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Old 22nd November 2008, 09:14 PM   #1
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Default Dummy transformer question

I have 2 identical transformers. Each 500VA, 120v primary and 24v secondary. Can I, series the the primaries and parallel the secondaries?

And if so, what kind of current capability might I have after rectifying, filtering and regulating to about 12 volts?

Want to power up Ham equipment.

Thanks,
David
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Old 23rd November 2008, 01:30 AM   #2
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You have to current limit each primary to about 4.2 Amps.
So each secondary will be good for about 250VA at 12 Volts.
But it will be a very stiff 12 Volts.
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Old 23rd November 2008, 03:43 AM   #3
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Kevin,

Thank you for the reply. I'm not sure what you mean by "You have to current limit each primary to about 4.2 Amps." Does that mean a separate fuse for each primary?

After further searching, I realize I should probably re-think the whole set up. I'm looking at this circuit and going with 24 volts instead of 12.

Is it possible to add parallel transistors for more current out? 20 amps would be OK, but I want to take advantage of all the iron I have.

Also, I said, this is a dummy question. Can I just divide the secondary voltage into the VA rating to get the current capability of the transformer?

Your (or any one's) thoughts? Snide remarks?

Thanks again,
David
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Old 23rd November 2008, 06:27 AM   #4
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Default Re: Dummy transformer question

Quote:
Originally posted by davidlzimmer
I have 2 identical transformers. Each 500VA, 120v primary and 24v secondary. Can I, series the the primaries and parallel the secondaries?

And if so, what kind of current capability might I have after rectifying, filtering and regulating to about 12 volts?

Want to power up Ham equipment.

Thanks,
David
Hi from Z37HWX!
For a Ham Transceiver you should have minimum 13.8 Volts to 14.2, not so critical, but 12V is not enough for transmitting, you'll get a smaller output power.
Your transformer of 500VA should gave around 20A maximum on the secondary if this is enough for the case.
To get a 13.8 to 14.2 volts of regulated out you should lift the common pin of voltage regulator with adding 3 in series , say 1n 4004 diodes from common pin to the ground.
If you use transistors take a good care of a heatsink, cos with 5 amperes per transistor the dissipating heat is too high.
I will use 6X 2N3055 for 20A.
73 de Zoki
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Old 23rd November 2008, 07:06 AM   #5
Eva is offline Eva  Spain
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A linear regulator fed with rectified 24V AC is going to dissipate too much morer, in fact more power than delivered to the load.

A practical solution involving these transformers would require a switching regulator.
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Old 23rd November 2008, 07:39 AM   #6
poynton is offline poynton  United Kingdom
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Default Re: Dummy transformer question

Quote:
Originally posted by davidlzimmer
I have 2 identical transformers. Each 500VA, 120v primary and 24v secondary. Can I, series the the primaries and parallel the secondaries?

And if so, what kind of current capability might I have after rectifying, filtering and regulating to about 12 volts?

......
What input voltage are you using ? 240v or 120v ?

If 120v, parallel the primaries.
If 240v, Series the primaries.

Use a bridge rectifier after each secondary and parallel the + and - rails.


Quote:
Originally posted by davidlzimmer
Kevin,

Thank you for the reply. I'm not sure what you mean by "You have to current limit each primary to about 4.2 Amps." Does that mean a separate fuse for each primary?

After further searching, I realize I should probably re-think the whole set up. I'm looking at this circuit and going with 24 volts instead of 12.

Is it possible to add parallel transistors for more current out? 20 amps would be OK, but I want to take advantage of all the iron I have.

Also, I said, this is a dummy question. Can I just divide the secondary voltage into the VA rating to get the current capability of the transformer?


The current output through the each of the secondaries will be approx 20A max. So for 2 secondaries in parallel, about 40A max.

The circuit you want to use will work and, yes, you could add more transistors to increase available current (to the 40A max) or reduce dissipation in each transistor.

But you will need a large heatsink. At 40A/12v output, you will be dissipating more than half the total power ie 500w.
It will get HOT

You will have to use the correct fuse on both the primaries and secondaries.


Andy
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Old 23rd November 2008, 04:02 PM   #7
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Many thanks for all the replies. I have a lot of thinking to do now. But now with good advise to base it on.

THANKS ALL!
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Old 23rd November 2008, 04:22 PM   #8
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Quote:
Originally posted by davidlzimmer
Many thanks for all the replies. I have a lot of thinking to do now. But now with good advise to base it on.

THANKS ALL!
Here is a simpliest way to get high current on 13.8Volts and crazy 65 Amperes!, as I told U beforethis is taken from http://www.qsl.net/yo5ofh/projects/h...20projects.htm)
This is not my design, and I never test this schematic, I saw 65A which sounds a bit crazy and Transformer is 45 A rated at 18 V, probably the guy mean a short peak Amperage of 65A!

Click the image to open in full size.

Here is interesting modification of computer power supply to get 15 amperes on 13.8 V:

http://www.qsl.net/yo5ofh/projects/1...r%20supply.htm

QSL Mr.?
Best 73,s de Z37HWX
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Old 23rd November 2008, 04:40 PM   #9
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Quote:
13.8Volts and crazy 65 Amperes!
Wow!!! That's GREAT!!!

I could strap the two secondaries to get 48 volts with a center tape. NO?

That's crazy easy! A chassis, large heat sink, and a few small parts and I'm in business!

Many thanks to Z37HWX and to all for your help.

David
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Old 23rd November 2008, 05:02 PM   #10
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Quote:
Originally posted by davidlzimmer


Wow!!! That's GREAT!!!

I could strap the two secondaries to get 48 volts with a center tape. NO?

That's crazy easy! A chassis, large heat sink, and a few small parts and I'm in business!

Many thanks to Z37HWX and to all for your help.

David
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TNX David, UR RST is 599 plus 10dB...
Well I think that if you go with linear power supply your transformers are not quite match for this because usually secondary voltages needed is around 18Volts AC.
If you are able to exchange your transformers for one big transformer which will give you 18V and say 25 Amperes on the secondary.
Dissipation with 24 volts transformer will be to high, when you rectify will be 1.4*24, over 30 volts!.
I 'v made similar power supply around 15 A at 13.8v, but I lost that schematic, but i could find, it was taken from very good HAM book( over 800 pages from DR. BOZO METZGER, YU2BR, and I have no info if he still in this world, that book was from MID 80's.
That schematic also has a current limiter, which protect the PS. from over currents or short circuit, but it can be done by simply thermal fuse, which is not so fast, as we know that the fastest fuse are the semiconductors, active elements!!!
I wish you good luck, and remember all power, amperes is needed when you transmitting, for the receiving and stand by mode you don't need more than 1A. depends on the rig you will use, for the present rigs without vacuum tubes is needed small current when RCV or stand by mode.
Regards,
Z37HWX, now CL
73, Zoki
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