Noise Control Question

[dallypost]

I am working on a revision of an existing product. This new revision will be powered by a 12 volt automotive type battery. This battery will also power a 1/3 hp dc motor. Noise is a concern.

I am attempting to add a little noise protection to my power supply. After reading everything I can find, I have designed the power supply circuit to look like the attached image. V1 is a 12vdc to 5vdc regulator and V2 is a 5vdc to 3.3vdc regulator. Additionally, I have a .1uF cap placed next to all vcc pins and the avcc pin for the avr and the vcc pins for all other integrated circuits on the board. Is there more that I should do?

Thanks

 

[(*steve*)]

I would recommend that you add some sort of protection to this circuit from voltage spikes that the motor may generate.

Since you have at least 5 volts to dissipate anyway, you could place a series resistor that will drop about 3 or 4V at maximum load in series with the 12V and place a large capacitor after this. That will provide a time constant which will protect your power supply from short spikes.

Adding a (say) 18V zener across the capacitor will assist in preventing higher energy, longer spikes through.

 

[Harald Kapp]

Protection from spikes is a good idea. But (sorry steve) I'd recommend an inductor instead of a series resistor. An inductor will additionally help to filter high frequency noise but will pose less resistance to DC.
A Zener is a good idea, too, especially in an automotive environment where large voltage spikes due to load dump are a common problem. Make sure the Zener is after the inductor (or resistor) since the series element is required to limit the current through the diode.

Harald

 

[Rleo6965]

You might try to connect both Voltage regulator IN to +12V. You connected the 2 voltage regulator in series.

 

[Harald Kapp]

Putting the 3.3 V regulator after the 5 V regulator minimizes the 3.3 V regulator's power dissipation. The power is now mostly dissipated in the 5 V regulator. Depending on the exact power relationship it may suffice to add one heatsink to the 5 V regulator instead of having two heatsinks.
Also, the 3.3 V regulator may not be rated for 12 V input - but we do not know which types dallypost uses.

Harald

 

[Rleo6965]

I am working on a revision of an existing product. This new revision will be powered by a 12 volt automotive type battery. This battery will also power a 1/3 hp dc motor. Noise is a concern.

How about placing noise filter capacitor on dc motor terminal ( source of noise ).