Han Higasa wrote:
>GM all
>
>As Steve told whatever loop (open or closed) has been chosen
>a situation where a relay contact will not close and inrush-current
>limitation resistor R will never be shorten can happen.
>
>To address this a "step-start duration" timer should be added;
>and to monitor this a "step start duration timer" timer should also be?
>
>Instead, robust R and an operator can do this by perceiving
>"it takes too long and the step-start never lasts"
>
>I am using a 30-ohm 100-W resistor for the R now.
>I think I should disconnect my step-start delay timer and measure
>the voltage across R in time elapsed, to make sure the R will not
>barn out.
>
A much simpler solution is a slow-blow (T) fuse in series with the
current limiting resistor. This idea has been used by Alpha and others
for many years.
The fuse rating can be about 25% of the long-term short circuit current,
eg for 230V and 30 ohms it could be a 2AT fuse. If the normal step time
is less than about 1 second, this fuse will never blow unless the
step-start "hangs". The step-start resistor must be capable of handling
the "hang" current (the steady value when the resistor is tuck
permanently between the mains and the amplifier) for the several seconds
that it takes to blow the fuse, but it doesn't need to be rated to
survive forever - the fuse will protect it.
Incidentally, most step-starts are much longer than they need to be, as
the big current pulses only occur during the first few cycles of the
mains. The aim should be to make the first and second current surges of
equal size, so that neither one is "worse" than the other. A very long
step time doesn't really help with this - it doesn't affect the size of
the first current surge, and doesn't do much to reduce the second
current surge, but it puts much more stress on the resistor.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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