[Amps] soft start, how to determine if needed?
TexasRF at aol.com
TexasRF at aol.com
Sat Feb 27 12:36:41 PST 2010
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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