have excluded the transient peaks since I was again using a breadboard.
Clearly from this the noise at the 12V rail is substantially more. The 5V rail being a linearly regulated one along with proper decoupling shows much lower noise it would be much more cleaner on a PCB. Now most of the times may be you wont use the +12V rail in your circuit. But sometimes say if you do want to include both the rails (for high voltage applications) you definitely want to do something about the noise.
This is the same noise with the scope speed up. The switching frequency of the switching regulator can be clearly seen.
Simple solution is just decouple the +12V rail as well.
With input decoupling capacitor.
The Noise on the 12V rail is greatly reduced.
This is a simple and effective way of having two power rails within your circuit.
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