Filtering isn't the problem. Dynamic regulation is a problem with a choke.
You get severe voltage drops when current is drawn at an audio rate from the
power supply. A swinging choke is even worse than a regular choke. They
should never be used in an SSB power supply.
Put a scope on the high voltage output of the supply with a choke (thru
appropriate divider resistors) and you will see some pretty nasty looking
voltage drops with modulation.
Only a tuned choke is desirable as it does not have the problems of a
regular choke.
Series resistance of the transformer is the second problem for voltage
regulation in a power supply used for SSB. We want as low a resistance as we
can get. Most of the older power transformers are not well suited for SSB
operation because of their rather high series resistance.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of jeremy-ca
> Sent: Tuesday, March 18, 2008 5:43 PM
> To: garyschafer@comcast.net; 'Pat Barthelow'; 'Jim Tonne'; 'Amps
> @Contesting.com'
> Subject: Re: [Amps] Power transformer for 600 watt rig
>
> SSB is less critical than a Class AB or B modulator as far as PS
> requirements which is why a single capacitor is capable of sufficient
> filtering.
>
> For a modulator a choke input is used and it is not a fixed value
> smoothing
> choke nor is it a tuned choke at amateur power levels. What is used is a
> swinging choke that maintains critical inductance at a light load but
> changes (swings) as the current is increased. These are typically built
> over
> a 5:1 ratio; 4-20; 5-25, 6-30 Henries, etc.
>
> By maintaining critical inductance the voltage will not soar with a light
> load.
>
> All you ever need to know about designing this style of supply is found in
> the old Handbooks from the AM days, especially the Bill Orr series of
> Handbooks.
>
> By using a choke input filter SS rectifiers can be used with the old style
> transformers designed for tube rectifiers without cooking them as usually
> happens with a single fat cap.
> Instead of a unloaded DC voltage of 1.4X RMS with a bridge and a cap the
> output will be .9. However the regulation will be far superior to the cap
> version which may drop well below .9 on peaks.
>
> Swinging chokes in the 500-1000 ma CCS range are readily available on the
> used and surplus market. I recently picked up a 5-25H @ 1A with 7500V
> insulation for $70. I cant wait to watch the 304TL's glow on voice peaks!
>
> Carl
> KM1H
>
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer@comcast.net>
> To: "'Pat Barthelow'" <aa6eg@hotmail.com>; "'Jim Tonne'"
> <tonne@comcast.net>; "'Amps @Contesting.com'" <amps@contesting.com>
> Sent: Tuesday, March 18, 2008 3:14 PM
> Subject: Re: [Amps] Power transformer for 600 watt rig
>
>
> > Hi Pat,
> >
> > The biggest problem with a choke input filter is the transient response
> of
> > the choke with SSB. It will allow the voltage to dip quite a bit on some
> > of
> > the voice peaks as the amplifier demands current. A supply with no choke
> > doesn't have those kind of problems. A tuned choke on the other hand
> will
> > act like a capacitor input filter and will not have the transient
> problems
> > either.
> >
> >
> > 73
> > Gary K4FMX
> >
> >> -----Original Message-----
> >> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On
> >> Behalf Of Pat Barthelow
> >> Sent: Tuesday, March 18, 2008 12:12 PM
> >> To: garyschafer@comcast.net; 'Jim Tonne'; 'Amps @Contesting.com'
> >> Subject: Re: [Amps] Power transformer for 600 watt rig
> >>
> >>
> >> This is quite informative...Question, From Gary's comment about
> Secondary
> >> Resistance, and choke input power supplies, not sufficient for SSB
> >> dynamic
> >> regulation, if one has available an older power supply, that is choke
> >> input, you need relatively low secondary resistance....Does the choke
> >> resistance have a lot of influence there also? What are some typical
> >> values of HV choke resistances? If so, then I think what you are
> >> saying,
> >> is some (most?) choke input power supplies will have regulation issues
> >> with SSB.
> >> Yes/No?
> >>
> >> All the best,
> >> Pat Barthelow (916) 315-9271
> >> aa6eg@hotmail.com
> >> http://www.jamesburgdish.org
> >>
> >>
> >> > From: garyschafer@comcast.net
> >> > To: tonne@comcast.net; amps@contesting.com
> >> > Date: Tue, 18 Mar 2008 11:08:59 -0400
> >> > Subject: Re: [Amps] Power transformer for 600 watt rig
> >> >
> >> > Hi Jim,
> >> >
> >> > No I don't have LTspice.
> >> >
> >> > What I was saying about what type of service the power supply is to
> >> expect
> >> > to see matters a great deal. A transformer (a good one) that is to be
> >> used
> >> > with a voltage doubler might have a secondary resistance in the
> >> neighborhood
> >> > of 15 ohms.
> >> >
> >> > A transformer to deliver the same power supply voltage using a choke
> >> input
> >> > filter can get by with a transformer secondary resistance of a couple
> >> > of
> >> > hundred ohms.
> >> > The dynamic regulation of the latter will be fine for an AM or FM rig
> >> but
> >> > not acceptable for SSB.
> >> >
> >> > 73
> >> > Gary K4FMX
> >> >
> >> >
> >> > > -----Original Message-----
> >> > > From: amps-bounces@contesting.com
> >> > > [mailto:amps-bounces@contesting.com]
> >> On
> >> > > Behalf Of Jim Tonne
> >> > > Sent: Monday, March 17, 2008 9:47 PM
> >> > > To: garyschafer@comcast.net; 'Amps @ Contesting.com'
> >> > > Subject: Re: [Amps] Power transformer for 600 watt rig
> >> > >
> >> > >
> >> > > Gary:
> >> > >
> >> > > Do you have LTspice? I will plug those numbers
> >> > > into the transformer model to complete the design.
> >> > > The numbers I quoted are adequate for the problem
> >> > > as stated.
> >> > >
> >> > > - Jim Tonne
> >> >
> >> > _______________________________________________
> >> > 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
> >
>
> _______________________________________________
> 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
|