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Old 20th July 2007, 01:54 AM   #1
jarthel is offline jarthel  Australia
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Default using a choke on a negative supply

someone has told me that only the positive supply supplies the current. if that is true, if I use an CLC (or LC) filter on the positive supply, do I also need to use a choke one the negative one? It's easy enough to use power resistors on the negative supply to make the voltage even.

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Old 21st July 2007, 06:23 AM   #2
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the voltage and current laws hold in both directions. a CLC filter on the positive supply will not make the negative supply work better. filtering will be based on circuit needs.
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Old 21st July 2007, 03:22 PM   #3
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Hi Jarthel,

I assume by positive supply and neagtive supply you mean a wire supplying current to a circuit and a negative return wire. Electric currents always flow in closed loops. But the path of the loop isn't always obvious. In the case of a noise filter, noise currents can either be returned to the source before reaching sensitive cicuitry via a parallel capacitor, or converted to another type of energy such as heat in the case of series resistor, or a magnetic field in the case of a series choke. Whether the series element is connected in the source or return wire is mostly irrelevant in a simple circuit. In a more complex circuit currents flow through many different paths both within and outside of the piece of equipment, thus the only effective place for a series filter element is usually in the positive supply wire.

However, if the circuit has a 3 wire supply; positive, negative and a return 0V wire, the positive and negative are both considered to be supplying the circuit with current and the main current loop can be thought of as broken into two sections; positive supply with a 0V return, and negative supply with a 0V return. Thus filtering needs to be added to both the positive and negative wires, but again usually not in the 0V return for the reasons mentioned.

Cheers, Ralph
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Old 22nd July 2007, 10:46 AM   #4
jarthel is offline jarthel  Australia
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thank you
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