Stupid question re: Center Tap

[mfrimu]

Hello,

I have a 18v CT (9v-0-9v) transformer rated at 1.5A

Can I draw 1,5A on just one side of the CT (I just need 9v). Unfortunately it's not a 2x 9v windings, but a CT winding.

And I assume I can't use the 'other side' for another application (reverse polarity?)

Thanks.

 

[AndrewT]

The 1.5A is actually an AC current rating and applies ONLY when a resistive load is driven by the sinewave from the secondary.

If you need DC to power a circuit, the current rating is very dependant on how you draw the current through the rectifier.

 

[paulb]

Yes you can draw on just one side of the CT (with the restriction that Andrew noted). And yes, you could use the other side if they share a common ground and both grounds are connected to the CT.
You might want to read some of the power supply articles at
DIY Audio Articles

 

[mfrimu]

Thank you both !

 

[mfrimu]

Is there a way to get two positive supplies from a CT transformer. Instead of bipolar. Maybe if they were 'floating' amd not use CT as ground?

 

[sofaspud]

It depends on how you intend to use the two positive supplies. +9v and +18v could use the same ground, for example. What circuits are you thinking of powering?

 

[mfrimu]

I need just +9V but could use 2 of them. I'd like to take full advantage of the transformer in terms of current output. Somehow, using only 1/2 the secondary winding does not sound like 100%.

 

[sofaspud]

In terms of power (or power capability), it wouldn't be 100%. In terms of current (9v @ 1.5A) it would be.
It would probably be best if you posted a better description of the circuits you have in mind. Or better yet a diagram; a block diagram showing needed power supplies and loads may be all that is necessary to provide an adequate idea of the situation.

 

[mfrimu]

I need 2 separate regulated +5V supplies, using LM317. I was hoping to use just one transformer. But if no big deal. Am I loosing 50% of 'capacity' by using only half the secondary winding? I measured the primary resistance at 27 Ohms, so it looks like a 40VA or so transformer.

 

[sofaspud]

If the supplies and loads are truly separate then you have more options.

Using only half the secondary loses half the possible voltage, and therefore half the possible power. The two halves of the CT secondary are in series, so the possible current doesn't change.

18V@1.5A = 27VA

 

[sreten]

Hi,

Read up on power supplies. This is inane. You use use both secondaries
for a simple DC supply referenced to the centre tap, via diodes and a cap.
It is just so wrong to just use one when you can use both.

rgds, sreten.

 

[sofaspud]

This is inane.

The OP was polite enough to include a warning in the title.🙂
But yes, I agree that full-wave center-tap rectifier is the short & sweet.

 

[mfrimu]

Thank you !

That was what I was looking for - Full Wave Center Tap Rectifier !

So simple.

 

[woody]

What i would do is use a full wave rectifier NOT a full wave bridge rectifier. This picture shows one without the filter cap. When you add a filter cap to this circuite and use your transformer you should get about 12V DC.