[Boatanchors] Inrush Current Limiter

[Brian Clarke]

Hi Alex,

This is on the way to being a great idea. One tiny problem - using a single 
diode on the mains side of the transformer sends a dc pulse back into the 
mains. Now, from one little radio, no big deal; but if we all did this, the 
power distribution authorities would get a bit upset.

So, here's what I do, which is very similar to your idea and appears in 
many ARRL Handbooks - unfortunately, the ARRL Handbook doesn't 
give you the theory:

Choose a Normally Open [NO] relay that operates from ac and that 
requires 90% of the mains rated Voltage before it pulls in. So, if you are 
in a 100V country [Japan] choose one that pulls in at about 90V; in 
America, choose one that pulls in at about 104V; and, in those parts of 
Europe and its unwilling dependencies where some impression of 
230/240V is foisted on us, choose a pull-in of about 210V. If you live at 
the end of a long distribution run and get all kinds of mains sags, choose 
a lower pull-in Voltage. You may have to test a few relays with a Variac 
to find one with the appropriate pull-in Voltage rating - manufacturers' 
specs may not be accurate enough.

Wire this relay's coil directly across your mains transformer primary. Then 
use the high power resistor across the relay's NO contacts as Alex suggests. 

What value resistor? You can use the t = RC formula. Here R = the startup 
resistor + the transformer's primary resistance + [1/N]^2 x secondary 
resistance, where N = turns ratio of the trannie; C = [1/N]^2 x input filter 
capacitor [assuming capacitive input filtering].
As the C charges up, the load resistance comes into play and slows down 
charging. So, use this approach as a start.

If you are running a 400W Class B set from 28V, the filter cap may be 
100mF [that's milliFarad]. The turns ratio in America would be about 4:1 or 
less. The primary resistance might be about 3 Ohm and the secondary 
resistance about 0.5 Ohm. If you want startup to take about one second, 
then R = 16 x 1s/ 0.1F = 160 Ohm. Subtract [3 + 1/16 x 0.5] Ohm. So, 
start with a 150 Ohm resistor across the relay contacts.

73 de Brian, VK2GCE.
  Alex asked:

  Have you thought about alternative inrush control methods?

  For example, I use small DC relays for the purpose in my amplifiers. They're
  fed through a diode rectifier from the AC line and use a series RC
  time-constant to set the time until they kick in. A high power WW resistor
  is placed in series with the transformer primary. It's ultimately shorted by
  the relay, thus feeding normal AC line voltage to the xfmr. Works like a
  champ.

  Vy 73, AI2Q, Alex in Kennebunk, Maine
  http://users.adelphia.net/~alexmm/ai2q.htm