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

To: RICHARD SOLOMON <w1ksz@q.com>
Subject: Re: [Amps] soft start, how to determine if needed?
From: Roger <sub1@rogerhalstead.com>
Date: Sat, 27 Feb 2010 21:47:29 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>

RICHARD SOLOMON wrote:
> 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.
>   
You can purchase time delay relays, build a small RC circuit to turn on 
a transistor that keys a relay.
Essentially you have a circuit that has a resistor in series with the 
primary of the plate transformer. A set time after power was applied 
(usually 3 to 5 seconds) the relay closes, shorting out the resistor and 
applying full voltage, or more correctly removing the current limiting 
from the circuit. 

I like the idea of using a triac to ramp up the voltage instead of just 
switching a resistor in and out, but they work.
It's not just HV supplies either.  I have one of the large 50A 
conventional Astron power supplies. If you switch it on near a voltage 
peak it has that typical THUMP...Whunnnngggg!  Any supply that does this 
would certainly benefit from a soft start.  An additional point about 
these large, low voltage supplies is the pass transistors are exposed 
and even a screw dropped down the side can short them out.

However there are two places to use a soft start. One of course is the 
power supply to limit the initial on switching current.  I didn't call 
it the inrush current because there is more to it than just the inrush 
current through the diodes charging the capacitors. Although the diodes 
are the most fragile part and need protecting, the magnetizing current 
from a switch on at the voltage cycle peak and wrong part of the B/h 
curve can be far more than the inrush current.  This can cause arcing in 
the power switch which can cause some very high voltage spikes on the 
transformer output which isn't good for anything down stream.

The second place is the filaments in the amplifier. Nearly any power 
tube can benefit from a soft start although the Thoriated Tungsten 
filaments will benefit the most.  This is why I prefer the triac 
approach to ramp up the voltage rather than a step start.

73

Roger (K8RI)
> 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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>>>    
>>>       
>> --
>> 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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