Voltage selection options for dual-primary toroidal transformer?

snipped-for-privacy@yahoo.com wrote: (snip)

Sure. Wire the two primaries in series and the volts per turn for all windings falls by half. The current still has to pass through both windings in the same direction it did when they were in parallel, so you have to disconnect both ends and reconnect one end of one winding to the end it wasn't connected to of the other one. The reconnection can be wired to a double pole double throw switch, so you can do it on the fly.

If each of the 33 volt windings is rated for 7 amperes, and you keep them in parallel, that makes a 33 volt winding rated at 14 amperes. If you change the primaries from parallel to series, you retain that amp rating, though the total power output falls to half, because of the lower voltage.

You permanently connect the hot (after an on off switch and a fuse) to (for instance) end 2, and the neutral permanently to start 1. End 1 goes to the center contact of one of the poles, with high voltage position to end 2 and the low voltage position going to start 2. The center contact of the other pole goes to start 1 and the high voltage position goes to start 2. Nothing connects to the low voltage position of that pole.

It would create a lot of heat if it was done in any way that wastes the voltage. At this current level, I would be thinking in terms of an inductor input filter, instead of connecting the rectifier output directly to the filter capacitor. This will reduce the voltage to the average of the rectified waveform, rather than try to pump the caps up to the peak voltage of the waveform. It also greatly lowers the ripple current heating in the capacitors and slightly raises the current rating of the DC supply because the current is not concentrated in brief pulses near the peak of the wave. The inductor will have to be about as big as the power transformer, to be effective in this service.

Yes, Essentially you have 2 identical transformers(hopefully) and you can combine them in 8 possible ways. Your voltages will be either 66V, 33V, or

VP/VS = NP/NS = IS/IP

where the P represents the primary side and the S the secondary side.

You have said that VP is 115 for one primary and 33v for one secondary. This implies that NP/NS ~= 3.5. We can take NS = 1 and NP = 3.5 for conviences as long as the two primaries are identical as also the two secondaries.

In that case can see that if we hook the primary side up in parallel we do not increase the turns on that side but allow for double the current. If they are in series we double the voltage but cut the current(since resistance doubles). If we only use one primary then we do nothing to that side.

The same logic holds for the secondary side. So basically you have

a/b*NS/NP*VP = a/b*33V = VS b/a*NP/NS*IP = b/a*7A = IS (I'm assuming 7A is measured from the secondary)

a and b are indicators that tell you which side and which configuration the inductors are in. a is for the secondary and b for the primary. If a = 1 then its parallel and a = 2 means series. If you only use one primary or secondary then the formulas won't work but you can use the same logic to get the voltages.

If you have the primary configured as parallel and the secondary as series then a = 2 and b = 1. This gives your secondary voltage as VS = 2/1*33V =

If you configure the transformers with primary in series and secondary in parallel then then b = 2 and and a = 1 so that VS = 16V and IS = 14A.

So if you want to drop the voltage you can increase the mas current rating. This isn't entirely true and I have no idea how well it works in the real world as you can saturate your core which makes the transformer less efficient and it can burn up.

The main thing to note is that when you hook a side up in series you increase its voltage that it supplies or that it takes(depending on which side). If you hook them up in parallel then you increase the current that it can take but decrease the voltage.

Yeah, you can do this many ways and a switch would be easy. Just remember that you could ruin the switch if you don't account for inductor kickback.

Maybe just use a resistor. If you need to drop a few volts, say 10v at 14 amps then thats 140W. (so a lightbulb would do. A bit hot though.)

Well, I hope that info helps somewhat.

Jon

Toroidal transformers have the advantage that you can usually add a few extra turns to either the primary or secondary to tweak the voltage a bit. A 600 VA transformer like this probably has about 0.5 volts per turn, so ten turns of #16 to #14 wire can give you an additional 5 VAC that can be added or subtracted from the output. This would be a lot more efficient than burning up the extra voltage in a resistor.

Remember that most transformers are specified at the rated current, at which point the output voltage will be 2% to 10% lower than at open circuit. Also, at the rated line voltage, the primary will be saturating just a bit at the voltage peaks, whereas this will not happen with the primaries in series. So if you read 33 VAC at 120 VAC with parallel primaries, you will probably read 17 or 18 VAC with a series connection, and the waveform will have less distortion (no flattened peaks).

It is possible to drive a transformer with a properly designed triac or SCR circuit, which can even be controlled in a feedback loop to produce a very well regulated DC voltage. This is a very efficient method that is used in some very high current supplies, but it is important to have a solid low impedance source and a well balanced phase firing circuit. Any imbalance in the phase angle will cause a net DC current and can lead to instability.

The most efficient safe way to adjust the output is by means of a variable autotransformer (Variac or Powerstat).

Paul

Paul, and everyone who responded (to both my recent threads) -- thank you. This information helps a lot.

Best regards, Chris

If the secondaries are in series, the output will be 66 volts with a current capability of 7 amps, not 3.5 amps.

How would the core become saturated?

The tranny has two independent primaries and two independent secondaries. So if each primary seeing 115V causes each secondary to output 33V, then wiring the primaries in series should cause each secondary to output 16V, right? It's like wiring the transformer to have a center-tap.

I'm the newbie here, so may be wrong...but that's the way I was thinking about it...

-Chris

And they do wire them like that for a center-tap; it's quite common.

Cheers! Rich