[Amps] soft start, how to determine if needed?

RICHARD SOLOMON w1ksz at q.com
Sat Feb 27 13:11:24 PST 2010 

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 at aol.com
> Date: Sat, 27 Feb 2010 15:36:41 -0500
> To: alexeban at gmail.com; amps at 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 at 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 at gmail.com] On Behalf Of Felipe
> Ceglia - PY1NB
> Sent:  Wednesday, February 24, 2010 2:22 PM
> To: Edward Swynar
> Cc: Alex Eban;  Amps at 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 at gmail.com>
> > To:  <felipe at dxwatch.com>; <Amps at 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 at contesting.com 
> >> [mailto:amps-bounces at contesting.com]  On Behalf Of Felipe Ceglia - 
> >> PY1NB
> >> Sent: Tuesday,  February 23, 2010 3:34 PM
> >> To: Amps at 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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