toroidal Transformers

[wjfiles]

I am about to build a power supply for the HobbyCNC 4 axis board.
On searching suppliers for a transformer to give 24 volts AC at 10 Amps I am unable to relate to the specs on the labels on the transformers.
I am advised!!! that there could be more than one that will give the required out put depending on how it is wired.
Could some good samaritan explain how to wire the 2 outputs and the centre taps. Also explain VAC.
Example 18v 0 18v 150 VAC. What can I get from this one.
I understand that once rectified the no load voltage increases, but by how much.
I just want to build the classic supply as recommended by Dave.

[wjfiles]

Since posting this question I have found some answers on other posts.
They are related to specific problems.
A brief explanation of Toroids would be a help.
Is there any advantage or disadvantage to using a slightly larger capacity transformer.

[john_s]

Yes there is more than one way to wire transformers. If you need 24V, you could use a double wound 24V transformer and connect the 24V windings in parallel. Or you could use a double wound 12V transformer and connect the windings in series (12+12 = 24).

VAC is Volts AC (alternating current).

One thing to take care with a toroid is to ensure that the monting bolt does not short to the (assuming metal) case at the free end, as it will act as a winding on the transformer, and then you have a short circuit.

Advantage of going slightly hight capacity is that it will probably run cooler. Disadvantage is that it will cost more.

Edit to add: The increase in the voltage comes about as the AC voltages are RMS voltages. The voltage after rectification and across the smoothing capacitor will be 1.414 times the rated voltage of the transformer, as the capacitor charges to the peak voltage coming out of the transformer.

[Al_The_Man]

There is typically two types of secondary windings you will see on a transformer, in your case 18-0-18 is a centre tapped winding and these cannot be uses in parallel.
You can use a full wave centre tap common which will give you 18x1.414= 25vdc, or ignore the centre tap and use a bridge rect. connected to the outer ends and get 53vdc.
(less a couple of volt drop for the rect).
The other type is two distinct windings, if both 18v, they can be connected in series or parallel, but must be phased correctly, there are numerous previous posts here to give you more info on this subject as well as modifying the windings on a Toroid to adjust the output voltage.
Also see the tech page on the Plitron transformer site for combinations.
Al.

[wjfiles]

thanks guys for the helpful responses

[Megahertz]

The first classification I would place on a secondary winding of a transformer is "step up" or "step down". Does the secondary produce more or less voltage than the primary winding. Then I would talk about how many secondary windings, their center taps, and wiring configurations.

[wjfiles]

Thanks again guys,
One last question, if the transformer has a VA 300 rating I divide this by the output volts it will give me the output amps ?

[wjfiles]

Connecting in series or parallel toroids must be correctly phased ??????
How is this done , how can they be tested .

[MaxHeadRoom]

Originally Posted by wjfiles Connecting in series or parallel toroids must be correctly phased ?????? How is this done , how can they be tested .

There are lots of previous posts here on how to do this safely.
Al.

[Ron111]

Try the below thread and go to post 29 and 30, I attached campbell's pdf power supply file and put a link to pmimno's schematic which show's a power supply for this board. I built a PS from this schematic and it worked well and this link has the sources where to buy the components. It's not hard, just take your time

http://www.cnczone.com/forums/showthread.php?t=23095

Ron