[ARC5] 24vdc power

Mon Sep 5 16:54:38 EDT 2022

Man....all this info is great!

And with regards to building an xfmr 'brute force' supply, do you really 
need a bridge rectifier or will half-wave do?

Do dynamotors and filaments really care if there's a bit of ripple?  I 
have some large diodes and caps I could put to experiment with.  Kinda 
the KISS method.

de kn7sfz

On 9/5/2022 12:42 PM, Bob via ARC5 wrote:
> Do we worry too much about temperature ratings. As noted by KB8TG ".Hams
> have been “abusing” transformer ratings this way pretty much
> forever and ever.
>
> If you look at some of the ratings on imported magnetics they are all 
> in the 100C and up category. Semiconductor rectifiers can also take a 
> lot of heat.  and a bit of a heat sink on a bridge will keep them 
> happy most of the time.  As to filtering, not that many decades back a 
> 100,000 ufd 75 vdc capacitor was a rare item now you can buy one on 
> Ebay for trifle.  While some of the folks on this list are lamenting 
> the diminished marketplace for boat anchor stuff,  Swap meets are 
> still great venues for that gear and material.  Just look under the 
> tables for the gems.
>
> I agree with a lot of older folks, like myself, there is nothing 
> sweeter than the song of a dynamotor spooling up when you push the PTT 
> on your WWII based station.  But like many contributors to the list 
> have said in essence there is no free lunch and in may cases just 
> float a couple of cheap storage batteries across the brute force DC 
> supply, watch the electrolyte levels and temperature when operating 
> and enjoy the moment
>
> Cheers
>
> Bob, KE6F
>
> -----Original Message-----
> From: Bob kb8tq <kb8tq at n1k.org>
> To: Charles <charlesmorris800 at centurytel.net>
> Cc: ARC-5 <arc5 at mailman.qth.net>
> Sent: Mon, Sep 5, 2022 12:22 pm
> Subject: Re: [ARC5] 24vdc power
>
> Hi
>
> Are you really running a cap input? In a lot of cases, you might
> not be.
>
> It depends very much on the size of the cap. If you are getting a
> voltage boost ( due to the cap charging ), you don’t get as much
> current. Power out of the secondary and power into the load
> need to add up ( … no free lunch). Cap not big enough to count,
> not much voltage boost, not much current penalty. No, that’s not
> quite the whole story, but it’s a part of it.
>
> Unless you have an absolutely giant cap, you likely are running
> effectively a “no cap / no choke” circuit when driving it into your
> motor at start up. You get a bunch of ripple in the output. The motor
> pretty much does not care … it starts turning. Since the diodes in
> the bridge are happier running this way (larger conduction angle),
> it does have other advantages.
>
> Further complicating things, transformers are generally rated on
> some sort of continuous duty basis. It’s a good bet that they will
> put out a bit more for a short while. Motor startup inrush current
> is very much in the “short term” range. Will it do 20% more or 2X
> more? Depends a lot on how long (seconds vs minutes vs hours)
> and the voltage drop you can tolerate ( is a 20% drop ok? ). Hams
> have been “abusing” transformer ratings this way pretty much
> forever and ever.
>
> Lots of variables ….
>
> Bob
>
>
>
>
> > On Sep 5, 2022, at 10:04 AM, Charles via ARC5 <arc5 at mailman.qth.net> 
> wrote:
> >
> > It is also necessary to remember that the transformer rating does 
> not give DC ampere output in a 1:1 ratio unless you have a choke input 
> filter. From the Signal Transformer Catalog 892340 (available for 
> download online), on page 38:
> >
> >> The secondary currents shown in the tables are RMS ratings. 
> Depending upon rectifier circuit configurations, the RMS secondary 
> current is different from the DC output current. This is indicated in 
> the chart below:
> >>
> >> Full-Wave Center-Tap Choke Input = RMS Secondary Current is 0.7 x 
> DC Amps
> >>
> >> Full-Wave Center-Tap Capacitor Input = 1 to 1.2 x DC Amps
> >>
> >> Full-Wave Bridge Choke Input = DC Amps
> >>
> >> Full-Wave Bridge Capacitor Input = 1.6 to 1.8 x DC Amps
> >>
> >> For example, in a F.W. Bridge circuit with a capacitive filter, if 
> the load is 1 Amp DC, the RMS Secondary current is 1.6 to 1.8 Amp RMS.
> >>
> >
> > So I needed a 250 amp (5 KVA 20 volt) transformer to deliver 150 DC 
> amps... and at that current level, the bridge rectifier repetitive 
> forward rating is non-trivial, as is the heat generated. I used a big 
> heat sink and a muffin fan.
> >
> > -Charles,WB3JOK/0
> >
> >
> > On 9/5/22 19:31, arc5-request at mailman.qth.net wrote:
> >> From: Bob kb8tq<kb8tq at n1k.org>
> >> To: MICHAEL ST ANGELO<mstangelo at comcast.net>
> >> Cc: ARC-5<arc5 at mailman.qth.net>
> >> Subject: Re: [ARC5] 24vdc power
> >> Message-ID:<4F446E03-9F04-4E3F-A0FC-8BE25F92746B at n1k.org>
> >> Content-Type: text/plain; charset="utf-8"
> >>
> >> Hi
> >>
> >> One option is to stack some number of smaller transformers:
> >>
> >> This would be one candidate (based on a very quick search):
> >>
> >> https://www.antekinc.com/an-10425-1000va-25v-transformer/ 
> <https://www.antekinc.com/an-10425-1000va-25v-transformer/>
> >>
> >> The same folks who made that come up with a wide variety of
> >> stuff up into the 1.5 KVA range. What they have today likely
> >> will not be what you find in a couple of months.
> >>
> >> If you bought four of the ones above, you would be up
> >> around 150A into your load. Voltage would be a tad high.
> >> A lower voltage / higher current version likely will pop
> >> up at some point. They have had them ( = 20V versions)
> >> in the past.
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