Questions about pairing 2 transformers.

[DWSmith]

Making a power supply for ham radio QRP use.

I have 2 transformers of the same part number.  They are 120 volts for the primary, 9-0-9 volts for the secondary, and each rated for 2 amps.

I need 4 amps at 18 volts.

As long as they're matched for input and output phases is it safe to tie these together?  That is, will the advertised voltages match closely enough there won't be a problem? Should I tie the center taps together or not?

Should I just buy an 18 volts, 4 amps transformer instead of doing this?

Yeah, I know.  Stupid beginner question from a stupid beginner but I have to start somewhere.

Thanks,
Dan

[CaptDon]

Same part number, same manufacturer, should be no problem!!

[johansen]

a typical 20 to 40 watt transformer is 70% efficient at best, so even if you did have a 10v and 9v transformer in parallel each feeding their own rectifier,  you would have a good match under load.

i would rectify the 18v from each transformer separately and not tie the centers together, just in case the voltage difference is more than .7 volts ( extreemly unlikely)

[CaptDon]

DW, you are very correct about matching the phases correctly! Parallel the primaries together, no problem. Parallel the secondaries making sure they are in phase. I did exactly what you are doing to power a 2 Meter Ham Radio. (Kenwood TS-7850) and my transformers were the same as yours. 120vac primaries, 9-0-9 secondaries. I ended up with about 22 volts D.C. on my filter capacitors and I was passively regulating down to 13.5vdc. With that much voltage headroom between the filter cap voltage and output voltage the series regulator transistor wanted to run very warm during transmit. The heatsink from my junkbox wasn't really sized for long transmit cycles. Make sure to fuse the primary and D.C. output. Cheers!!

[DWSmith]

Thanks for the feedback guys!

[Doctorandus_P]

It's very likely to work, but you should verify this none the less. So just try it out.

What I would do:
First verify that both transformers work properly, (a bit depending on their origin).
Put the primaries in parallel. (that is always safe if it's for the rated voltage).
Keep an eye on the primary current of both transformers if you connect the secondaries. If the current increases, you have some extra losses.

Put some decent load on the transformers while keeping an eye on currents and temperature. A current clamp is best, but you can also add a shunt resistor to both transformers, as long as it's voltage drop is low, for example a 1 Ohm resistor in both of the primaries.

Also: You should give each of the transformers a separate primary fuse of a suitable value.

And please stop calling yourself stupid. Nobody is benefiting from that, you're just wasting the time of the people reading that. 

[Konkedout]

Keep in mind that (Probably) your 4 Amps requirement is rectified DC current, while the transformer current rating is RMS current.

Depending upon the rectification-filtering method (inductor versus capacitor input) the RMS current in the transformer output windings will exceed the DC rectified average output current.

Now...maybe this is not a problem if your 4A number is only a peak number and not an average.

This analysis is easily done with any circuit simulator.  LTSpice for example, or an Excel spreadsheet, or I even SIMULATED this with a programmable calculator about 50 years ago.

And yes the transformers can normally be connected in parallel if they are perfectly matched.  (A difference in turns will cause circulating current.)  Using separate rectifiers is not a bad idea.

A buck switching regulator can give you a reduction in transformer current for a reduction in output voltage.  But care is required especially in radio applications as the buck switching regulator can make RF noise.

[Konkedout]

Just for the heck of it....Just now I found an Excel simulation of a 50 Hz AC source (with series resistance) feeding a bridge rectifier to a capacitor and constant current and/or constant power load.  I put this together in 2009 but just now added calculations for transformer RMS current and average DC output current.

Numbers are shown in Engineering notation (with exponents being multiples of 3).

1) Nothing about this spreadsheet is protected so it can easily be screwed up.  The intent is to enter variables in the green area at top.  It was intended for my own use and is not "idiot proof".

2) It actually is a simple simulation, with 1000 time steps.  It starts the output voltage at 28V so it looks only at steady state operation and not the startup from 0V.

3) For the calculations shown, it calculates an RMS transformer current of 1.407 Amps for an average DC output current of 706 mA.  That is about 2:1!!

I am open to any questions about it.  LTSpice would have probably been easier but I was not using that when I originally put this together.  And this shows the calculations, which LTSpice generally does not.

Errors are possible but I believe it is mostly good.

[MrAl]

Making a power supply for ham radio QRP use.

I have 2 transformers of the same part number.  They are 120 volts for the primary, 9-0-9 volts for the secondary, and each rated for 2 amps.

I need 4 amps at 18 volts.

As long as they're matched for input and output phases is it safe to tie these together?  That is, will the advertised voltages match closely enough there won't be a problem? Should I tie the center taps together or not?

Should I just buy an 18 volts, 4 amps transformer instead of doing this?

Yeah, I know.  Stupid beginner question from a stupid beginner but I have to start somewhere.

Thanks,
Dan

Hi,

Should work, but use fuses.  Transformers that get overloaded can get so hot they cause fires.  It takes a while for them to get hot, but then can get very hot.

[DWSmith]

Transformers were purchased new for this project several years ago and have never been used.

Four amps would be available but never needed. Even while transmitting I wouldn't need 0.5 amp for the transceiver (I use CW and the duty cycle would be low).  I'd use the power supply while building/testing the transceiver but when operating my station I would be running it and my accessories in the field from a battery.

This is a Doug DeMaw (W1FB) designed power supply.