# Transformer with 3 secondary taps

Can I use more than one of the taps on a transformer like this at the same time? If my total load (Amps) is with in the maximum that it can handle?

Yes, it is possible to use more than one winding at the same time, but you have to be careful that you violate neither the power rating (18 VA) nor the current rating (1 A) at any of the four taps.

This means, as an example, that you can use the 12 V and 15 V taps if you need a voltage of 3 V, but you can only use 1 A at these taps, which equals to 3 VA - not 18 VA.

Also, there is an isolation between the primary and secondary windings, but the three secondary windings are not independent from each other (i.e. not isolated). Probably not a problem for you, but still something to keep in mind.

The data sheet has a circuit diagram of your transformer and includes the current rating (1 A max).

• Thanks nice simple answer just what I needed. I don't think the isolation is going to be a problem and if I pull 1A other things are going to blow before this so they will be fused. Oct 27, 2011 at 12:06

Yes, you can use multiple taps on a transformer simultaneouely.

BUT there are a few points to watch - some less obvious than others.

Isolation: You need to remain aware that taps on the same winding are not isolated so that if you eg connect the 12v tap to ground then the 0v point will now be at -12VAC* relative to ground and the 15V tap will be at 3VAC relative to ground and the 18V tap will be 6VAC relative to ground.

If separate windings are provided, as they sometimes are, then isolated supplies may be created subject to the insulation strength, eg if you have two low voltage windings and make an eg 12 VDC supply ay ground potential and a 12VDC supply at mains potential the interwinding voltages MAY cause insulation breakdown. Primary to secondary insulation is necessarily designed with this effect in mind, Secondary to secondary insulation will usually have much lower voltage breakdown specs due to relaxed manufacturing methods.

• Note: AC is AC and a sign is not usually used - but I said -12VAC rather than 12VAC as it will have the opposite phase to the other outputs due to the "top" of the 0-12V winding being grounded, whereas the 12V-15V winding has it's "bottom" grounded.

Power & current ratings

As you note, total wattage must not exceed total transformer rating This is for "magnetics" reasons as well as due to copper losses.

BUT if you eg ground the 0V point then current drawn from the 12 VAC point AND current drawn from the 18V point will BOTH flow in the 0-12 winding section. This will not "bother" the overall magnetics issues as the core is shared by all windings BUT may cause extra copper losses in the common winding.

LOSSES:

If this transformer was rated at say 18 VA you may expect it to be able to supply either 18VAC at 1A or 12VAC at 1.5A or 3VAC at 6A (from the 12-15 V taps of the 15-18V taps). Chances are that the designer has built this with 18VAC at 1A in mind and the taps are a bonus. Windings may well have been sized for 1A so the 6A when used as a 3VAC source may cause more losses than expected.

When run at full VA capacity copper losses increase linearly with decreasing output voltage eg losses at 3V 6A are 18/3 = 6 times larger than at 18V 1A.

• The total copper losses when run at full power are inversely proportional to winding voltage
ie if running at 18V 1A caused 1 Watt of copper losses
then running at 12V, 1.5A causes (18/12) x 1 Watt = 1.5 Watt copper loss
and running at 3V, 6A causes (18/3) x 1 = 6 Watt copper losses.

However, the per winding length losses rise with current squared. As copper losses are mainly I^2R the loss per unit winding length at 6A compared to 1A is 6^2 = 36 times higher. This may cause enough local hot-spotting to cause damage or magic smoke.