[Amps] Baffling DUAL HV problem ! We need some help.

[Gary Schafer]

Hi Jim,

If I understand your circuit correctly the problem is simple. You have two
separate bridge circuits. One from zero on the transformer to a hi tap. The
other bridge from zero on the transformer to a lower tap. When no current is
being drawn all the diodes behave ok with normal and reverse bias as a
bridge should. However when current is drawn some of the bridge diodes will
be biased differently and will short out the winding taps. 
Remember that the lower tap is out of phase with the high tap as far as the
hi tap bridge is concerned. 
Essentially the second bridge has one of its diodes biased backwards  when
current is drawn on the other bridge which causes the short. It is the phase
relationship between taps that causes the problem.

Another way to look at it, the B- sides of the two bridges are tied
together. The B- side of a bridge would normally be the same as a "center
tap" as far as the transformer is concerned. But now you have a second
bridge tied to one common end of the transformer and a second tap which
alternately gets switched to B- and you have a difference voltage by the
amount of voltage that exists between taps. Each "center tap" is trying to
be at B- at the same time when current is being drawn.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Jim Thomson
> Sent: Sunday, January 17, 2010 5:48 AM
> To: amps at contesting.com
> Subject: [Amps] Baffling DUAL HV problem ! We need some help.
> 
> Ok,  here's one that has myself and a friend  really baffled.
> 
> We  decided to try and hang  TWO  x FWB's  and  TWO x HV  filter caps...
> both from the SAME plate xfmr.
> 
> Supply #1  uses  a FWB  [ 10 x 6A10's per each of the 4 x legs]  and a
> high C  filter.. [ 12 x lytics in series]
> 
> Supply #2     also uses  a  FWB  [ but more  6A10's per leg]   and also
> a high C filter  [ 20+  x ltyics]
> 
> Dahl plate xfmr  has  sec taps   like  0-2300-2900-3900-4600-5000-5400.
> 
> Supply #1   hangs off   the  " 0  and  2900 taps"
> 
> Supply  #2  hangs off the    "0  and   5400 taps"
> 
> RVS connected  6A10's wired between  B-  and chassis.. on both
> supplies.
> 
> Standard Pri setup, with contactor #1  applying 240 vac.    Contactor
> #2  shorts out the step start R, after XXX  seconds.
> 
> Good so far.   When testing the supply's ...ONE AT A TIME  [ IE:  fwb on
> UN-used supply disconnected from  sec]  everything is fine.
> 
> ## supply #1 =  aprx  4180 vdc          Supply #2 =  aprx 7800 vdc
> 
>  OK, now we tie down BOTH  FWB's  to the sec.     B+  of power  supply
> #1  is  temp disconnected  from  VHF RF deck.
> B-  of  supply #1  IS  connected to  VHF  RF deck.       BOTH  B+  AND
> B-  of  supply  #2  IS  connected to HF  RF deck.
> 
> Ok,  both supplies work fine.  B+ meter's everywhere read  what they
> should.   Plus a  Fluke  0-40 vdc probe available.
> 
> Now,  here's the part  when all hell breaks loose.     With  B+
> connected to VHF deck... [ at this point, B+  and B-   from each supply,
> are connected to their
> respective VHF /HF  rf decks]   and   B+  switched on... this is what
> happens.   Voltage as seen on VHF deck  sails  up PAST 5 kv !   This is
> during the 20 x second  step start delay
> sequence.  Before he could hit the off switch on the power supply,
> BOTH  the    AC  HV fuse  for  supply #1  AND DC  HV fuse, also for
> supply #1...  blow open!
>   In both supplies,  a  HV AC fuse is located between sec  and input to
> FWB  [ one leg only]   A  HV  B+  fuse is located just prior to the
> respective glitch R.
> Supply #2 isn't even scathed, nothing happens to supply #2  [ no fuses
> blown]
> In addition, the big 240 vac breaker that feeds the input of the plate
> xfmr, ALSO  trips open.
> 
> To sum up, everything is  fine, UNTIL  the B+  gets plugged into  VHF
> deck .    We traced this thing 6 x ways to Sunday, and can't  find any
> back door path  anywhere.
> Funny thing though.  With the Fluke HV probe  going... we noticed that
> supply #1  showed  4200 vdc, when measuring directly between  B-  and B+
> .  However, when probe was between
> B+  and CHASSIS... then we measured  5 kvdc !!
> 
> So what are we doing wrong here ???    What am I missing ?   The concept
> was to obtain two simultaneous, but  different B+  voltages, and not
> have to buy a 2nd  plate xfmr.
> Only one RF deck was to be  driven,  at any time.  Moot point, since the
> plate xfmr  can handle the load of both Simultaneously.
> 
> There is NO resistor's  between  B-  and chassis.. on either supply.
> [Only rvs connected  6A10 diodes]  Rvs connected diodes are also located
> in both RF decks, also between  B- and each RF deck chassis.
> The rvs connected diodes ensure the B- can never float more than  +/-
> .8vdc  from chassis potential.
> 
> Individually, EACH  RF deck  works superb.. as long as ONLY the mating
> B+ supply is hooked up to the sec of plate xfmr.
> 
> Is the  dual supply /one plate xfmr  concept flawed ???     We  really
> don't want to have to install a 2nd plate xfmr  for  Supply #1.    Plan
> B was to install vac relays, and wired such that only one FWB is tied to
> the
> plate xfmr sec... at one time.
> 
> The only thing common to both supplies is the chassis.   Also, both  RF
> deck chassis's are well bonded together.    Only the  '0'  tap of  the
> plate xfmr is  common to each FWB.
> 
> baffled........ Jim   VE7RF
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