Harold Mandel wrote:
> Dear Steve,
>
> You raise a very good point in your question about what happens to a
> switching P/S
> in an arc situation.
Hi Hal. I understand all the points you make - for sure anything with
semiconductors in it has to be potentially more fragile than a simple
mains transformer, but I think it also depends on situation. Here's my
reasoning (and I'm wide open to correction):
Where there's a heater and indirect cathode, a well designed amplifier
should have protection diodes so that the cathode doesn't shift by more
than a few volts during an arc, so the switcher shouldn't be exposed to
any big voltage spikes whether or not one end of the heater is tied to
the cathode. Arc fault current shouldn't flow through the heater circuit
at all. The heater winding will be an isolated winding, no ground
reference and with 5+kV insulation to ground so even if eht does reach
the heater somehow, the energy transferred into the electronics will be
through the interwinding capacitance, and I think a pair of beefy FETs
will probably handle that.
73, Steve
>
> In a Class A power supply (ferroresonant, etc., ) there's a chance the
> filter cap(s) might puncture the dielectric (and their case(s) ), the
> rectifier
> diodes might open or short, the "glitch" resistor might explode violently,
> etc., etc.
>
> In a switching power supply all bets are off.
>
> In the Telco world, seeing HV (ESD) and lightning damage to
> switching supplies for several years has led to the following observations:
>
> The microprocessor rectifier-controller gets wiped out and one result is
> a +24 volt switcher bank pushes +58 VDC into a set of VRLA battery cells,
> cracking them open and pushing out caustic goo, the radios on line all
> frying
> from more than their rated input. All new gear needed.
>
> Another observation was that the switchers just went all dead like there
> was no a.c. input. The microprocessor controller (MCA) looked the same way.
>
> The radios all worked until the batteries depleted. Since the MCA was dead
> there was no Low Voltage Disconnect operation until the battery voltage
> couldn't
> keep the solenoid magnets pulled in: All new batteries, rectifiers and MCA.
>
> Yet another scenario seen in Syracuse: The switchers shorted out and the
> resulting overvoltage boiled out the floating cells and the hydrogen gas
> ignited and caused a Halon discharge. The fuses blew in the radios because
> they were ancient. Modern radios would have fried.
>
> Hal
> W4HBM
>
>
> Tom W8JI wrote:
>
>>>I'm aiming for a floating supply as the current project is a tetrode
>>>running with the screen at ground potential. I initially thought about a
>>>dc supply where the switching frequency can be filtered to a minimal
>>>level, but then I thought about using the ac (more or less square wave)
>>>instead for simplicity. Capacitive coupling from the heater to the
>>>cathode is quite high impedance, and the cathode is bypassed to ground
>>>so I figure that the likelyhood of heater voltage getting transferred
>>>onto cathode-control grid bias voltage is low.
>>
>>
>>How will a switching supply handle a HV to ground fault if the tube arcs?
>
> For what I have in mind, the heater supply will come from the secondary
> of a fully isolated transformer. 5kV insulation is easy to achieve and
> fault current shouldn't flow through the heater. I don't see any major
> difference from what happens with a regular mains transformer.
>
>
>
>
>
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