Re: Fw: working voltage measurement
Yes, it's important to recognize the limitations of a test method before relying on it. In this case, the A-B method was used at power frequency and below, to observe discharge time for UL testing. The MAIN advantage was not balance so much as that the chassis was isolated from the potential being measured by the impedance of its input amplifiers. This is an additional safety measure not provided by battery operation. The _previous_ method used had been to remove the safety grounding terminal (pull the third pin) and float the whole 'scope at line voltage. The possibility of a mistake hooking things up made operators uneasy. An expedient to make channel gain difference less of a problem is to measure the point under test with both channels at once, in A-B mode, and as high a gain setting as possible without clipping in the input stage, then adjust their relative gains so that the offset on the screen is within a tolerable amount. This is often adequate for troubleshooting or specific tests. Cortland My thoughts alone are what I write Not my employers', on this night; I wish list members gathered here A happy, prosperous, New Year! --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
RE: Fw: working voltage measurement
Just to add to what Rich has stated - the CMRR will also vary with frequency for both passive and active systems. These variation are caused by small differences in the LCR coupling of the probes and the finite bandwidth of any amplifier. I remember using a very high gain product (A Thermal Imager - the noise figure was down around (800pV/Hz^0.5)and the end of runway radar bursts of RF were demodulated by the combined non-linearities and finite CMRR to wipe-out the IR image. I think this story might also be relevant to the debate about spurious emission in safety applications - although I believe that 'proper' safety circuits will be designed so that a single fault (such as a junction s/c or capacitor o/c)will not affect system safety. Best regards and Happy New Year Gregg -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Rich Nute Sent: Monday, December 31, 2001 1:46 PM To: Cortland Richmond Cc: xin...@cesi.ac.cn; emc-p...@majordomo.ieee.org Subject: Re: Fw: working voltage measurement Hi Cortland: > One can also use a pair of probes known to be well balanced and take the difference between the A and B channels, or > use a differential input accessory or plugin. There is then no possibility of high-voltage on the instrument chassis. "Balance" is really Common-Mode-Rejection-Ratio, CMRR. There are two schemes for difference measurements. One is measuring the high and low signals independently (through Channel A and Channel B), and then subtracting the two (A and B) signals. This requires both channels to have exactly the same gain (and phase shift) throughout the frequency range of interest. A 1% gain difference yields a 100:1 CMRR, which is quite poor -- almost useless. Another scheme is the difference amplifier, where the high and low signals are applied to the + and - inputs of the same amplifier. A good difference amplifier is capable of 100,000:1 CMRR at low frequencies. The CMRR of a difference amplifier falls off as the frequency increases. A difference amplifier may not have sufficient bandwidth to accurately measure the high frequencies in a SMPS. I have found the portable, battery-operated scope to be the most accurate and easy way of measuring waveforms within a SMPS. Best wishes for the New Year, Rich --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Fw: working voltage measurement
Hi Cortland: > One can also use a pair of probes known to be well balanced and take the > difference between the A and B channels, or > use a differential input accessory or plugin. There is then no possibility > of high-voltage on the instrument chassis. "Balance" is really Common-Mode-Rejection-Ratio, CMRR. There are two schemes for difference measurements. One is measuring the high and low signals independently (through Channel A and Channel B), and then subtracting the two (A and B) signals. This requires both channels to have exactly the same gain (and phase shift) throughout the frequency range of interest. A 1% gain difference yields a 100:1 CMRR, which is quite poor -- almost useless. Another scheme is the difference amplifier, where the high and low signals are applied to the + and - inputs of the same amplifier. A good difference amplifier is capable of 100,000:1 CMRR at low frequencies. The CMRR of a difference amplifier falls off as the frequency increases. A difference amplifier may not have sufficient bandwidth to accurately measure the high frequencies in a SMPS. I have found the portable, battery-operated scope to be the most accurate and easy way of measuring waveforms within a SMPS. Best wishes for the New Year, Rich --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Fw: working voltage measurement
Hi Xing Weibing: > 2. supply the SMPS from an isolating transformer where > both supply conductors are isolated from ground. > 3. connect secondary's ground and primary 's ground (neutral and ground > are connected together) This is the correct connection for measuring the primary-to-secondary voltage. > What I mean originally is what waveform of primary and secondary for SMPS > should be : > Switching waveform , sin waveform moduated by high-frenquency switching > waveform or other . The correct SMPS waveform between primary and ground (i.e., grounded secondary) is a switching waveform superimposed on a (mains-frequency) sine wave. Best wishes for the New Year, Rich --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Fw: working voltage measurement
Dear Mr. Rich Nute: Thanks for your e-mail we have made measurements of primary voltages with respect to secondary circuits as follows:: 1. using a mains-operated scope, 2. supply the SMPS from an isolating transformer where both supply conductors are isolated from ground. 3. connect secondary's ground and primary 's ground (neutral and ground are connected together) 4. we adjust trigger level and time base to obtain the full waveform we are interested in What I mean originally is what waveform of primary and secondary for SMPS should be : Switching waveform , sin waveform moduated by high-frenquency switching waveform or other . Best Regards Xing Weibing - Original Message - From: "Rich Nute" To: Cc: > > y d > > Hi Xing Weibing: > > > One of the problems of making oscilloscope measurementsg > within switching-mode power supply circuits is that the > common side of the oscilloscope *MAY* change the waveform. > > There are two ways to minimize the effect of the common > side of the scope from affecting the waveform: > > 1) If you are using a mains-operated scope, supply > the SMPS from an isolating transformer where > both supply conductors are isolated from ground. > In this way, you can connect the scope common > lead to any point in the SMPS with minimum effect > on the waveform. > > 2) Use a battery-operated scope such as the Fluke > Scopemeter. You can connect the scope common to > any point in the SMPS with minimum effect on the > wavefrom > > When a scope measures the RMS value of a waveform, it > does so over the time displayed on the screen. If the > time base is changed, then the RMS value *WILL* change > because the displayed waveform is changed. > > Likewise, when the trigger level is changed, the display > changes the starting point for RMS calculation, and the > RMS value may change if the waveform is not repetitive > for the duration of the time base. > > An accurate RMS value is obtained when the scope measures > complete, full cycles of the waveform. If the number of > cycles is not full, then the RMS value is the value for > the number of full cycles plus the partial cycle, and will > not represent the RMS value of interest. > > To get a useful RMS value of a complex waveform, use a > very long time base, such as 0.1 second/division or > longer. You can verify that this is a useful RMS value > if the RMS value does not change significantly with > trigger level or time base setting. > > If you are making measurements of primary voltages with > respect to secondary circuits, then you must ground the > secondary circuits, and operate the SMPS from a grounded- > neutral supply. (If you use an isolating transformer for > primary-secondary voltage measurements, there is no > reference for the primary circuit, and all measured > voltages are incorrect.) > > Good luck! > > > Best wishes for the holiday season, > Rich > > > > > > Hi Group > > > > I have a question regarding working voltage measurement of IEC 60950 > > > > we have a E.U.T.(switching power supply) > > I want to clarify the measurement of working voltage > > > > 1. Using an oscilloscope having an adequate bandwith and using a high > > impedance probe (100Mohm), and adequate integration time to measure > > working voltage. > > The load on the secondary circuits is to be varied in order to find > > highest > > voltage across the insulation. Floating secondary outputs (capacitively > > connected to > > earth) > > are earthed. > > > > 2. don't make connection between primary winding and secondary winding. > > > > 3. we will get a stable waveform on the oscilloscope. > > > > working voltage we measure are as follows: > > The waveform we get by the above method is a kind of waveform modulated by > > high-frequency > > switching waveform. > > MEASURED voltage: 246V(rms),576V(peak) > > When we change trigger level and time base to obtain stable switching > > waveform, we get > > different rms voltage > > with different trigger level. the highest rms voltage we get is 380V. > > > > My question > > 1. the above steps are correct or not? > > > > 2. For switching power supply, what waveform of working voltage is correct > > for primary > > and secondary > > How to obtain? > > > > 3. Which one(246V, 380V) is correct for working voltage measurement? or > > other methods? > > > > > > Thank you for any comments in advance > > > > Best Regards > > > > Xing weibing > > > > 2001-12-17 > > > > > > --- > 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: > majord...@ieee.org > with the single line: > u
Re: Fw: working voltage measurement
One can also use a pair of probes known to be well balanced and take the difference between the A and B channels, or use a differential input accessory or plugin. There is then no possibility of high-voltage on the instrument chassis. Cortland (my own thoughts, and nor those of my employer) Rich Nute wrote: > There are two ways to minimize the effect of the common > side of the scope from affecting the waveform: > > 1) If you are using a mains-operated scope, supply > the SMPS from an isolating transformer where > both supply conductors are isolated from ground. > In this way, you can connect the scope common > lead to any point in the SMPS with minimum effect > on the waveform. > > 2) Use a battery-operated scope such as the Fluke > Scopemeter. You can connect the scope common to > any point in the SMPS with minimum effect on the > wavefrom --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Fw: working voltage measurement
Hi Xing Weibing: One of the problems of making oscilloscope measurements within switching-mode power supply circuits is that the common side of the oscilloscope *MAY* change the waveform. There are two ways to minimize the effect of the common side of the scope from affecting the waveform: 1) If you are using a mains-operated scope, supply the SMPS from an isolating transformer where both supply conductors are isolated from ground. In this way, you can connect the scope common lead to any point in the SMPS with minimum effect on the waveform. 2) Use a battery-operated scope such as the Fluke Scopemeter. You can connect the scope common to any point in the SMPS with minimum effect on the wavefrom When a scope measures the RMS value of a waveform, it does so over the time displayed on the screen. If the time base is changed, then the RMS value *WILL* change because the displayed waveform is changed. Likewise, when the trigger level is changed, the display changes the starting point for RMS calculation, and the RMS value may change if the waveform is not repetitive for the duration of the time base. An accurate RMS value is obtained when the scope measures complete, full cycles of the waveform. If the number of cycles is not full, then the RMS value is the value for the number of full cycles plus the partial cycle, and will not represent the RMS value of interest. To get a useful RMS value of a complex waveform, use a very long time base, such as 0.1 second/division or longer. You can verify that this is a useful RMS value if the RMS value does not change significantly with trigger level or time base setting. If you are making measurements of primary voltages with respect to secondary circuits, then you must ground the secondary circuits, and operate the SMPS from a grounded- neutral supply. (If you use an isolating transformer for primary-secondary voltage measurements, there is no reference for the primary circuit, and all measured voltages are incorrect.) Good luck! Best wishes for the holiday season, Rich > Hi Group > > I have a question regarding working voltage measurement of IEC 60950 > > we have a E.U.T.(switching power supply) > I want to clarify the measurement of working voltage > > 1. Using an oscilloscope having an adequate bandwith and using a high > impedance probe (100Mohm), and adequate integration time to measure working > voltage. > The load on the secondary circuits is to be varied in order to find highest > voltage across the insulation. Floating secondary outputs (capacitively > connected to > earth) > are earthed. > > 2. don't make connection between primary winding and secondary winding. > > 3. we will get a stable waveform on the oscilloscope. > > working voltage we measure are as follows: > The waveform we get by the above method is a kind of waveform modulated by > high-frequency > switching waveform. > MEASURED voltage: 246V(rms),576V(peak) > When we change trigger level and time base to obtain stable switching > waveform, we get > different rms voltage > with different trigger level. the highest rms voltage we get is 380V. > > My question > 1. the above steps are correct or not? > > 2. For switching power supply, what waveform of working voltage is correct > for primary > and secondary > How to obtain? > > 3. Which one(246V, 380V) is correct for working voltage measurement? or > other methods? > > > Thank you for any comments in advance > > Best Regards > > Xing weibing > > 2001-12-17 --- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Fw: working voltage measurement
Hi Group I have a question regarding working voltage measurement of IEC 60950 we have a E.U.T.(switching power supply) I want to clarify the measurement of working voltage 1. Using an oscilloscope having an adequate bandwith and using a high impedance probe (100Mohm), and adequate integration time to measure working voltage. The load on the secondary circuits is to be varied in order to find highest voltage across the insulation. Floating secondary outputs (capacitively connected to earth) are earthed. 2. don't make connection between primary winding and secondary winding. 3. we will get a stable waveform on the oscilloscope. working voltage we measure are as follows: The waveform we get by the above method is a kind of waveform modulated by high-frequency switching waveform. MEASURED voltage: 246V(rms),576V(peak) When we change trigger level and time base to obtain stable switching waveform, we get different rms voltage with different trigger level. the highest rms voltage we get is 380V. My question 1. the above steps are correct or not? 2. For switching power supply, what waveform of working voltage is correct for primary and secondary How to obtain? 3. Which one(246V, 380V) is correct for working voltage measurement? or other methods? Thank you for any comments in advance Best Regards Xing weibing 2001-12-17
