As David said step starts circuits are simple. There is no need for a timer
etc. In a couple of supplies that I have I just place a low value resistor
in series with the mains and place the relay coil, of the relay that shorts
out the resistor, across the transformer primary. When the capacitors charge
up enough the voltage across the primary will also rise. When it gets high
enough to pull in the relay you are done.
The only problem with doing this is that the relay will have some chatter
due to the rising pull in voltage. It would be better to use a DC relay here
and a diode from the transformer primary so that the relay pulls in
smoothly.
This simple approach does the same thing as sensing the high voltage without
the associated problems. It also has built in delay too.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of David Cutter
> Sent: Sunday, February 03, 2008 8:41 AM
> To: 'AMPS'
> Subject: Re: [Amps] Step start
>
> I have a 240V 15A variac which would consistently blow a 13A plug fuse at
> switch on, unloaded. The spec for these plug fuses states they must blow
> in
> 30s with 2.1 x nominal current applied, ie >27A: these always blew
> instantly. Slow blow fuses did not work either. The variac looks like a
> short across the mains until the magnetising current is established.
> (Large power supplies have large transformers which look similar). My
> simple
> cure was to fit a mains relay across the variac and 2W 1k resistor in
> series
> with the supply to both the relay and variac. The relay contacts shorted
> the resistor. The inherent time delay within the relay allowed the
> magnetising current to rise slowly enough to avoid that large surge. I
> don't know how vulnerable filaments are to switch on surge, I don't recall
> reading anything about this in valve specs, but it's been a while.
>
> David
> G3UNA
>
>
>
> > It does not matter for the HV caps how fast they are loaded.
> > The reason for a step-start circuit is to limit the supply current to a
> > value below that of a typical household circuit breaker.
> > As long the step-start resistor is in circuit the HV cap will never
> reach
> > full voltage.
> >
> > 73
> > Peter
>
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