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@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Jim Thomson
> Sent: Sunday, January 17, 2010 5:48 AM
> To: amps@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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