Transformers "reuse"

[patroclus]

I have several not working power suplies. Is there any way of identify what kind of transformer are there in, so I can desolder them and use them for making a new power supply??
I opened the power suplies, but the transformers have some numbers on them that give me no info... thanks

 

[Nigel Goodwin]

I have several not working power suplies. Is there any way of identify what kind of transformer are there in, so I can desolder them and use them for making a new power supply??
I opened the power suplies, but the transformers have some numbers on them that give me no info... thanks

Assuming they are conventional mains transformers, simply connect them to the mains and measure the output voltages. Working out the current or power isn't so easy, probably the easiest way to estimate it is to measure the size of the transformer, or weigh it, and compare to similar transformers in catalogues.

Generally any numbers on a transformer aren't going to be much help.

 

[Klaus]

Before you unsolder them identify the primary and the secondary leads.
Find out if it has a multi tapped primary winding and which wire is connected to the mains plug (via a fuse) if it has.

If the secondry winding has a much thicker wire than the primary winding you can assume it has a low voltage secondary. Connect it up and measure it, keeping in mind that this voltage will drop somewhat when you connect load to it.

 

[patroclus]

I've been examining one of them. It seems to have 4 pin input and 2 pin output? Is it possible? Maybe I could post a photo?

 

[ChrisP]

Sure -- transformers are offered in many different configurations, including dual-primary and dual-primary/dual-secondary. Here is one example of how the same dual-primary power transformer might be used in two different applications:

**broken link removed**

 

[patroclus]

Ah, yes, I understand.
So, when you get a transformer that says "220V to 12 V, 6VA", that means that has power of 6 watts?? So, as P = V x I, you get 6/12 = 0,5 Amps at 12V??

It seems like very little current.. specially because I've seen 2,8VA transformers (around 0,23amp). Bigger transformers, 24VA, cost more than $10. I need 1,5 amp at 12 VAC... I supose 9VAC won't let me get 15V DC, will it??
Thanks.

 

[ljcox]

6VA is the Apparent Power. So if the phase angle is zero, then it can deliver 6 Watt. You are correct, at 12 Volt, it can deliver 0.5 A.

9VAC if full wave rectified will give you about 12 VDC. For 15 VDC you need at least 15/Square root 2 = 11 VAC

Len

 

[patroclus]

So, for a 15V DC, 1.5Amp power supply, I would need 12VAC transformer, 18 VA, and a full bridge rectifier. Is that it?

 

[ChrisP]

Yes -- plus of course filters for smoothing the DC...

 

[ljcox]

So, for a 15V DC, 1.5Amp power supply, I would need 12VAC transformer, 18 VA, and a full bridge rectifier. Is that it?

That is true assuming that you only want an unregulated supply. Also note the point by ChrisP.

If you want to regulate the voltage to 15 Volt, you will need at least 18 Volt DC in order to provide sufficient "head room" for the regulator. See the data sheet of whatever regulator you buy.

Len

 

[patroclus]

Ok, I understand. Thank you

 

[patroclus]

I got a non working PC power supply that I don't need, and I opened it. I saw three(?) transformers inside. One is the biggest, and the other two are smaller... does anyone know if I can reuse any of them. I suposse they have high VA.

 

[Nigel Goodwin]

I got a non working PC power supply that I don't need, and I opened it. I saw three(?) transformers inside. One is the biggest, and the other two are smaller... does anyone know if I can reuse any of them. I suposse they have high VA.

They are switchmode transformers, not 50/60Hz ones, it's a totally different level of technology - not something to mess about with if you don't understand them.

 

[patroclus]

How can I diference which transformer is swithmode, or 50/60Hz??

 

[Nigel Goodwin]

How can I diference which transformer is swithmode, or 50/60Hz??

Switchmode transformers will be much smaller, lighter, and use a ferrite core. 50/60Hz mains transformers will be much larger, heavier, and use a core made of steel laminations.

They don't really look anything like each other. Also, if the transformer is in circuit, simply check the wiring - a 50/60Hz transformer will have mains going directly to the transformer. With a switchmode transformer the mains goes through a bridge rectifer, is smoothed by a large electrolytic, before connecting to only one terminal of the transformer, the other one goes to the switching device.

 

[patroclus]

Cool, I know how to diferenciate them now. Two of the transformers I have go through rectifier and capacitor before. I will avoid those.

 

[Nigel Goodwin]

Cool, I know how to diferenciate them now. Two of the transformers I have go through rectifier and capacitor before. I will avoid those.

If you don't you'll get a loud BANG!! when you plug it in :lol: