toroid rewinding-adding a tap

[Bloy2004]

a while ago I rewound a toroid (big one with #10 wire) bringing the output after rectification from 97VDC to 70VDC. I'm willing to do it again ....
Question: Can I unwind some of the coils and add a tap at the point where 50VDC (after rectification) would be put out ?
I have two other motors that SHOULD be run at around 50VDC and the 70VDC is for running two other motors.

Is this the way to do it? Of course, this time I won't cut the coils as I unwind and then have only 40 VDC output.....and then adding fresh wire.

[Al_The_Man]

Its not the best way but if you do, put the bridges across each each secondary half, then you should be able to common the bottom end to ground.
Al.

[Bloy2004]

Is this what you mean? Pretending the spiral is the secondary coil of the toroid, and the extra tap at 35.7 volts is added.
Put a bridge between the 50VAC ...and another between the 35.7VAC wires?
....and then add separate caps to get two DC supplies from the Large toroid?

[Al_The_Man]

Yes, except the common I was referring to was the DC common, which would be the -ve end of the bottom bridge, not a winding common. It would esentially be two supplies in series.
Al.

[gar]

050817-1629 EST USA

Al:

Did you assume he would have two secondaries that are isolated from one another?

I concluded he was going to put a tap on his single secondary.

.

[Bloy2004]

Right Gar, I'm just planning to unwind a bit of the now 50VAC coil (secondary), add a wire out at 35.7VAC and rectify both of the voltages separately...getting a 70 VDC and a 50VDC from the same single secondary coil.

I just don't know if this will put a strain or unusual action to the 50VAC portion as the 35.7VAC is drawing from the same coil.

[Al_The_Man]

I should have added that to ignore the negative end of the top bridge and connect the top cap positive to the +bridge terminal and the top cap negative to the bottom cap positive.
Unfortunately that makes the top supply half wave.
Al.

[Bloy2004]

If its okay, I would like to keep the DC supplies separate coming out of the toroid winding.

(By the way Al, I was counting on you chime in on my electical question and provide answers...as you have for me in the past....Thanks again!)

[gar]

050817-1904 EST USA

Bloy2004:

A single secondary with a tap and two bridge rectifiers won't work. Draw the circuit.

As Al indicated you can make the top rectifier a half wave unit, but this increases transformer heating and the capacitor needs to be double.

Do it the correct way as it is not much more difficult. Create two independent secondaries.

One to generate the 50 vdc output, and the other for the 70 vdc output and appropriately size the wire for the VA load on each secondary.

Or make one a 50 vdc output and the other 20 vdc and series these outputs. Again appropriately size the wire for the VA load on each output. Here the 50 vdc unit has to support the current load of all motors, and the 20 vdc unit only the current of the 70 v motors.

(edit) Your total DC VA (volt-ampere) load must be less than the AC VA rating of the transformer. I might suggest 80% of the AC rating. (end edit)


.

[Bloy2004]

I see....thanks.
Would I be able to just go over the top of the existing coil and add that second coil to get the 35.7 VAC? I mean...just take the tape off the toroid (remember this is about 8" diameter with #10 coiling) and start another coil on the outside? This would be the easiest if it's okay to do that.

Bloy

[gar]

050818-0708 EST USA

Bloy2004:

See the other thread you started.

Basically yes if you can meet the volt-ampere requirements.

.

[Bloy2004]

all-righty then!
Thanks all