[Amps] re: Magic values for "Glitch" Resistors in HV Pwr Sply

Jim's EBAY Account Jim's EBAY Account" <jimhomer@email.com
Sun, 22 Dec 2002 09:16:32 -0800


To the collective intellect abroad at "AMPS"...  <}:<}

Regarding the use of surge limiting resistors in high voltage plate
supplies:

Once, somewhere, I read that the surge limiting resistor should be of a
value
that will drop at least 25% of the plate voltage at 5 times the normal "max"
current of the supply.  And, the wattage rating should be exceeded by a
factor of 10 or more at that current (so it will fail as needed - like a
fuse).

Let's do some math.

If this is correct, a typical application might be:

50 ohm, 100 watt ceramic power resistor in negative lead of a 5000
vdc 1.0 amp CCS Power Supply

...where,

UNDER NORMAL CONDITIONS (under 1 ampere load):

Plate B+     "Normal"         Volts dropped         Watts used at
Voltage       max  Iplate      at "normal" Iplate     "normal" Iplate

5000 vdc      1.0 amp         50  volts                  50 watts
                                                                        (1/2
the resistor's rating!)

UNDER SURGE CONDITIONS (5 times normal current, or 5 amp surge):

Plate B+     "Surge"            Voltage drop @ 5x         Wattage dissipated
Voltage       max  Iplate      "normal" Iplate                 under 5 x
"normal" current

3750 vdc     5.0 amp          1250 vdc drop                 6250 watts
                   (5 x "normal")  (25% of P.S. Voltage)      (62.5 x the
resistor's rating!)


I realize that winding resistance, core flux collapse and etc will have a
limiting
effect as well, but I have 160 UFD of filter capacitance, so there are a
large number
of joules available from the filter caps!

It is fairly obvious that the resistor will probably fail before a surge
reaches, say,
5 amps because 5 amps takes the resistor to over 62 times its power rating.

Here's the question...

What is the best "type" of power resistor for this?  I read somewhere that
normal
ceramic power resistors can stand about 500 volts of "drop" per inch, but
that
they tend to be very "forgiving" with short duration surges, meaning they
fail slowly.

Many commercial HV power supplies I have repaired have large 'carborundum'
type resistors - (expensive and hard to find surplus in the right values) in
series
with special HV fuses (also expensive and hard to find).

Would the "dale" type power resistors that bolt to a chassis work better or
worse?
( I have attached a picture..! - Virus-free! Promise!)

What are the pros and cons of each type?  It seems the "dale" bolt-down
would
tend to minimize 'shrapnel' in the power supply (if it blows) due to its
basic design.

What are the "rules of thumb" with regards to joules (watt-seconds) ratings
in such
a case?  My present project application will produce 5000 VDC at 1.0 amp,
and I want to be such the tube is protected.  I am currently considering 50
ohms
at 100 watts.

I can do the math, but lack experience in this area. Any advice is
appreciated.

Thanks for any guidance, gentlemen.

Jim Homer W6JNG







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