Toroid Question

[jiminy]

Hi,
I would like to put together a power supply using my surplus toroid transformers.. I have 2 of these. They are 120V primary, 88 V Center tapped (44V) 15A , with a second secondary of 12V.

I want to keep things general so I can power 4 G320 Geckos. Should I use 1 or 2 of these transformers in my supply.. these are HEAVY units.. any suggestions? Can anyone suggest what size rectifier and Caps I should use?

The original Toroids are center tapped with only 1 wire .. I have carefully peeled back the outer plastic insulation around the secondary wires and see that there are 4 copper wires soldered to the single center tap cable coming out of the transformer.. Should I seperate the secondary at the tap point to produce two separate secondaries of 44 V? If so, how should I proceed? Two separate rectifiers and caps (1 for each of the new separate secondaries?).. is there a better way?

A lot of questions for you guys... hope someone can help!
thanks, Jim

[Al_The_Man]

You could parallel the two centre tap windings, you would have to separate every end and identify it with is companion other end and then phase everything before hooking up in parallel. for phasing, only connect the end of one winding with another and then measure the two open ends and there should be close to zero voltage, if it is double the expected voltage then use an alternative end of one winding and try again, when you get zero voltage, you can parallel both windings by then connecting the two ends that read zero voltage.
Clear as mud, I know!
Al.

[jiminy]

Al,
what do I gain by paralleling the two center tapped windings? Is this better than say running 2 gecko's off 1 center tap and 2 from the other?
What is your opinion of using these transformers for this purpose? I should be able to get 44x1.414 = approx. 58V from one half of the secondary winding... isn't that right?

thanks again,
Jim

[gar]

060102-2028 EST USA

jiminy:

You can use the transformer as is as a full-wave center tapped rectifier power supply.

I would make the center tap common (negative), use two diodes, one to each outer end of the transformer. You do not connect the outer ends of the transformer as this would short the secondary. To each outer transformer end connect a diode anode. Connect the two cathodes together. This gives you a full-wave rectified positive output at the cathode node relative to the transformer center tap.

This method slightly increases the power loss in the transformer vs a full-wave bridge. Your DC output with a capacitor input filter will be approximately the AC VA rating of the transformer. AC VA is 88 x 15 = 1320. The capacitor input filter DC output voltage is approximately 1.414 x 44 = 62 v. So the approximate continuous DC load current is 1320/62 = 21 Amps. Maybe derate this to 85% and you have 18 Amps. Possibly use a 40 Amp 200 V common cathode rectifier.

.

[jiminy]

Wow! Never thought of that idea! That's the easiest solution.. no need to do surgery on the toroid! Why is the loss greater?
regards.Jim

[gar]

060103-0803 EST USA

jiminy:

The full-wave center tapped rectifier was the most common type in the vacuum tubes days where a transformer was used. In ACDC sets half-wave rectification directly from the line was used with no transformer.

In the full-wave center tapped rectifier each half of the secondary has a half-wave rectifier. For a sine wave no filter and a resistive load the average DC ouput current is 0.5 * 0.636 of the AC peak current for one half of the secondary. This is a more peaked waveform than a sine wave and thus the RMS current is higher.

When we look at a capacitor input filter on the rectifier output we get a very peaked current compared to a resistive load. Thus, this produces more heat in the transformer secondary than than a sine wave for the same average output current.

A full-wave bridge and a full-wave center tapped rectifier with the same capactive load have almost the same peak current on each half cycle. When the same amount of copper window is used in the transformer for the secondary, then the source resistance for the full-wave bridge is 1/2 that of the full-wave center tapped. This results in a somewhat higher peak current for the full-wave bridge, but the relative peakiness is worse for the full-wave center tapped circuit and its source resistance is higher as viewed on a half-wave basis.

You would need to run a temperature rise measurement in the transformer secondary to determine what actual derating should be applied.

.

[Al_The_Man]

One of the down sides with full wave centre tapped it is not as an efficient use of the transformer as a full wave bridge, as each half of the secondary is only used half the time.
You would need a transformer 1.4x larger when using full wave center tap over a full wave bridge for the same current capacity.
Al.