power supply and earth

[lynch]

Hi
I just designed a simple power supply which steps down mains 230V to +15V and -15V, i have rectified, smoothed etc the voltages and all is well.

My only concern is that the secondary voltages are essentially floating with respect to Earth.

I was told i could simply capacitively couple (1nF 2kV) my 0V on the secondary to Earth.

Is this correct or are there other standard methods?

Many Thanks
Chris

 

[A.Anand Srinivasan]

a capacitor would add some voltage across it when current passes through it and i dont think this is the right way... i believe shorting the 0V of secondary to earth is the right way....

 

[IanP]

It isn't a good idea, nor is it a common practice to connect the output 0V terminal of a power supply to EARTH ..
This would mean that the DC power supply is no longer earth free on the output ..

In most cases this will not be a problem ..
However, you have to be careful if the circuit to which you connect the power supply to has also some connection to earth, e.g. if you cascade two power supplies in order to have positive and negative voltages ..

So, consider this: make connection beween EARTH and 0V(output) of your power supply with 1n-10nF capacitor in parallel with 1MΩ-10MΩ resistor ..
Adding resistor is important, as it equalizes potentials between EARTH and your power supply and cap provides good return path for RF signals ..

Regards,
IanP

 

[lynch]

Guys
Many thanks for both your replies, i will use IanP suggestion and go with the capacitor and resistor.

Funnily enough when i asked two engineers at work one suggested that all i needed was capacitor and the other believed just the resistor would do

Regards
Chris