[ARC5] 24vdc power
kn7sfz
kn7sfz at gmail.com
Mon Sep 5 22:26:50 EDT 2022
Great info...thanks Brian!
kn7sfz
On 9/5/2022 7:20 PM, Brian Clarke wrote:
>
> Half-wave rectification causes noise on the mains because of the
> sudden changes in current flow. And such rectification of the
> secondary also reflects DC back onto the mains supply. Most
> electricity supply authorities frown on such misbehavior, even though
> many cheap, Asian computer power supplies use half-wave rectification.
> Why do electricity supply authorities frown? Because of magnetisation
> of the pole pig that reduces its efficiency and causes unwanted
> heating beyond the normally designed-for iron and copper losses.
>
> So, be kind to your neighbours; use full-wave rectification.
>
> 73 de Brian, VK2GCE
>
> *From:*arc5-bounces at mailman.qth.net
> [mailto:arc5-bounces at mailman.qth.net] *On Behalf Of *Bob kb8tq
> *Sent:* Tuesday, 6 September 2022 7:58 AM
> *To:* kn7sfz
> *Cc:* ARC-5
> *Subject:* Re: [ARC5] 24vdc power
>
> Hi
>
> Given that bridge rectifiers are pretty cheap compared
>
> to the rest of the parts. There is no real reason to go with
>
> half wave for cost reasons.
>
> A half wave will effectively put DC on the transformer
>
> secondary. That can / will / might magnetize the core.
>
> This isn’t going to help you get full power out of the device.
>
> If you put half wave onto something like a filament, you
>
> have a 50% duty cycle compared to full wave. In a “no cap”
>
> approach, the voltage would need to go up to compensate
>
> for this.
>
> Simple answer: go full wave.
>
> Filaments are happy running on AC current. Ripple filled
>
> DC isn’t any worse on them ( provided the voltage is right)
>
> than AC. Motors have a lot of rotating mass. That will
>
> “smooth out” any ripple issues. Field windings are big chokes.
>
> They will also reject ripple. Again, you don’t want to get the
>
> (effective) voltage to far out of line.
>
> Why keep harping on this no cap stuff? With a cap, as noted
>
> in a lot of places ( including an earlier post) the voltage will idle
>
> at 1.4X the RMS secondary voltage. At very heavy load ( as
>
> it goes to “no cap”….) the effective voltage goes to the RMS
>
> value ( or just a bit less …). You get a less well regulated
>
> supply as a result.
>
> With no cap, the diodes conduct over the full cycle. ( ok, one
>
> conducts half the time and then the other one comes in, one or
>
> the other always has current flowing). With a big cap, the diodes
>
> conduct only at the peaks of the waveform. This puts more stress
>
> on the diodes and increases various losses. If you don’t *need*
>
> clean DC, why put up with those issues ….
>
> Bob
>
>
>
> On Sep 5, 2022, at 12:54 PM, kn7sfz <kn7sfz at gmail.com> wrote:
>
> 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> <mailto:kb8tq at n1k.org>
> To: Charles <charlesmorris800 at centurytel.net>
> <mailto:charlesmorris800 at centurytel.net>
> Cc: ARC-5 <arc5 at mailman.qth.net> <mailto: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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