output transformer connections

[Mike Brett]

Hi all
I have a small output transformer for a speaker and have mislaid the paper regarding connections. One side of the transformer has a reading of 492 ohm and the other side reads just 1.6 ohms, this side also has a wire earthing one of the terminals to the transformer casing. Could someone tell me which side I should connect to the speaker.
Many thanks Mike

[DAC10 Collector]

The transformer side reading 1.6 ohms should be connected to the speaker.

[Ian - G4JQT]

Almost certainly the 1.6 ohm side should go to the speaker.

Incidentally, the resistance reading (492/1.6 ohms) are just that, the DC resistance of the windings. The DC resistance usually gives little to no indication to the actual turns ratio, just that one side has a lot more than the other.

Ian

PS: Seems DAC10 just beat me to the post!

[GrimJosef]

Depending on what you're going to do with it it may also be useful to know the relative orientation of the windings. If your circuit uses negative feedback taken from the secondary then the primary will have to be connected the right way round. Which way that is depends on lots of things and a lot of people simply 'suck it and see'. But they keep their finger very close to the power off switch at switch-on because if they've got the orientation wrong the now positive feedback turns the amplifier into a full power howling oscillator !

Cheers,

GJ

[Mike Brett]

Many thanks for the information people.
Cheers Mike

[G8HQP Dave]

Actually the DC resistance ratio can give a reasonable estimate of the actual impedance ratio, assuming equal winding space for each winding. Twice as many turns will require half the wire cross-sectional area so the resistance will be four times as much - the same as the impedance ratio for 2:1 turns.

Taking the example above, 1.6 vs. 492 could be a 15:4500 impedance ratio or thereabouts. I don't claim an accuracy of more than a factor of 2, though!

Measuring the inductance ratio may be more accurate, as that varies as turns^2 and is not disturbed by wire size.

[kalee20]

Originally Posted by G8HQP Dave

I don't claim an accuracy of more than a factor of 2, though!

Probably about right - even a 1:1 transformer will sometimes show different resistances for primary and secondary, if they are wound one on top of the other rather than side-by-side with a twin section bobbin. The winding which is wound first will have a shorter length per turn, thus lower resistance.

The transformer designed often does not take that into account, unless absolutely peak efficiency is being striven for, in which case different wire gauges may be used for the same number of turns.

[Herald1360]

If you've got a reasonable meter, just put about 12Vac on the primary and see what the secondary offers. Volts ratio = winding ratio = √(impedance ratio).

[G6Tanuki]

Measuring winding-resistance will surely only be of relevance to calculate turns-ratio if the windings use the same diameter of wire?

A traditional single-ended valve output-transformer invariably uses very fine [40-gauge?] wire for the primary and much thicker [22g] wire for the secondary. Thats why OPTs are so prone to burning-out the primary if the output-valve pulls excess current.

[kalee20]

Originally Posted by G6Tanuki

Measuring winding-resistance will surely only be of relevance to calculate turns-ratio if the windings use the same diameter of wire?

See post #6 by G8HQP Dave above.

Transformers are often designed with the same bobbin area for primary and secondary - this gives nearly optimum efficiency.

In this case, with different turns, the wire diameter definitely won't be the same. But as wire cross-section area is then inversely proportional to turns (to give the same area of bobbin occupation), you just take the turns ratio as the square root of the resistance ratio - and treat the answer as a very crude approximation.