The AL-1200 has a simple Soft-Start circuit, one resistor and a relay.
Download the manual from the MFJ site and see how simple it is.
73, Dick, W1KSZ
> From: TexasRF@aol.com
> Date: Sat, 27 Feb 2010 15:36:41 -0500
> To: alexeban@gmail.com; amps@contesting.com
> Subject: Re: [Amps] soft start, how to determine if needed?
>
>
> Hi Alex, I didn't see anyone reply to your suggestion to add a one ohm
> resistor in the plate transformer primary so I will.
>
> This is a really bad idea from voltage regulation viewpoint. It would be
> the same as having a one ohm source impedance from the AC line service.
>
> As we have covered several times in recent weeks, the peak current in the
> secondary and primary will be five or more times the average current. This
> is because all of the needed power is drawn from the line during and near
> the ac peaks.
>
> With a 20A average load, peaks can be 100A or more. The peak voltage drop
> across the resistor could be 100 volts in this case. The output voltage will
> drop right along with the primary voltage as much as 2000v peak. If the
> transformer is normally 2500vac, peak voltage is 1.414 X 2500 or 3535v.
> loosing 2000v of this leaves 1535v peak or 1085v rms. Not good!
>
> I have recently experimented with using resistors to set the full load dc
> voltage from a twt power supply. .1 ohms in this power supply primary
> (120vac source) will drop the output voltage about 10%, from 5600vdc to
> about
> 5000vdc. By the way, the .1 ohm resistor gets very hot. During standby, a
> 200
> ohm resistor is used to keep the no load voltage from soaring. This
> resistor gets pretty warm but not really hot.
> capacitor charging seems to work very well in practice.
>
> A typical transformer has quite a bit more primary and transformed
> secondary resistance than one ohm. So, adding a one ohm resistor to control
> turn
> on surges is not enough. With the inherent transformer resistance, the
> capacitor bank will need several cycles to charge. 1A diodes can only take so
>
> many cycles like this before they are destroyed. Most power supplies built
> today use 3A or more rated diodes because of this.
>
> A 25 ohm resistor in the primary and waiting a few seconds seems to work
> well in practice and will limit the charging surge to about 10A in the
> primary and 1A in the secondary. A solid state timer and shorting relay makes
>
> all this painless and automatic.
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 2/25/2010 1:03:57 A.M. Central Standard Time,
> alexeban@gmail.com writes:
>
> Yes, there is.
> The most endangered part are the rectifiers. The specification includes a
> current value called " non repetitive peak current", usually with a time
> limit of about a cycle or so. Use that value, assume a dead short at the
> capacitors and calculate a value for the resistor, such that the peak
> current at the peak of the input waveform voltage is less than this
> current.
> Keep in mind that the transformer action changes the current so that 1
> ampere at 2500V translates to 10 amperes at 250V!
> At 240 VAC, the resistor comes out around 1 ohm or less so that it can be
> wired in series and left there, even without a shorting relay. Use a wire
> wound resistor, such as Dale's and install on a heat sink.
> Alex 4Z5KS
>
> -----Original Message-----
> From: Felipe Ceglia [mailto:felipeceglia@gmail.com] On Behalf Of Felipe
> Ceglia - PY1NB
> Sent: Wednesday, February 24, 2010 2:22 PM
> To: Edward Swynar
> Cc: Alex Eban; Amps@contesting.com
> Subject: Re: [Amps] soft start, how to determine if needed?
>
> Folks,
>
> Thank you for your replies.
>
> Is there a way to calculate what R values and relay contact Amp rate should
> I need?
>
> Also, if the primary is feed with 110+110 volts, I should use one R on each
> primary leg winding, right?
>
> 73 tu,
>
> Felipe - PY1NB
>
>
>
>
> Edward Swynar wrote:
> > Hi Guys,
> >
> > FWIW, I had to re-build my rectifier stack at least twice before I
> > finally incorporated a soft-start / step-start scheme in the primary
> > of my B+ transformer...
> >
> > The p.s. is a full-wave rectifier job, applying 2.5 KV to a pair of 813s.
> >
> > Since incorporating the scheme, I have had no further troubles in this
> > regard.
> >
> > As someone said before me, it's "...cheap insurance", and not all that
> > daunting a task to add-in, even after the fact.."
> >
> > ~73~ de Eddy VE3CUI - VE3XZ
> >
> >
> > *******************************
> >
> > ----- Original Message -----
> > From: "Alex Eban" <alexeban@gmail.com>
> > To: <felipe@dxwatch.com>; <Amps@contesting.com>
> > Sent: Wednesday, February 24, 2010 3:39 AM
> > Subject: Re: [Amps] soft start, how to determine if needed?
> >
> >
> >
> >> As far as possible, USE ONE!!!!!
> >> First, the cold resistance of a tube's filament is as low as one
> >> tenth of the warm filament, so inrush currents can reach 10 times the
> >> operating value.
> >> Second, inrush current of a HV power supply can reach a hundred
> >> amperes, measured with a Tektronix current probe. If nothing else,
> >> that current can weld the switch contacts!
> >> I don't have to tell you that the stresses on the rectifiers don't do
> >> any good and the capacitors are also badly stressed.
> >> Today, almost all integrated circuits intended for power supply
> >> control incorporate some form of soft start circuitry, precisely for
> >> reducing
> >>
> > these
> >
> >> stresses.
> >> Use step start or soft start, it's very cheap insurance.
> >> Alex 4Z5KS
> >>
> >> -----Original Message-----
> >> From: amps-bounces@contesting.com
> >> [mailto:amps-bounces@contesting.com] On Behalf Of Felipe Ceglia -
> >> PY1NB
> >> Sent: Tuesday, February 23, 2010 3:34 PM
> >> To: Amps@contesting.com
> >> Subject: [Amps] soft start, how to determine if needed?
> >>
> >> Hi folks,
> >>
> >> Regarding this recurring topic about soft starting...
> >>
> >> How can one determine if it is needed or not?
> >>
> >> Should HV be soft-started as well?
> >>
> >> What would be the best math to determine the soft-start dumping
> resistors?
> >>
> >> 73,
> >>
> >> Felipe - PY1NB
> >>
> >> --
> >> Felipe Ceglia - PY1NB
> >> ---------------------------------------------------------------------
> >> -----
> >>
> > --
> >
> >> ---------
> >> PR1T team member /// Rio DX Group member /// Araucaria DX Group
> >> member http://www.dxwatch.com /// http://reversebeacon.net ///
> >> http://riodxgroup.dxwatch.com
> >>
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> >>
> >
> >
> > _______________________________________________
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> >
>
> --
> Felipe Ceglia - PY1NB
> ----------------------------------------------------------------------------
> ---------
> PR1T team member /// Rio DX Group member /// Araucaria DX Group member
> http://www.dxwatch.com /// http://reversebeacon.net ///
> http://riodxgroup.dxwatch.com
>
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