Need help figuring out taps of a torodial transformer?

[rwgast_lowlevellogicdesin]

So a while back I was given a pretty big torodial transformer it's about 8lbs so im guessing it's somewhere between 200 and 500 VA. The thing is I just can not figure out the coil/tap configuration. Usually id guess tap wires of the same colors are the main wires for each coil and the thicker gauge taps are the primarys, but those assumptions dont make sense when I test the taps out for continuity and when I try to test the thing using a signal generator the ratios I get just make no sense.

Here is a picture of the transformer with colored dots indicating groups of continuity.

The blue, black, and white taps are the lowest gauge.

Coil 1 is, Black, Blue, White, Orange, Red
Coil 2 is, Orange, Violet
Coil 3 is, Red, Yellow, Brown

Lastly there is a label on the transformer, which turns up nothing on google
080-0017-0
BMI G857

So how do I figure out which damn wires are the the primary coil?? At first site I thought the Black,Blue/Red,Orange where dual primarys for 120/240 but that doesn't explain the white wire or why all 5 are on the same coil so maybe those 5 wires are a secondary but why would the wires be thicker than the primary?

[Cubdriver]

Do you have an ohmmeter that's sensitive enough to try to determine the resistance between the taps on the various coils?  That could help to determine the order of the taps.

My guess (and this is purely a guess, and also worth exactly what you've paid for it) would be that the multiple-tap winding might be the primary.  White and black could be the 120V section of the winding, one of the others could be a tap for 100V, and the remaining two be for operation at perhaps 208V and 240V, or maybe 230V and 250V.

The others would then be a single voltage winding and a center tapped winding.

Without knowing more about the configuration of the transformer, the raw thickness of the wires doesn't necessarily tell you much.  A step down transformer will often have thicker secondary windings than primary (voltage stepped down, delivers much higher current than the primary); conversely a step up transformer (less common in our low voltage modern world) would have thinner secondary windings as the increase in output voltage will result in a lower current capability.  The actual tap wires that connect to the outside world will often be larger than absolutely needed for physical strength reasons.

-Pat

[Paul Rose]

I agree with Pat on wire thickness.

Stepping down the voltage steps up the current.

The lower voltage side (i.e. the secondary for step down) carries higher current, and it would be normal for it to have the thicker wire.

[Cubdriver]

One more caveat that just occurred to me regarding the suspected multi-tap primary - a sensitive ohmmeter can help you suss out the windings and taps based purely on the resistances, but there is one more thing to keep in mind.  For a given power, current drops as voltage increases.  If the transformer has a multi-tap primary winding as I suggested it might in my previous post, it is also possible that that primary is not would with the same gauge of wire all the way through.  Knowing that doubling the voltage will halve the current for a given power level, it is possible that the manufacturer might use a heavier gauge wire for the 0-120V section of the primary to handle the higher current that will flow when it is fed by 100-120V, then switch to a lighter gauge wire for the part 'above' that which would only be in play at 200+ volts and only need to carry about half the current that the bottom part must for 120-ish V operation..  If that's the case, then the pure resistances won't tell the whole story as the 'high' section of the primary would have more resistance than the 'low' side because of the higher resistance of the smaller gauge wire.

If this is the case, simple resistance measurements will give you an idea of things, but not the entire picture.  This knowledge in combination with your signal generator should at least allow you to determine the order of the taps and get an idea of the turns ratios.

-Pat

[edpalmer42]

You don't actually need to use a signal generator - since this is a mains transformer, you can use a low voltage (maybe 6-10V) transformer as your source.  Whichever you use, measure the ratios of the windings.  If there are multiple taps on a coil, see if any of them are centered.  Watch for coils that give the same output voltage as you're feeding in.  Now play 'what if'.  i.e. If I connected mains voltage to this coil, what would the voltage be on the other coils & taps?  See which combinations give you some rational results.  Be careful when you make these measurements.  If you put 6V into a low-voltage winding, you'll have line voltage at the primary windings!

Five wires on one coil suggests a multi-tap secondary with a center tap.  Image two bipolar supplies with full wave rectification and a grounded center tap.  The higher voltage pair is for the output drivers and the lower voltage pair is for the low-level circuits.  You might see this in a high-power audio amp.

On the primary side, the 'magic' voltage ranges are 220 - 240V, 110 - 120V, and 90 - 110V.  The last range is for Japan where the nominal voltage is often 100V.  I've seen transformers where only one of the two primary coils has a tap for 100V.

Once you've figured out the primary and secondaries, look in a catalog and find a toroidal transformer that's about the same size and weight as the one you've got.  Ideally you want to know the dimensions of the core, but that's often hard to measure because it's hidden under the windings.  That will give you an idea of the total power you can draw.

If you have more than one output winding, determining the rating of each coil can be tricky.  One source I saw suggested that a 10% drop from the no-load voltage was reasonable.  Smaller transformers allow a larger drop, while for larger transformers, 10% might be too much.

It's not an exact science, but transformers are pretty rugged so a bit of overload here or there shouldn't cause any problems.  Remember that a fully-loaded power transformer will likely heat up to the point that you definitely don't want to put your hand on it!

Ed