Hello,
For a product we make we are looking into a single connector that provides RS-485 communications, 24VDC control power, and 120VAC heater power. The connector would be a Harting or Epic style connector (if you search "epic connector" the top images are representative). I am concerned with EMI between the RS-485 and 120VAC lines. The subject of EMI is largely out of my depth, but I have a basic understanding.
The 24VDC is fairly low and steady current of at most 1A, so I am not concerned with EMI on these lines. But the 120VAC line will be switched - right now it's half-wave zero-cross switched (low harmonics), but in the future it could be phase angle (high harmonics). Up to 15A can flow through this line, so EMI is a concern here. As for the RS-485, the current baud rate is 460800 but I would like to provision for up to 2M.
Regarding the connector, we would prefer to use a standard mono block type due to cost and ease of assembly. (There is also a modular design that can include a fully shielded portion for the communications and here I would not be concerned with EMI). But for the mono block type, their would be no shielding and no twisted pair across the connector, a length of about 4 inches. We would place the RS-485 lines at the opposite end of the connector pinout as the 120VAC, separated by about 2 inches. The overall connector housing would be shielded, but the AC line is within that shield.
How can I verify interference will be of an acceptable level?
I did some calculations that show that the electromagnetic frequency in copper with a wavelength of 4 inches is about 2GHz, which is well in excess of the RS-485 baud or slew rate.
I looked at it geometrically and with equal but opposite currents on the AC line it should produce a relatively symmetric magnetic field that should cancel out at the RS-485 differential receiver.
I am looking into testing. I could produced a phase-angle switched load of 15Apk and measure the impact on the RS-485 lines with an oscilloscope. In that case I could possibly use a transformer with a low voltage secondary to reduce the load power needed for that much current. That is, only if the 120 VAC electric field is not important, and only the 15A magnetic field is. If the electric field is important then I need a much higher powered load resistor.
I have also contacted an EMC test lab who could perform a similar test, but at a relatively higher cost.
So is testing needed, or are the analytical methods I used sufficient? If testing is needed, do I need 120V/15A on the AC line or just 15A at any voltage?
Thanks!
For a product we make we are looking into a single connector that provides RS-485 communications, 24VDC control power, and 120VAC heater power. The connector would be a Harting or Epic style connector (if you search "epic connector" the top images are representative). I am concerned with EMI between the RS-485 and 120VAC lines. The subject of EMI is largely out of my depth, but I have a basic understanding.
The 24VDC is fairly low and steady current of at most 1A, so I am not concerned with EMI on these lines. But the 120VAC line will be switched - right now it's half-wave zero-cross switched (low harmonics), but in the future it could be phase angle (high harmonics). Up to 15A can flow through this line, so EMI is a concern here. As for the RS-485, the current baud rate is 460800 but I would like to provision for up to 2M.
Regarding the connector, we would prefer to use a standard mono block type due to cost and ease of assembly. (There is also a modular design that can include a fully shielded portion for the communications and here I would not be concerned with EMI). But for the mono block type, their would be no shielding and no twisted pair across the connector, a length of about 4 inches. We would place the RS-485 lines at the opposite end of the connector pinout as the 120VAC, separated by about 2 inches. The overall connector housing would be shielded, but the AC line is within that shield.
How can I verify interference will be of an acceptable level?
I did some calculations that show that the electromagnetic frequency in copper with a wavelength of 4 inches is about 2GHz, which is well in excess of the RS-485 baud or slew rate.
I looked at it geometrically and with equal but opposite currents on the AC line it should produce a relatively symmetric magnetic field that should cancel out at the RS-485 differential receiver.
I am looking into testing. I could produced a phase-angle switched load of 15Apk and measure the impact on the RS-485 lines with an oscilloscope. In that case I could possibly use a transformer with a low voltage secondary to reduce the load power needed for that much current. That is, only if the 120 VAC electric field is not important, and only the 15A magnetic field is. If the electric field is important then I need a much higher powered load resistor.
I have also contacted an EMC test lab who could perform a similar test, but at a relatively higher cost.
So is testing needed, or are the analytical methods I used sufficient? If testing is needed, do I need 120V/15A on the AC line or just 15A at any voltage?
Thanks!
