[amsat-bb] Re: Linear vs. switching and HV DC

[Edward R. Cole]

At 12:26 PM 2/18/2011, Vince Fiscus, KB7ADL wrote:
>At 12:01 PM 2/18/2011 -0900, "Edward R. Cole" <kl7uw at acsalaska.net> wrote:
> >I am rebuilding my 4.2kV PS for the 2m-8877.
>
>
>That'll get you into AO-51.  ;-)

Not me!  I'm not getting the blame for melting the AO-51 receiver.



> >I was 14 years old when working on my DX-35 with 850vdc on the 6146 plate.
> >
> >I agree that many hams these days aren't not trained to handle
> >high-voltage with all the 12vdc solid-state electronics
> >available.  But QRO sspa are running at 50vdc and higher.  There are
> >rules for safe work on HV...time to review them.
>
>
>I know that's true.  Even the large caps in high current 13v supplies. I
>thought I had things discharged and just to be sure before I started poking
>around with a screwdriver checking other things, I thought I would be sure
>the filter caps discharged, so I put the screwdriver across the
>terminals  .....

You know arc welders run at low voltage,,,but lots of 
current.  Discharging a large filter cap is a potential welding 
unit!  Same goes for getting across the terminals of a large 
battery  (a wrench or screwdriver laying across a car battery will 
cause a lot melting and possible overheat the battery enough to explode!


>You should have seen the spark.

And a loud "bang" or "pop"?
You ought to hear what 4kV sounds like when it arcs to ground!

Watch for burn and eye damage situations if working around high 
capacity/voltage systems.

Rule of HV:  NEVER touch a HV circuit until it is verified that no 
voltage remains.  Hooking/disconnecting HV probes to measure HV is 
always done with power-off/discharged.  Connect the HV probe/meter 
and do not touch them when HV is applied.  In commercial HV systems 
there is dead-man hook: a metal hook like a shepards crook with a 
cable to ground attached to a long insulated handle.  You do not hold 
it to short HV circuits.  YOU toss it onto the circuits and step 
quickly back.  I was taught this at NASA where we had a walk in 100kV 
PS (think it was housed in a semi trailer).  It ran the anode on 20kW 
s-band klystron amps.

ALWAYS inform someone when you are about to test HV (in case you need 
medical assistance).  INSTRUCT them to NEVER touch you or  go near 
the equipment if you get shocked/electrocuted.  Show them how to shut 
down the Main Breaker on the house (but CALL 911 FIRST, because a lot 
of cordless phones quit when the power is off).  DO NOT attempt CPR 
if you are not absolutely sure that power is off.  In most 
electrocutions the heart goes into fibrillation and ordinary CPR will 
not revive a person.  It will extend life if it can be done 
safely.  Be sure to tell 911-operator that an electrical shock has occurred.

Most electrocutions above 240v are fatal.  NO SECOND CHANCE!  Sort of 
like falling off that 100-foot tower - you only do it once.




73, Ed - KL7UW, WD2XSH/45
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