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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