paralleling power transformers

[W4AMV]

I have a set of identical Stancor RT-204 transformers. They are rated at 12V/8A or 24V/4A. I need to achieve 28V and with a bit larger filter capacitor then mentioned in their data sheet, hope to achieve the required say 30V. I would like to use the transformer in CW service... so the power demands are less severe. However, I need to achieve a 10A capacity. Thoughts on placing the secondaries in parallel and primaries in parallel, can this be safely done and the required precautions? I am concerned about the reactance load of each xmfr on its neighbor. Thanks!

Alan

[WD8BIL]

Just a thought, build 2 seperate dc supplies on the same chassis and tie the dc supplys together after the rectifiers? That will keep the ac phasing problems out of the picture.

[w8khk]

Buddly has a good idea here, but I would like to add some additional suggestions...

First, place the diodes such that the cathodes are the positive output terminal, and the anodes are the negative output terminal.  It appears in Buddly's diagram that the outputs are connected to the transformer, and the bridge input terminals are connected to the load.

Second, I do not believe there is a need for decoupling diodes after the bridges, and they will typically drop another 7/10 of a volt.  If the goal is to provide the highest output voltage, skip the additional diodes and lump all the capacitance together.

Third, using bridge rectifiers, you pay a penalty of voltage drop of two diodes at all times.  This is approximately 1.4 volts.   A simpler and more efficient method would be to connect the primaries in parallel, then series the secondaries.  Ground the common secondary terminal (effectively a center tap, if one transformer was used).  Then, using just two diodes, implement a full-wave center-tapped supply.   This effectively uses both transformers, but reduces the voltage drop to .7 volts, as the current needs to flow through only one diode at any point in time.

The only down side I see in this arrangement is that the current flows through each transformer winding less than 50 % of the time, so run some tests before building to verify that transformer heating is not significant.  Hope this helps.

[W7TFO]

I've run transformers in parallel many times over the years with no problems.  A simple test is to hook them up as you say, all in phased parallel, and use an AC milliammeter in the secondary of one to see if there is any difference current.  It should be very low if indeed they are identical.

i.e, <50 ACmA in a total secondary available current of 10 ACA wouldn't be an issue.

I have seen a bit more than that current present in factory multiwinding transformers that were designed to be used in either series or parallel hookups.

73DG

[WD8BIL]

Yup, you're right Rick. I drew it up in a hurry and didn't pay attention. Good catch.

Fixed!

Your suggestion with the quazi centertap configuration is a good one. It'll make for a more consistant voltage out of the supply. The 2 seperate supplies would need near identical outputs to load share.

Of course his final voltage will depend on the actual line voltage at his house but assuming he really gets 24VAC from the transformers his caps will charge to @33VDC so I just figured the diode drops would be no big deal. In practice I've found most 24VAC transformers really do 25+ most of the time.

[w4bfs]

hi Alan ... those RT series Stancor trannies are rated for rectifier service .... I've used one for about 40 years in a power supply fb ... I would connect each for low voltage / high current and then put them in series .... to a bridge rectifier each will contribute what it has to offer without balance problems ....a bit more worrisome is that you have 192W of transformer and you have a 300W load .... I wouldn't put a brick on the key of yer xmtr

[W4AMV]

All great answers and information! Thanks! I'll study these comments and ideas and then incorporate them and let you know what results. I am surprised after looking through several years of the Handbook, that this subject had not been treated or I simply could not find the year in which it was discussed.

[WA1GFZ]

The full wave CT will save 1 diode drop but you have to compare it to the transformer running double current for 50% duty cycle so resistive losses could eat up the .7 volt saved. You won't know until you try it. I have a 24 volt transformer in my present HPSDR final with a bridge. The output drops to 28 volts under heave load with 130,000uf worth of caps. Resting up around 34 volts.

[k4kyv]

I would be concerned only about differences in the two transformers. Even though the nominal voltage is the same, the actual voltage may be slightly different. One of the transformers may have a fraction of a turn more than the other, due to normal variations in the manufacturing process.  

Wire them in parallel, and measure any current flow between the two transformers.  If it is more than about 5% of the current rating of the transformers, that may raise a red flag.  In that case, one possibility would be to use equalising resistors (or better still, reactors) between the two winding where they are connected in parallel. Use enough resistance to cut the current down to a fraction of the current rating of the transformers, but not enough to cause any significant voltage drop at the current you are pulling from the paralleled transformers. The best configuration would be to use two low-resistance resistors in series, and let the common connection between the two serve as the midtap, and pull the load off that point.

[Opcom]

I'd put a bridge on each one and have each one set to the highest voltage so the bridge's drop would be the smallest percentage, then put those sets in parallel.
Those 30A bridges are about $2 each surplus and will bolt right to the chassis. Capacitor input definitely and if you still have hum, then a 50-200mH choke rated for lots of current followed by another big cap will get rid of that. I got a Motorola supply here, supposedly 12V, makes 15 no load, 13.5V @ 25A. It uses that topology and the choke is the size of a 12V/3A heater transformer. Too bad choke input won't do the voltage beyond Lcrit.

[KA2DZT]

Put the two 12V 8A windings in series (phased correctly), put one FW bridge rectifier on the two windings.  Now, if you have another spare xfmr, like a 5 or 6 volt filament xfmr at 8amps, put a FW bridge rectifier on the filament xfmr.  Connect the FWB diode OUTPUT of the filament xfmr DIRECTLY to the (ungrounded) negative terminal of the FWB diode of the two (connected in series) 12 volt xfmrs.

The DC pulse output of the filament xfmr will add directly to the DC pulse output of the two 12 volt xfmrs.  This will give you about 30 volts.  Now add the filter caps.

If you have a 10 volt filament xfmr or even a third 12V 8A xfmr, use it instead of the 6 volt filament xfmr.  This will give you about 36 volts.  Then you can use a choke input filter (10 or 20 mH at 10amps) with the large filter cap.

This set-up will give you at least 30 VDC (with full load) well filtered.

Fred

[W2VW]

FWIW I've been paralleling similar power transformers for over 25 years without a failure.

This includes transformers with 6 kv secondaries.

[WD8BIL]

I've been paralleling similar power transformers for over 25 years without a failure.

Yu da man, DAVE!

[k4kyv]

I have seen small inductances inserted between large transformer windings when they are wired in parallel.  It should work equally well with separate transformers as with multiple windings on the same transformer. I have a huge variac in my junk collection with two identical toroidal sections in parallel that uses that arrangement. It should be more efficient than using resistors.