Thanks a lot Mr. Neil Barker, First I have made one mistake on my description. The bigger LCD is powered by 12v (other power supply). Both the [GND 5V] and the [GND 12V] are connected near the supply unit.
These power supplies are within the equipment. The supervision module power cable has 2,45m and the LCD power cable has 1,45m. And both LCD have internal GND<->Frame that I believe it is for security. The metal frame from each LCD is metal to metal connected with our machine for security. We have on our supervision module bypass capacitors (5v to GND) at the entry point but they can not clean the disturbance. It seems that the LCD demands significant rush currents. We have also tried to make a ramp circuit in order to slower the LCD power and this seems to work but as before we are a bit on the Test and Try without carefully thinking. Maybe some ferrites on the power lines at the power supply will help!? I know that USB power distribution uses this. One comment from what you say: "I suspect that all your GNDs are not as near EARTH as you might like to think they are, so your extra GND->EARTH is probably just helping to stabilise the potentials in the system and also to provide another path for transient currents. " Is this bad!? That is can we distribute more points GND-EARTH? Also can this means that we may have a security problem because the LCD frames are not correctly connected to EARTH? Once again thanks a lot. Joao Martins -----Original Message----- From: Barker, Neil [mailto:[email protected]] Sent: quarta-feira, 24 de Julho de 2002 12:59 To: Joao Martins; [email protected] Subject: RE: gnd and earth connections You do not say whether the LCD display is supplied from a separate power supply or from the host equipment. You also do not say how long the interconnecting cable is. Remember that a cable has significant inductance and capacitance. Any changes to the current flow in the cable will very likely result in a significant transient. I suspect that all your GNDs are not as near EARTH as you might like to think they are, so your extra GND->EARTH is probably just helping to stabilise the potentials in the system and also to provide another path for transient currents. I would not worry about EARTH current, as this is only a transient effect. There will not be any change in EARTH leakage current. You may well be able to reduce this effect by paying more attention to power supply decoupling in the units as this could help reduce the magnitude of the transient. Careful attention to terminating the cable in a matched impedance could also be beneficial as the mismatch may be increasing the effect of the transient due to reflection. Best regards, Neil R. Barker Compliance Engineering Manager E2V Technologies Waterhouse Lane Chelmsford Essex CM1 2QU U.K. Tel: +44 (01245) 453616 Fax: +44 (01245) 453410 E-mail: [email protected] > -----Original Message----- > From: Joao Martins [mailto:[email protected]] > Sent: 24 July 2002 10:49 > To: [email protected] > Subject: gnd and earth connections > > > > Hi, > > I am new to this group so I forgive me if I do anything wrong! > > I am an electronic design engineer with very phew knowledge and > experience about EMC. > > Where I work we must connect together some OEM modules with our own > modules. The whole must comply with EMC requirements of course. > > After this introduction here is my problem: > > We have on LCD that has the GND connected with the EARTH(protective). > This LCD is power by 5v and can be disconnected / connected during > operation. > We also have another LCD smaller in our supervision module and here we > have again the GND<->EARTH. All GNDS are the 5v GND. > The problem is that when we switch on the exterior LCD some > disturbances > arise on the GND that triggers the voltage supervision chip on the > Supervision board. > > We have solved this by making another connection GND-EARTH near the 5v > Power source. We have done all the tests including security > tests again > and it seems ok but I do not know if this is correct since I > believe we > are increasing the electrical current on earth(probably not > on the earth > that comes outside the machine). > > So, I will really appreciate any comments on this matter. > > Thanks, > Joao Martins > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" >
<<attachment: Neil_R._Barker_(E-mail).vcf>>
