centre tap mains transformer

[jestaudio]

More curious than anything, as i under stand it a centre tap is essentialialy a tap in the secondry coil which effectivly gives two sepearate coils each with the centre reference to earth or ground(depends your terminology), i would assume if you lifted the centre tap putting the secondrys in sieries on say a 290-0-290 you would get 290+290, taking it a step further if you had a twin secondry traansformer with 290-290 and no centre tap you could link them i series then use the common as a tap and get 290-0-290

[Jana]

It sounds like you understand it.

A 290-0-290 gives you the option of using a full wave or a full wave bridge rectifier.

Your only option for a 290-290 is a full wave bridge.

If you have two windings, each one is 290-290, then yes, you can tie the lead from one winding to a lead from the other winding to make a 290-0-290. But, one thing you must do is get the polarity of the windings correct.

[jestaudio]

It sounds like you understand it.

A 290-0-290 gives you the option of using a full wave or a full wave bridge rectifier.

Your only option for a 290-290 is a full wave bridge.

If you have two windings, each one is 290-290, then yes, you can tie the lead from one winding to a lead from the other winding to make a 290-0-290. But, one thing you must do is get the polarity of the windings correct.

that could get awkward, how would you work out the polarity of the windings,

[Jana]

It's not awkward, just potentially dangerous.

Label the leads from the transformer as follows:

For the first 290 winding, label the leads A1 and A2.

For the second 290 winding, label the leads B1 and B2.

Okay, connect A2 and B1 together and insulate the connection.

Connect your multimeter to A1 and B2, set the meter for high voltage AC.

Apply power to the transformer primary. You will either read about 580 VAC or close to 0 (it might be a few volts AC but it will be very low).

If it reads 0, don't let that fool you into thinking it is safe to touch the wires! There is a high voltage there! It is the differential between the two leads you are measuring.

If it reads 580, you are set. You know which is now the centertap (the two wires connected together).

If it reads a low voltage, swap B1 and B2 so that A2 and B2 are connected together and you are measuring across A1 and B1.

Be very careful with this, take your time--we want you to come back tomorrow and ask another question.

[martin manning]

If you have a diagram of the transformer the windings may be marked with a dot to show which end is which. You want to connect a dotted end to a non dotted end to form the CT. You could also do as Jana describes, but apply a small AC voltage (from a 6.3VAC filament transformer say) to the primary instead of using line voltage.

[jestaudio]

It's not awkward, just potentially dangerous.

Label the leads from the transformer as follows:

For the first 290 winding, label the leads A1 and A2.

For the second 290 winding, label the leads B1 and B2.

Okay, connect A2 and B1 together and insulate the connection.

Connect your multimeter to A1 and B2, set the meter for high voltage AC.

Apply power to the transformer primary. You will either read about 580 VAC or close to 0 (it might be a few volts AC but it will be very low).

If it reads 0, don't let that fool you into thinking it is safe to touch the wires! There is a high voltage there! It is the differential between the two leads you are measuring.

If it reads 580, you are set. You know which is now the centertap (the two wires connected together).

If it reads a low voltage, swap B1 and B2 so that A2 and B2 are connected together and you are measuring across A1 and B1.

Be very careful with this, take your time--we want you to come back tomorrow and ask another question.

just tried it and got lucky first time, used my DMM with small croc clips and got 500 hundred odd unloaded