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 <[email protected]> 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 <[email protected]>
To: Charles <[email protected]>
Cc: ARC-5 <[email protected]>
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 <[email protected]> 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, [email protected] wrote:
>> From: Bob kb8tq<[email protected]>
>> To: MICHAEL ST ANGELO<[email protected]>
>> Cc: ARC-5<[email protected]>
>> Subject: Re: [ARC5] 24vdc power
>> Message-ID:<[email protected]>
>> 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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