In any design Clean DC is a must. and noisy PC Power supplies can be a real pain. Here are a few rules that will help you "Clean up your act"

1. Always use .01 -.1 uF/50v Ceramic disc capacitor Between the VCC & GND Lines At every chip As close to the chip as possible.

2. if the power supply is more then say 10 inches away from the cicuit being powered, then try using 100 - 4700 uf/50V Electrlytic
capacitor Across the VCC/GND at the entry Point to the Circuit Board, to help Filter out the Ac Noise component.

3. Try wrapping 4-8 turns of the power supply cable to your target system using a torroid core (those gray donutS) 2"-4" in diaeter. this helps reduce HF Oscillations produce by the power supplies Switching osc that are being induced to into the Circuit.

4. if the above steps dont help you

use a 7805 3 Voltage regualtor with a pair of 1uF electrlytic decoupiong caps accross the input and output pins to the regualtors Ground pin. use the output of the 7805 to power your Target system while using 7-10v dc this will help produce a clean dc Voltage to your Circuits.


Phil!



have access to a SCOPE connect it to the output of the Pwer supply and see if you get any Noixd..