[Amps] 240 vac primary protection..again

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Thursday, November 25, 2010 6:59 AM
Subject: [Amps] 240 vac primary protection..again


> Carl mentioned about using .01 uf caps  from each  hot leg of the 120/240 
> line to chassis..and tingling.
> That scheme was used on my real old drakes..except that they only had a 2 
> x prong
> 120 vac plug ! [ caps wired hot to chassis..and  neutral to chassis].  One 
> day I had nothing  connected to the
> r4c/t4xc....  and got  tingling, when touching the r4c chassis..and 
> external grnd  [ another piece
> of gear that was well grnded. ]
>
> If u  re-draw it, there is 2 x .01uf @ 1kv disc ceramic caps in series, 
> with their CT  bonded to chassis.
> The 120 vac is dropped across  both of em.   IE:  60 vac  across  each 
> cap.    That  explains why I measured
> 60 vac  between  drake chassis....and  external grnd.    In normal 
> operation, all the radio  gear chassis's are
> bonded via interconnecting coax braids, and also the myriad of phono to 
> phono  rca connectors on the small
> coax used  for the various functions between the Tx  and RX box's.

Ive yet to see a regular disc let loose even on 240V if its properly sized. 
There are hundreds of thousands amps out there using them. Most use a 
.0047/.005 on 240V

Any .01 disc will give you a tingle, especially on 240. The reason for the 
.0047 across the diodes and to ground is that not everything gets suppressed 
at the input. Those very fast rise time spikes get into the transformer, 
pulses are lengthed, and may cause a problem....its certainly not a given. 
With the crappy utility lines in the US these days I dont like taking 
chances especially when the electrolytic string is close to the voltage 
edge.

>
> Apparently, per W8JI, IF  .01uf caps are  wired between 120 v hot leg and 
> chassis, and also neutral to chassis,
> [or between each hot and chassis on a 240 vac input device]... the caps 
> must be  UL rated caps for just that
> purpose.    The common garden variety  .01uf  @ 1 kv  disc ceramics  do 
> not cut it any longer.


Those never did except on 120V. The NANNY state is what forced the change 
since many elcheapo consumer products cut corners plus there were still 
plenty leaky paper caps in use in older equipment. A Sprague Black Beauty 
makes a nice display when it lets loose.

Many in the vintage radio business or individual collectors use the 3 cap 
method these days, especially with a 2 wire cord. One cap between each leg 
and ground and one between the 2 leads. This is only for a transformer 
operated device.


  The UL  rated caps
> for this transient / RF bypass protection  are  2500 V rated devices. 
> Apparently too many fires started with non
> UL rated caps..as many would fail shorted.   A similar thing happened with 
> garden variety  130 v rated MOV's....
> too many house fires.  Modern MOV's  for 120/240 v use, will  have a 3rd 
> lead sticking out [offset to one side]
> and an internal fuse built into em.


MOV's dont catch on fire, its the closely packed other components that 
ignite from the heat. That the reason I never use those CL type surge 
suppressors, they run too hot in normal use and if one lets loose I dont 
want to be responsible.


>
> I see no reason why .01uf  @ 2500 v rated /UL rated bypass caps can't be 
> used from each hot leg of the 240 line
> to chassis, fused or otherwise, in a typ B+  HV supply.   Then also well 
> grnd the chassis of the same B+ supply.
> [either from a  standard 4 x conductor ...'2 x hots, 1 x neutral, and 1 x 
> grnd]...or a separate wire from chassis of
> B+ supply to station grnd [SPG], RF deck, etc.


If youre buying .0047's for the B+ then get 2 more for the input and dont 
worry about it.


>
> Now this  should kill  any transient, before it even gets into the primary 
> of the plate xfmr, alleviating the need
> [or perhaps in addition to]  for a HV  bypass cap  across each leg of a 
> FWB.   Carl had mentioned about using
> a .0047 uf @ 6/10/15 kv disc ceramic across each leg,  4 in total.


4 in a FWB only.
Plus another to ground at the HV output.


>
> In the last Orr book,  they used a RC snubber [ which I think was a 100 
> ohm, 25 watt ww in series with a .01uf/
> .005uf  @ 3/6/10/15 kv rated disc ceramic]... and snubber was wired 
> directly across the sec of the plate xfmr.


That was popular for awhile but probably took out its share of transformers. 
The ARRL HB quit showing it also.


>
> I see no reaon why a .01/.0047 uf @ 3/6/10/15 kv cap couldn't be wired 
> directly across the plate xfmr SEC.


Same reason as above. It would work at LV with a HV cap, it was in all 
vibrator radios as a buffer cap and usually 1600-2000V in the .005 to .01 
range to suppress the ~ 115 Hz vibrator hash. The B+ was around 250V. You 
would want at least the same voltage difference in an amp supply.
>
> Correct size movs  on the 240 v side  should also work.

Put them in a sealed metal box. I have a couple of older USA made outlet 
strips that I built in a line filter that reall works ( A QST/HB  DeMaw 
article) and MOV's. Every couple of years I lose the MOV's from a close 
lightning strike. Open the case, remove the residue and soot and wire in new 
ones.

Carl
KM1H



>
> Later.... Jim  VE7RF
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