Dummy transformer question

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davidlzimmer

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Joined 2006
#1
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
 
#3
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
 
#4
davidlzimmer said:
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
Click to expand...

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
 
#5
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.
 
#6
davidlzimmer said:
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?

......
Click to expand...

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.


davidlzimmer said:
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?
Click to expand...


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 :hot: :hot:

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


Andy
 
#8
davidlzimmer said:
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!
Click to expand...
Here is a simpliest way to get high current on 13.8Volts and crazy 65 Amperes!, as I told U before🙁this is taken from 🙁http://www.qsl.net/yo5ofh/projects/ham radio projects.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!

power_supply.gif


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

http://www.qsl.net/yo5ofh/projects/13.8v_pc_ps/13_8 v - 15a from a pc power supply.htm

QSL Mr.?
Best 73,s de Z37HWX
HPE CU on the Air.
Ham's - moving power of the electronics.
We are the best.
 
#10
davidlzimmer said:


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
KJ4HLN
Click to expand...

KJ4HLN De Z37HWX
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
 
#11
Zoki

Well then back to my original question. If I put the two primaries in sereis, that would bring me to 12v in the secondary. After rectifying and filtering, it would be about 16.8 volts.

Enough?

Also, I'm having a problem finding diodes that will handle this current. Do I parallel each?


Thanks again. I should really know all this but it's been a long time since I've dealt in electronics other than building a few wonderful gain clone audio amps.

David

PS I see from QRZ that you have a lot of mountains to propagate over. 😉
 
#12
davidlzimmer said:
Zoki

Well then back to my original question. If I put the two primaries in sereis, that would bring me to 12v in the secondary. After rectifying and filtering, it would be about 16.8 volts.

Enough?

Also, I'm having a problem finding diodes that will handle this current. Do I parallel each?


Thanks again. I should really know all this but it's been a long time since I've dealt in electronics other than building a few wonderful gain clone audio amps.

David

PS I see from QRZ that you have a lot of mountains to propagate over. 😉
Click to expand...

No you can't connect that primary's in series to get lower voltage as you think.
I can here connect the primary 's of your two transformers in series in order to feed them with 230Volts which is our present standard voltage here.
Your transformers is calculated to operate on 120V (U.S) standard.
If you put them in series you'll feed them with half voltages each or 60Volts.
Serial connection of the two primary's is only in order to double value of voltages, and you must have exactly the same transformers, and the final power will be approx. as one of them.
Series and parallel connection is used in the secondary windings of two transformers in order to double the voltage with same current, or double the current with same voltage.
About the paralleling the rectifier diodes is not problem and you can do that.
16.8 volts as you think is not enough for the 13.8 V regulated output, at least the unregulated DC voltage must be 5 or more volts higher than the desired regulated output voltage.
with 18 volts transformer we 'll have 25V which is 25-13.8=11.2 volts higher voltage which will be a voltage drop of the series (paralleled output power transistors ) VCE and from this directly depends power dissipation of them or heating loss.
To achieve a good regulation of the output voltage you must have at least 5v higher input(than desired output) before the transistors but with 25 volts input is an ideal input to get 13.8V, a kind of compromise for a good stability and heating loss.
My suggestion is to make a tested project (schematic), even the schematic before posted seems to work O.K.

You are wright about my city position, Kicevo is surrounded with quite high mountains, but I have no problems to work North America west and east coast.
Sometimes I got reports of 599/59 plus.
Telegraphy is my favorite and I am very fast, and in my past I was on many High speed telegraphy competitions, and now just in QSO and I can make a QSO about 50 WPM(words per minute) or more than 200 letters or CPM .
My favorite band is 20 meters(14MHz), with My TH6DXX- THUNDERBIRD beam from "HIGH GAIN"
I hope to meet you on the radio, now Cheerio
73, Zoki
 
#13
Starting to get confused again.:bawling: How bout if I take ONE 500VA transformer at 120v primary and 24v secondary and use a regulator circuit like yours and fuse it at 20 amps?

20 amps will be fine and I could then build another PS with the remaining transformer if it works real well.

Thanks,
David

PS Do you ever sleep? 😀
 
#14
davidlzimmer said:
Starting to get confused again.:bawling: How bout if I take ONE 500VA transformer at 120v primary and 24v secondary and use a regulator circuit like yours and fuse it at 20 amps?

20 amps will be fine and I could then build another PS with the remaining transformer if it works real well.

Thanks,
David

PS Do you ever sleep? 😀
Click to expand...


O.K.🙂
Than try with the schematic you watched:
This schematic using 24 volts of the transformer.
than the current of your 500VA tr. will be approx. maximum 20A, and you can try and parallel secondary winding of the other transformer but if the secondary voltages are exactly the same, if not than the current will be flowing from one to the other!.
12v30a.png
 
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