Jim Thomson wrote:
> Date: Mon, 22 Mar 2010 02:55:04 -0400
> From: Roger <sub1@rogerhalstead.com>
> Subject: Re: [Amps] big capacitor in HV PS
>
>
> John Lyles wrote:
>
>> The stored energy in that capacitance at 3.6 KVDC is 8.1 kilojoules.
>> This is a tremendous amount to deal with for a power supply. Roughly
>> equivalent to the mechanical force of a part of a stick of dynamite when
>> it lets loose. I would not want to have to deal with the safety aspects
>> of this design. For instance, if there was an internal arc in the power
>> supply, before the series resistor (mandatory) to the load, it would do
>> a lot of damage, such as rupture capacitors, blow holes in metal.
>>
>
> ## well, I have seen > 5 kilo joules short to chassis, by error, and sure
> it made one helluva bang, but it's not that bad. [ b4 glitch]
>
>
>
> This
>
>> is just from the stored energy. An active crowbar might be needed if a
>> series resistor
>> isn't enough to prevent more than a few Joules of energy to be dissipated
>> in a thin wire shorted across the power supply output, to simulate a tubes
>> grid wires.
>>
>
> ## The secret appears to be using the CORRECT sized HV fuse.. just prior to
> any
> 'glitch R. I use the Buss HVU series hv fuses, filled with silica,
> designed to interupt
> super high faults currents /energy levels. They also make HVJ series for up
> to 20
> kw dc/30 kw AC.
>
>
>
>
>
>
>> Then there is the inrush current. Rectifiers would need to be designed for
>> high peak currents, 500 amps or more.
>> Step start would need to be carefully designed to prevent taking out the
>> diodes.
>>
>
> ## standard 6A10 /10A1 diodes work just fine. When t=0... the entire 240
> vac gets dropped across the
> step start R...leaving nothing across the plate xfmr primary. The real
> problem is the mag current in the
> plate xfmr primary. The mag current flows through the step start R... and
> there is always a drop across it.
> Caps will never get fully charged up, and when step start R shorted, you get
> a big sec surge. The trick is to
> use a lower value step start R, when using high C filters. Plan B is to use
> a small variac to ramp up, then it's
> relay switched out of the cicuit.... THEN u can charge the caps up to 102% of
> their normal value.
>
>
>
>>
>>
> Having that much stored energy in a small area just isn't safe if
> something goes wrong.
>
> ## If it's all wired correctly, it's really not a problem..or an issue.
>
>
>
>
>
>
>
>> Seems like a lot of trouble to get stiffer high voltage that isn't really
>> needed for am amateur
>> amplifier for SSB, CW, AM. I would not recommend going above 1/10 of that
>> capacitance, unless prepared
>> to deal with the effects mentioned above. I have a 250 uF capacitor at
>> 28,000 volts at work, and
>> it took months to design the inrush and crowbar protection, to prevent
>> blowing a 30 AWG wire when shorted across the output.
>> This bank of capacitance is in a steel vault with chicken-wire reinforced
>> glass in the windows. Of course, this
>> is far more stored energy than 8.1 k, about 10X this amount, needed to
>> prevent more than a kilovolt of droop
>> during the pulses of an RF amplifier.
>>
>
> ### Up to 10 kvdc... a 50 ohm glitch R does the job as a current
> limiter..just fuse it..
>
>
>
>
>>
>>
> We used to use 50,000 (50 Kilojoules) MOVs across our power supplies.
>
> ## what voltage ? Used for what ? Why weren't the MOV's fused ?
>
They were used across the 480 primary of the power supplies.
They weren't fused as it was their job to protect the rest of the
system, acting more or less like crowbars. We'd have been happy had they
protected the 1200 amp SCRs, but I don't think I ever took out a good
SCR after the MOV blew. At least they dumped the power supply which was
capable of close to half a megawatt. We didn't destroy anything else
down stream.
> ## I use MOV's acoss the sec of the plate xfmr... fused. I also use em on
> the 240 vac side.,. also fused.
>
>
>
>
> When they went it was spectacular from both the light flash and almost
> unbelievably loud noise. We used #6 wire to them and you'd find the two
> wires sticking straight out and I do mean straight! They were even
> stretched a bit. If you were working in the cage next to the one
> containing a running PS when that MOV went it was certainly an adrenalin
> rush.<:-))
>
> ## all that shoukd have happened, is a blown mov fuse.
>
Our purpose was a bit different than using them in HV supplies at low
current. We did not use MOV fuses as that would most likely have
defeated the purpose of the MOV. A pair of shorted 1200 Amp (water
cooled) SCRs on a 480 volt line is an awful lot of power being dumped
into a transformer. The goal was to not turn what was on the other end
into a molten puddle of silver and copper. I don't know what they
weighed, but the transformers were large enough to let a standard
forklift do a wheelie if you weren't careful. After we built auto phase
back control into tap switching on the transformers we went through a
lot fewer MOVs and SCRs. Now those would have made some band
switches. The armatures were 4" wide X 1/2" thick copper, operated by
high pressure (High speed) pistons/ positioners.
73
Roger (K8RI)
> Jim
>
>
> 73
>
> Roger (K8RI)
>
>
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