>I'm using a 40 ohm and it seems to pull in almost right away. I still
>hear a thump when the power is turned on which leads me to think it's
>not delaying enough. I wonder if a 100 ohm might be a better choice.
>It's a AL80A amp.
>
>>Depends on the size resistor you use and how big the filter cap is in
>>the supply. I select resistor size so that it pulls in after a short
>>delay of a second or so. Too large a resistor and the voltage will
>>never get high enough to pull the relay in.
The delay isn't really a key parameter. The big current surge is over
within a 3-4 mains cycles. After that, the current slowly ramps down, so
you can short out the series resistor as soon as you like. When the
relay pulls in, there is a second current surge - a step-up in current
to the normal standby level.
The optimum is probably to make the first and second steps about equal.
If the series resistor is too low, the first step is too big, but the
second step is only small because the capacitors are already almost
charged up. If the series resistor is too big, the first step is small
but there's a big thump when the resistor is shorted out.
A good rule of thumb is to limit the first step to about the normal
operating current, which all the components should be rated to handle
comfortably. For example, if an amp runs from 230V and draws up to 13A,
the surge resistor could be as low as 230/13 = 18 ohms, or maybe 22
ohms. So even at worst, the first surge can never exceed the normal
operating current. You might go up to 30 or even 50 ohms, but there's no
point in going as high as 100 - that only increases the size of the
second step.
In an SB-1000 (fraternal twin to the AL-80A) I've had good results with
a 47 ohm resistor, shorted by a 12V DC relay connected to the existing
12V rail. This is a version of Rich's step-start - it works very well
and it couldn't be simpler.
No timing components are needed. Everything is supplied from the one big
transformer, so whenever the 12V rail comes up far enough to operate the
relay, that will be the right time to short out the resistor. In
practice this is a small fraction of a second.
If I'd had a suitable 33 ohm resistor handy, I might have used that in
preference to the 47 ohms - but definitely not 100 ohms.
The amp is permanently wired for European 230V mains, and is protected
by a fast 10A fuse which never blows.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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