AC power  IN-RUSH  surge current  problem and cures:                                

  (to be updated as tools arrive)

The reason this is a topic at all is just one thing, and that is transformer damage. (core and copper flex damage) and possible shorter life on that very rare now rocker switch.
The big reason would be that huge expensive and now rare big transfomer.

There are 3 surges, only one I worry about, true in-rush
  • True Inrush. (a.k.a , flux rush) or Remanence  flux.(10x higher than normal max primary current can happen !) 160mS long.
  • Filament heater warm up, see photo 1;  (the 3-500z 14.5amp filaments are (R=E/I) {5/14.5 =0.35ohms hot, less than 1ohm}  They are less cold utill seconds later land at 14.5amps.(each, 29amps both) The primary is 24 times less.(turns ratio of the Transformer.)
  • and HV capacitor bank charge up. ( at time zero, a cap is zero ohms resistance to any rise in voltage; that is what they do and that means current surge happens) The 98 % T= RxC on mine is  C=  30uF and  R =  2000vac transformer impedance.
Only #1 true in-rush concerns me.

"The core tends to retain the last magnetization field it experienced as it was being powered down. If the power up flux increases in the same direction as the stored flux, the core saturates, drawing a big pulse of primary current._
Some enginers use bleed AC capacitor on the secondary to  make the transformer ring down to zero (faster) after being disconnected from the line, to help demagnetize the core to prepare for the next start. (yes one more  cure  for sure)
The E-I core has no air gap so saturation is easy.
Cheap transformers (light) have no excess core sized to prevent this, lowering cost and for sure weight.
So the surge is fact real the (Knee surge) below.
Add to that a cheap rocker switch that loves to bounce and put you in to the knee at a drop of the hat, even  1 in 10 tries.

The trick is finding FLUX saturation in the core,
You can turn the amp on 10-20 times and miss it !  yup! (if the core is not pre- charged the Knee will no happen)
If you do hit what  I call true in_rush, you will hear the Transformer GRUNT loud. bingo ! (that sound you just heard is not good for the tranformer at all) This is what I want to end and cure (and did)
I have no fears at alll on the  easy filiament currents or HV BANK  CAP.'s charging.
This and  only this is what I want to cure, at the KNEE, and can be cured many ways, the hard way is processor + 1 zero crossing detector.
If you bounce the power button, you can get to  saturated transformer core easy, far too easy in my opinion.

First lets use my crummy old current loop CR17 sensor. (slow and inaccurate.
Photo#1(CR17)  ( the exact current below is not know by me, until I get better tools later) This is not core flux surge it is filaments + CAP.'s.  120vac all.
10mS per devision, 16.6mS 60HZ 120vac all. T=1/F  NORMAL this is and harmless.

Photo#2:CR17.  (it took great patience to find this hit) 10+ times normal, is FLUX induced In-RUSH.
My slow sensor I bet hides it going higher. Consider this just your first clue at to true in-rush flux mode. ( about 6 times normal surge and normal currents)

Below see my tool saturate and clip.(negative  in-rush)

Ham J8Ji , posts show this.  NO tools stated, (mine above are cheap crummy current clamp) (wish I knew if 120vac or 240vac , it matters ! The 120vac standby filiment current is 1.2amps,(and 29amps in the 2 tubes total)
I can tell his tool beats mine hands down on tool bandwidth performance. SW is switch on.  With 40 amps  he must be on 120vac. Seems to me.  (and true fux surge can be 80amps) 10 x nomral is  possible.

Simple CAVE MAN ,Power switches are lame, they bounce, all do, it is how they work !  (Marconi class) (one step above a knife switch)
This is very bad here, the 1st bounce closure,  can cause the core to fully saturate , (IF)  then next bounce you get this HUGE FLUX IN-RUSH, this is never good for any transformer like this. (all transformer makers know this over 0.5kW..

There are 4 cures, (silly resistor + relay cure) My themistor, cure (just 1 part cheap) and #3 the black box cure, a SSR and  digital button and 160mS delay before  turn on is allowed and now zero bounce allowed, and last #4 the zero crossing detector as seen on commeral gear endless)
Many use two, 22 ohm resistors in the power feed, and relays (  Relays are so wrong, and  2 resistors are never needed)
Others use  one 6 ohm resistor and a SSR (solid state relay) a wise cure for sure.  AS seen here.
My cure was easy. A modern INRUSH THERMISTOR. $2.56 cost (free when you ask for free samples to the maker , nice guys them)  SOLD HERE AT NON RIP OFF PRICES.
Just  one  RS1  is needed never  two. Just on the one  Hot 120vac line cut.  It is 5 ohms cold and when the filiments go hot. about 0.1ohms 1/10th of an ohm in silly fractions.  (I in fact tried many of these to best match my exact PA AC power current needs)
240 volt AC same answer asame fix but different part,  240 flow current from 2 hot lines only one needs limit, not both. The primaries are wired in series on 240v  mode.

The hardest cure of all , but 100 % perfect is, the zero crossing detector. into say a ARM or PIC processor (some simple code) and to SSR solid state relay. (like commerical rigs use).
This is not simple, but common in the world of 1kw transformers and up.

2kv Transformer timed perfectly wrong to make this flux hit,  70 amps,  from normal 5amp running system..  (timed right is 8 amps)  90degree delay from zero crossiong and 50% stepped

Note the 100mS duration of the STRESS to  the transformer, can shorten its life greatly do to flex inside. (alternate link for above)

One more (similar) cure might be and SSR , a solid state relay in a box and  fully debounced digital power on button, that turns the SSR on, or  off. and a delay for when turned off,  can not be turned back on for 1 full minute. (this lets the core ring down)
Now the PA actual rocker switch is left on for ever now and  can  not fail  like this.
Lets calls this a smart AC power box. (with Power jack AC on top, and line cord to wall.)
This box makes sure  the transformer core is never saturated ever. (at turn on)
Not tried by me, but will work !

There are many cures, I see most are using, 2 relays and TWO  6 to 22 ohm resistors, why do that? why use relays ever, in the year 2020.

How to make bettery measurments of current is below, lacking current probes $800 to $3000 from TEK.
Later I will add my 2 new sensor data.  (below)
A 120KHz rated band width and at 100 amp. My CR17 saturates far too easy and is slow, nor is it calibrated. (trike 3 me)

This is a fast hall sensor. Device.

Measuring current that is fast and high currents is not easy but tools to do so are more easy now.
The Core on this transformer I am told can hit 100amps  in-rush (for milliseconds only) when you land in the knee saturated core above. (I well do more testing soon)

Comments :
Flex happens at the  knee can in fact cause copper core wire to scuff off its insulation ! Flex is caused by core staturation ! for a few miliseconds. (guessing 16mS ,60hz period)
"Magnetizing inrush current in transformer is the current which is drown by a transformer at the time of energizing the transformer. This current is transient in nature and exists for few milliseconds. The inrush current may be up to 10 times higher than normal rated current of transformer."
PARTS I used to cure this:

Ametherm makes these wonderful parts.
I used this on , on my Heathkit SB200 too, The device has a cold spec. for ohms and goes near 0 ohms hot , my job is to pick a part that works from power on to standy.. (this page tells us what to do ! a starting point)
SSI = steady state current (I) and in our case we are in standby to get SSI/
The only down side to these method is , let the Device RS1 cool off first at turn off before turning it back on.

Note the 0.5kv transformer at 120vac many have 104 amp in-rush, current,  (to see true in-rush is not easy, it takes fast hand at the switch (mad hand) and a very good and  very expensive current probe.(for the scope) {most cheap current probes are slow, and do not see max}

Inrush max = 15amp (fuses) but this is slow acting fuse, and real in- rush is up to 10 times that. or more. (I have no calibrated current probes to measure  but I do have crude probe views here)
I run true 30amp 120vac service so there is no sag using robust 10AWG house main breaker power feed to my shack, I have no wall AC power v-drop, as will be seen on weak 14/20Amp feeds (wall) that means I must have the best protector.  My heaters run at 5v exactly. (my goal)
Example (I will later show best part tried) Select the smallest device that works at your SSI, is goal.
One part is the  SL22 4R014  4 ohms,cold  but at  1/2 load the ohms is at .12ohms and drops to 0.05 ohms fully hot and transmitting.  $1.56 (2019 cost)    Do not buy these parts from fleabay scaplers.
Later I went to a SL32 5R020. part. (5 ohm cold and way less hot)  5 ohms  has profound drop is in-'. This is my only purpose here is to stop the knee above from happening.
What I do is find a thermistor that gets hot fully from power on to standby.  (with highest possible cold ohms spec on the part, within reason) Keep in mind the wild 2 resistor relay fix is 22 ohms. but are too slow., see the 7ohm SB202 fix here.
The correct part, will be updated later here, I have a box full of these Amertherms and test them LIVE. for proper best preformance. (voltage drop across it very low, 1v or less, in standby (unkeyed) The 5R020 worked best in my SB200

rev 1   ++++12-19-2019