[Amps] soft start, how to determine if needed?
Alex Eban
alexeban at gmail.com
Sat Feb 27 23:58:56 PST 2010
By the way, an old trick that was used in tube type TV sets was to install a
thermistor in series with the mains. It had a cold resistance of a few tens
of ohms when cold, dropping to an oh or two after about 20 seconds.
The only problem was the at switch off you had to remember to let it cool
before switching on again.
The Sfernice corporation was manufacturing one type especially for TV sets.
(about 7W power dissipation) with values ranging from 27 to 1000 ohms.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Roger
Sent: Sunday, February 28, 2010 4:47 AM
To: RICHARD SOLOMON
Cc: Amplifier Reflector
Subject: Re: [Amps] soft start, how to determine if needed?
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 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
>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
>>>>
>>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at 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 at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>>
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
_______________________________________________
Amps mailing list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps
More information about the Amps
mailing list