tgkreimer@miavx2.ham.muohio.edu (Tom Kreimer) (02/05/91)
Hello! A service man, while working on a persistant and difficult problem with some of our new PCs, was grasping for straws and stuck his multi-meter in the AC outlet. He noticed 118v, ok, and then noticed between .2 and .8v between the Neutral and the Ground. He thinks it should be 0v and MAY be a contributing factor to our "problems". (The problems, not to bore you, are intermittent keyboard line failures during bootup on some DEC/TANDY PCs.) Now, one of our guys was checking into it and found that while the Hot was a clean sine wave, the Neutral was a bit "fuzzy". He also noticed that voltage between Neutral and Ground went to zero whent there was no load on the line. What I am asking you, the world, is this BULLSHIT, or NOT???? From my simple knowledge of electricty, I vote for Bull-doo-doo. Before we invest any further energy into this drain, what do you think? Am I right, or not? What is the best/simplest way to check the AC line? *** Bonus points for naming the movie in the subject line *** ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tom Kreimer --- Miami University --- Oxford, Ohio tgkreimer@miavx1.acs.muohio.edu --- tgkreimer@miavx2.bitnet <-- Use thease! <x> Standard disclaimer < > No flames < > Reply/w E-mail <x> Have a nice day!
henry@zoo.toronto.edu (Henry Spencer) (02/06/91)
In article <1991Feb5.005045.388@miavx2.ham.muohio.edu> tgkreimer@miavx2.ham.muohio.edu (Tom Kreimer) writes: >between .2 and .8v between the Neutral and the Ground. He thinks it >should be 0v and MAY be a contributing factor to our "problems"... >Now, one of our guys was checking into it and found that while the >Hot was a clean sine wave, the Neutral was a bit "fuzzy". He >also noticed that voltage between Neutral and Ground went to zero >whent there was no load on the line. > >What I am asking you, the world, is this BULLSHIT, or NOT???? It is Ohm's Law. When there is plenty of current flowing in the neutral wire, V = IR, so there will be a voltage drop along it. When talking about serious amounts of power, the resistance of a wire is *not* zero. Nor is its inductance, for that matter. What you are seeing sounds pretty much normal to me. If neutral were always precisely at ground, we wouldn't need a separate ground wire. -- "Maybe we should tell the truth?" | Henry Spencer at U of Toronto Zoology "Surely we aren't that desperate yet." | henry@zoo.toronto.edu utzoo!henry
brian@ucsd.Edu (Brian Kantor) (02/06/91)
It's been my experience that anything over a volt or so difference between the ground and neutral is an indication of an overloaded or high-resistance neutral, but frankly, with today's superior power supply designs, it's rare for that to be a real problem. I think your CE is totally stumped and looking for excuses. BTW, if all devices (say, a computer and its peripherals) are grounded together so that there's no potential difference between each other's grounds, the neutral to ground voltage on their feed is even less important. No neutral that has a load on it is ever going to show exactly zero volts; it's carrying current and therefore has a voltage drop; the ground isn't carrying current and won't. - Brian PS: how come my mailer can't figure out your address?
johne@hp-vcd.HP.COM (John Eaton) (02/08/91)
<<< < high-resistance neutral, but frankly, with today's superior power < supply designs, it's rare for that to be a real problem. --------- Todays switching supplies are superior in size,cost and efficiency but are lousy as loads. They act as negative resistance by drawing more current as the voltage decreases and tend to only draw current during the peak times of the AC cycle. This leads to problems with harmonics on the AC line. I have heard of one case where a large number of PCs on a line caused excessive current on the ground line and started a fire. Some safety agencies are looking into regulating the harmonics that a supply can put onto the AC line. <I think your CE is totally stumped and looking for excuses. ---------- Could be. I would check to see that all grounds are solid and non current carrying. Move PC's around to see if the problem follows a machine or an outlet. John Eaton !hp-vcd!johne
jcallen@Encore.COM (Jerry Callen) (02/14/91)
In article <2470012@hp-vcd.HP.COM> johne@hp-vcd.HP.COM (John Eaton) writes: >Todays switching supplies are superior in size,cost and efficiency >but are lousy as loads. They act as negative resistance by drawing >more current as the voltage decreases and tend to only draw current >during the peak times of the AC cycle. Wait a sec - isn't it true that ALL power supplies, once past the initial power-on state, only draw current during the peaks in the AC cycle? The filter caps will only draw current when the voltage out of the bridge is greater than the voltage on the cap, right? -- Jerry Callen jcallen@encore.com
cdl@chiton.ucsd.edu (Carl Lowenstein) (02/14/91)
In article <14050@encore.Encore.COM> jcallen@encore.Com (Jerry Callen) writes: |In article <2470012@hp-vcd.HP.COM> johne@hp-vcd.HP.COM (John Eaton) writes: |>Todays switching supplies are superior in size,cost and efficiency |>but are lousy as loads. They act as negative resistance by drawing |>more current as the voltage decreases and tend to only draw current |>during the peak times of the AC cycle. | |Wait a sec - isn't it true that ALL power supplies, once past the initial |power-on state, only draw current during the peaks in the AC cycle? |The filter caps will only draw current when the voltage out of the |bridge is greater than the voltage on the cap, right? ALL covers quite a bit of territory. Back in the olden days, we learned how to build power supplies with "choke input filters" that drew current from the AC source during the whole cycle. (except for a small time region as the voltage crossed 0 and the rectifiers switched). The modern switching supply tends not to have any 60-cycle iron-core components, thus saving weight and volume. But the external efficiency is lower, because of the peak current drain through the power grid. And, as pointed out above, the switching supply does act as a negative resistance load, and can adversely affect the stability of the power grid. -- carl lowenstein marine physical lab u.c. san diego {decvax|ucbvax} !ucsd!mpl!cdl cdl@mpl.ucsd.edu clowenstein@ucsd.edu