i'm trying to implement this SMPS in a design, but i'm not sure i totally understand why there is a connection to EARTH (the E in the input terminals on the left) in the schematic bellow.
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Can someone please help me out here? Thanks!
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1\$\begingroup\$ It's because the neutral (N is the neutral right?) is not at the same potential of the earth, and it's used for security reasons, to bring the potential of the case to the earth potential and avoid dangerous discharges over people. \$\endgroup\$– clabacchioCommented Feb 7, 2012 at 16:05
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\$\begingroup\$ @clabacchio does that mean it doesn't have to be there, but advisable? Is it possible to do without such an earth connection in the design? \$\endgroup\$– TiOLUWACommented Feb 7, 2012 at 16:10
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1\$\begingroup\$ IMHO there should be an RC connection to earth ground (C being to reduce noise, R being a relatively high value e.g. 1 megaohm to keep the return from floating too far from earth ground), and the user should be given the choice of whether to tie RTN to earth ground. \$\endgroup\$– Jason SCommented Feb 7, 2012 at 16:35
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\$\begingroup\$ @Jason: I agree in general, but we don't know where this schematic came from. It could be for a specific application where it was known the output must be earth referenced. \$\endgroup\$– Olin LathropCommented Feb 7, 2012 at 19:08
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2 Answers
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The reason the earth pin is at the input is because this particular power supply references its output voltages to it. Note that the power supply itself runs from the L and N (line and neutral) inputs, and that it's output is isolated from them. The outputs are then deliberately referenced back to the earth input line.
Some power supplies allow the output to float, others specifically reference it to something. This is one of the latter.
answered Feb 7, 2012 at 16:29
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\$\begingroup\$ does this mean that I could actually ignore the connection to earth and let RTN line just float? \$\endgroup\$– TiOLUWACommented Feb 7, 2012 at 17:39
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2\$\begingroup\$ @TiOLUWA: That would not be good as the return currents would have to take a round about path, adding noise, and it would also put current on the AC ground lead. All around bad. Don't do that. \$\endgroup\$ Commented Feb 7, 2012 at 18:57
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I have seen plenty of SMPS with inputs labeled L, N, and E/GND, yet they are designed to run on American style 240VAC (ie fed by two hot wires, both 120VAC from ground). I think more appropriate labeling would be L1 and L2, but that would confuse some people wiring in a hot and neutral.
When you consider how easy a product built with this supply could get powered with the N terminal connected to a hot line (240V circuit, incorrect cord or outlet wiring, etc.), it would almost be negligent to not tolerate that condition. By designing it such, the manufacturer reduces its liability and offers a more flexible product.
One possibility why the RTN is connected to E is to protect the isolation from ESD. That can be inconvenient for some applications. There are plenty of supplies available that float.
