Hi Rich: What I have been hearing from customers is that when they measure the secondary voltage (3.3VDC,5.0VDC)on the power supply with a scope after the surge, the voltage drops momentarily thus re-setting the processor. The power supply when tested as a stand alone unit passes EN 61000-4-5 because it continues to run.
Usually the client puts the onus on the power supply manufacturer. Best Regards Edward F. O'Toole Intertek Testing Services -----Original Message----- From: Rich Nute [mailto:ri...@sdd.hp.com] Sent: Tuesday, June 13, 2000 4:52 PM To: rbus...@es.com Cc: emc-p...@majordomo.ieee.org Subject: Re: Line/Neutral to Earth MOVS Hi Rick: > Somewhat related to this discussion is the opportunity I had last week to > perform surge testing on an off-the-shelf, rack mounted PC. The power supply > in question was from a reputable manufacturer and had safety, EMC and CE > markings. The system (power supply) passed the "Heavy Industrial" limits > when tested line to neutral but the computer would reset when tested L/N to > ground. My rationale for this anomaly was the lack of a suppression device > (MOV?) to ground. I can take some guesses as to what is happening: 1) The surge current path within the PC developed an EMI radiated signal within the PC that initiated a reset. 2) The PC was interconnected with other units; the surge current path divided between the PC's ground wire and the interconnected units' ground wires. A voltage developed across the signal ground wires (because they are smaller diameter and higher resistance the protective ground wires) which appeared to be a corrupt data signal. The PC reset. (However, I would have expected corrupt data, not a consistent reset, from this hypthesis.) 3) Same as 2, but an EMI radiated signal from the grounding path caused the reset. Whether or not the PC has a discrete suppression device, the surge current MUST return to its source (i.e., complete the circuit). The suppression device limits the voltage excursion, and probably actually increases the current. So, I'm not at all sure that a suppression device would prevent the reset In the absence of a suppression device, the current is due to the Y capacitors and stray capacitance in the unit. Initially, the surge current will be high, but then, through the capacitance, it can decay faster than with a discrete suppression device. But, the Y-capacitors will cause a double differentiation and there will be a negative voltage swing before the surge event is complete. A "suppression device" CAN be used to control the current path through the PC such that the surge current does not cause a reset. By the way, Y capacitors can also set the surge current path. Best regards, Rich ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
