Ways to find a transformer to fit specific needs?

[XOIIO]

Hey all, I was wondering what the best ways/online resources are to find a transformer to fit your application's needs, say for making a power supply, find one based on input voltage, output voltage, and current output?

[robimarko]

Basicly you just chose one that suits your input and output voltage and output current.
And thats it,not a lot of factors to count in

[XOIIO]

Basicly you just chose one that suits your input and output voltage and output current.
And thats it,not a lot of factors to count in

I'm wondering what sites a good for finding them where you could specify these sorts of things?

[mariush]

(later edit: apologies for typos below, my keyboard has a few keys that i have to press harder to work, so there may be some letters missing - already ordered replacement keyboard)

You won't find any website that will say "this transformer is designed for 12v DC up to 10A" or something like that.

The reason why you won't see this is because transformers can be used for several things and converting the AC output of the transformer to DC is optional. 
For example, you may want to use a transformer to light up an incandescent lightbulb and then the lightbulb doesn't need DC voltage, it can work just fine directly with the AC voltage.

Transformers also often have a center tap or the secondary winding is split into 2 equal sized secondary windings (and by joining one end of each winding you get a center tap). This gives you flexibility, you can either connect the windings (or just leave the center tap unconnected) and have one big AC voltage, or you treat the two secondary windings as two separate outputs.
This center tap is often used in audio transformers, which work with split power supplies (for example -12v and +12v).

Last, the output DC voltage varies depending on how you rectify the AC voltage and how much capacitance you have after rectification... the manufacturer of the transformer can't possibly know how you'll design the power supply.

Let me give you an example... let's say I want 12V DC at 3A  for an audio amplifier, so I need to pick a transformer and I don't want to use a switching power supply because it's "noisy", I want smooth 12v.

Let's say I chose a 12v AC rms transfomer like this 80VA one which has two secondary windings of 12v AC each : http://www.digikey.com/product-detail/en/VPS24-3300-B/VPS24-3300-B-ND/4746278

So I put the two windings in parallel and I have 12V AC rms up to 80VA.

There's a very simplified formula to figure out the maximum current on the secondary if you use a bridge rectifier.  Current Secondary AC= 80VA/12V = 6.66A , Current Secondary DC (rectified) =  ~ 0.62 x Current AC = 0.62 x 6.66 = 4.13A  so this 80VA transformer is more than adequate to provide at least 3A.

This is 12v AC voltage, so I need to rectify the AC to DC voltage using a bridge rectifier, so after rectification I get a DC voltage with a peak DC voltage of Vac x 1.414 = 12v x 1.414 = ~ 16.9v.   
But, as the bridge rectifier is made up of diodes and two diodes are always conducting at same time in the bridge rectifier, there's a drop in voltage equal to 2 x Vdrop on an individual diode in the rectifier. The diode voltage drop in a rectifier varies with the current and the temperature and any decent bridge rectifier datasheet  will show some curves with the Vdrop at least in relation to the current.
You may pick a bridge rectifier rated for 5A and that one may have a voltage drop of 0.3v at 0.5A but may have a voltage drop of 1V at 3A. 
Or, you may pick an over-sized 20A rated bridge rectifier  and that one may have a voltage drop of 0.5v at 3A and 1.2v at 20A.   

You see... the transformer manufacturer can't know what bridge rectifier you're going to use, if you're even going to use one, because you could just use a single diode for some applications (half bridge rectification).

But let's say we go with a rectifier that has a voltage drop of 0.8v per diode at 3A, then continuing with our scenario this means you now have a peak DC voltage of ~16.9v - 2x0.8v = 16.9v - 1.6v = ~15.3v

This is a PEAK, 100-120 times a second (2 times the mains AC frequency, which is 60Hz in US or Canada, 50 Hz in Europe, varies in other areas) the voltage will reach this peak voltage but most of the time the voltage will vary, it could even go down to 0 volts. So you need a capacitor to charge up when the output is close to the peak voltage and give the circuit energy when the output voltage is going down, therefore maintaining that voltage above the 12v you want (or a minimum voltage you want).



Now i have to think ahead a bit because if I use a super large capacitor, I will have 15.3v all the time but that's not necessarily a good idea, as I only need 12v and the device I want to power may not handle 15v so well. I also can't use too little capacitance because then my voltage may go under 12v at some points.
So the best course of action would be to use a big enough capacitor to keep the voltage above some voltage and use a linear regulator to always output 12v. 
Now there's another problem: all linear regulators need to have the input voltage above the output voltage by some quantity in order to provide a stable output. For a regulator like LM317 that's about 2v (but these don't do 3A), a regulator like 1117 only need 1.1v but can only do 1A.
Let's say I go with A LT1084 which is rated for 5A and has a voltage drop of just 1v : http://cds.linear.com/docs/en/datasheet/108345fg.pdf

So now the input voltage of the linear regulator needs to always be 12v + 1v, in order for the regulator to output 12v at 3A.   You have to pick a capacitor that will keep the voltage between 13v and 15.3v at all times, and for that you have a formula that approximates the size: Capacitance = Current / ( 2 x AC Frequency x Vripple)  where Vripple is the difference between peak and minimum voltage.
So for 13v at 3A, you would need  C = 3A / [2 x 60 Hz x (15.3v-13v) ]  = 3 / 276 = 0.010869F = 10869 uF
That's a pretty big capacitor.

Now you may either go this route and be forced to use over 10000 uF of capacitance (expensive, bulky), or you may just pick another transformer that outputs a bit higher AC voltage, for example 15v AC rms, which in turn rectified is ~ 21v, about 19v after the losses in the rectifier, so now you can use a smaller capacitor to keep the voltage above that 13v or you may even choose to use use a cheaper linear regulator with higher voltage drop, let's say 2-2.5v.   C = 3A / [2 x 60 x (19v - 14v) ] =  0.005F = 5000uF so you could just go for just one 4700 uF capacitor.

This second version may seem better, but it's not all positive things. I'm trading capacitors (space on pcb and cost) for an increased input voltage on the linear regulator at low currents.

In the alternative, the input voltage will hover between 14v and 19v but at low currents (let's say the amplifier is at low volume), the capacitor will be able to keep the voltage towards the 19v value.
This means that the linear regulator will dissipate more power as heat ... for example, at 1A  we may have 19v IN, 12v OUT at 1A, so the regulator will dissipate about 7W.  At 3A, let's say the capacitor will manage to keep the input voltage at 15-16v, then we'll have  (16v - 12v) x 3A = 12W.
For a classic audio amplifier, you may not care because you already have a big heatsink for the audio amplifier chips (or mosfets/transistors).

[Neilm]

In addition to the above, if your transformer is for a SMPS that will depend on many things for instance operating frequency, voltage ratio, topology, driving circuit to name just a few.

[andersendr]

I have listed a few reputable transformer manufactures below.  Otherwise a search on Digikey or Mouser will result in transformers to.  You can filter by input and output voltage.

SOLA:
http://www.solahevidutysales.com/

Acme Electric:
http://www.acmetransformer.com/en/products