[ARC5] 24vdc power

Mon Sep 5 20:15:50 EDT 2022

You don’t necessarily need a bridge, but it should be a full wave rectifier, otherwise it is very inefficient. In a high current supply, it would also cause a great deal of power dissipation in the filter capacitors.  Half wave rectifiers are best left for very low current applications.

Most battery charger use a full wave center tap rectifier , which requires only two diodes.  It is slightly more efficient than a bridge, since there are two fewer diode drops (0.6V @ 100 A would mean 60W dissipated per diode).   Dynamotors do not like raw rectified AC and will run hot.  Obviously, filaments don’t care, but the RMS value needs to be correct for proper emission. So, really, the KISS method is the minimum acceptable of a stiff, filtered, full wave supply.  If it is sufficiently sized, load regulation will probably be acceptable.

From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> On Behalf Of kn7sfz
Sent: Monday, September 5, 2022 1:55 PM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] 24vdc power

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  <mailto:kb8tq at n1k.org> <kb8tq at n1k.org>
To: Charles  <mailto:charlesmorris800 at centurytel.net> <charlesmorris800 at centurytel.net>
Cc: ARC-5  <mailto:arc5 at mailman.qth.net> <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 <mailto: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 <mailto:arc5-request at mailman.qth.net>  wrote:
>> From: Bob kb8tq<kb8tq at n1k.org <mailto:kb8tq at n1k.org> >
>> To: MICHAEL ST ANGELO<mstangelo at comcast.net <mailto:mstangelo at comcast.net> >
>> Cc: ARC-5<arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net> >
>> Subject: Re: [ARC5] 24vdc power
>> Message-ID:<4F446E03-9F04-4E3F-A0FC-8BE25F92746B at n1k.org <mailto: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/> <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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