1N4007 has a peak current rating of 30 amps for one cycle. 10 amp repetitive
peak current and 1 amp average current.
The 1N5408 has a peak current rating of 200 amps for one cycle. 50 amp
repetitive peak current and 3 amp average current.
If you look at Psu designer it shows the following:
I used a 20 ohm transformer at 800 volts in a full wave doubler. 3.3K load
which gives around 600 ma load current.
Since the voltage rating is not high enough for just 2 capacitors it takes 6
capacitors total (3 in each leg). So the Mfd will be 1/3rd in each half of
the doubler.
So with 125 Mfd capacitors (3 in each leg) the capacitance would only be
41.6 on each half. That would make peak turn on current 13.5 amps and peak
repetitive current would be about 5.6 amps
With 330 Mfd capacitors (3 in each leg) the capacitance would be 110 Mfd on
each half. That would make peak turn on current 25 amps and peak repetitive
current 5.8 amps.
Average current is around 600 Ma in all of the above.
With the 330 Mfd caps 1n4007's would still be ok provided the transformer
had 20 ohms of winding resistance. I don't know what the winding resistance
is on the SB200. It may be less than that so peak currents would be higher.
Note that by increasing the capacitors from 125 Mfd each to 330 Mfd each the
peak turn on current about doubles. This is with 20 ohms transformer
resistance.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Paul Kraemer
> Sent: Sunday, March 27, 2011 11:51 AM
> To: amps@contesting.com
> Subject: Re: [Amps] SB-200 power supply diodes
>
> Excellent point Gerald
> I also wonder if any one has done the math or actually measured the
> increase in diode / transformer current from arbitrarily increasing the
> original filter cap size from the original 125 mfd to say 330 mfd in
> those
> doubler supplies. I realize those original caps were probably 10 pct or
> more
> over the labled value and the new one will likely be 10 pct under.
> Still, it
> is a change and it does matter.
> Someday I'm gonna take the HP 428 and actually measure before and after
> transformer current, just out of curiousity.
> Paul K0UYA
> ----- Original Message -----
> From: <TexasRF@aol.com>
> To: <w7ry@inbox.com>; <4cx250b@muohio.edu>; <amps@contesting.com>
> Sent: Sunday, March 27, 2011 8:00 AM
> Subject: Re: [Amps] SB-200 power supply diodes
>
>
> > Think about this again. The SB200/220 both use a voltage doubler
> capacitor
> > filter hv supply. We know that peak currents in these type supplies
> can be
> > five to ten times the average current.
> >
> > So, with a .6 to .7 ampere average, those diodes are taking a 3 to 6
> > Ampere
> > hit 60 times every second.
> >
> > For the miniscule added cost it makes a lot of sense to use 1N5408
> diodes.
> > Reaming the mounting holes for the larger lead size is a trivial
> effort.
> >
> > If 1N5408 diodes had been available when those amplifiers were
> designed
> > no
> > doubt they would have been used. Especially so when you consider the
> > equalizing capacitors and resistors would not be needed. I would
> remove
> > them now
> > as the parts that are not there can't fail. Who knows what those
> resistor
> > values might be after upwards of 50 years use?
> >
> > 73,
> > Gerald K5GW
>
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