🚀 go-pugleaf

wtm@uhura.neoucom.edu (Bill Mayhew) writes:

> About 12 years ago, I worked in a tempest laboratory in Ft. Meade.
> We were testing various RF gear for EMI/EMC.  One of my jobs was to
> get some new RF screen rooms certified and running.  I was working
> with a crusty old-timer.
> 
> I was installing Topaz tap-switcher type regulating autotransformers
> on the 120 V bench power feeds for the insturment tables in the ante
> rooms.  I was using a Digitron DVM to check the outlets after
> installing the autotransformer.  I stuck the DVM leads in the wall
> socket and got an indication of 1,800 volts.  I called the
> old-timer over to take a look at my wiring, which turned out to be
> OK.  His assesment was that, "Damn DVMs aint no good anyway, let me
> get my Simpson 260."  Shortly, he returned hoping to teach me a
> lesson about the inferiority of new-fangled instrumentation.  He
> confidently set his 260 to (if memory is right) the 250 volt scale
> and stuck the probes in the socket.  There was a resounding,
> "Thwack!" as the needle pinned all the way to the right.
> 
> Out of curiosity, I got out a Tek 551 with an HV probe and checked
> out the socket.  Lo, there was a nice 1800 volt ~1 MHz oscillation
> where there should have been 120V at 60 Hz.
> 
> I took down the transformer to check it out.  I found that one of
> the tap switcher triacs was open and so was the fuse on the
> controller board.  I rearranged the triacs so that the highest tap
> was unused and put in a new fuse.  Nice 120V at the wall.  Topaz
> sent me a new control board.
> 
> I subsequently moved on to another lab, and had a chance to talk to
> the technician.  They had another transformer that failed the same
> way about six months after I left.
> 
> I didn't check to see how much current that bad transformer would
> have driven, but my guess is that it was not insignificant.  Would
> have been real interesting if somebody had plugged an HP network
> analyzer into that socket without checking the socket first.
> 
> 
> -- 
> Bill Mayhew      NEOUCOM Computer Services Department
> Rootstown, OH  44272-9995  USA    phone: 216-325-2511
> wtm@uhura.neoucom.edu   (140.220.1.1)

NO LOAD VOLTAGES ARE OFTEN HIGH IN TRIAC DRIVEN CIRCUITS DUE TO THE 
AUTOTRANSFORMER INDUCTANCE. BUT IF IT WERE LOADED< THE SPIKES WOULD HAVE 
DROPPED AND ALL INSTRUMENTATION CAN HANDLE 1KV spikes. Try a heater for 
loading when measuring Autotransformer tap settings.  You'll get more 
accurate results.. DVMs by the way tend to measure peak and assumes SINE 
and convert Peak to RMS, then Analog meters measure AVG and assume SINE 
to convert to RSM. THis is why you should never use DVM's for 
non-sinusoidal signals. Don't be afraid of new fangled 
instruments...understand them....

dino@inqmind.bison.mb.ca
The Inquiring Mind BBS, Winnipeg, Manitoba  204 488-1607