Amps
[Top] [All Lists]

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

To: alexeban@gmail.com, amps@contesting.com
Subject: Re: [Amps] soft start, how to determine if needed?
From: TexasRF@aol.com
Date: Sat, 27 Feb 2010 15:36:41 EST
List-post: <amps@contesting.com">mailto:amps@contesting.com>
 
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
>>
>>  _______________________________________________
>> Amps mailing  list
>> Amps@contesting.com
>>  http://lists.contesting.com/mailman/listinfo/amps
>>
>>  _______________________________________________
>> Amps mailing  list
>> Amps@contesting.com
>>  http://lists.contesting.com/mailman/listinfo/amps
>>
>>   
>
>
>  _______________________________________________
> Amps mailing  list
> Amps@contesting.com
>  http://lists.contesting.com/mailman/listinfo/amps
>
>    

--
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

_______________________________________________
Amps  mailing  list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps



_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>