# Transformer questions

Discussion in 'General Electronics Chat' started by 12stephen12, Jan 4, 2009.

1. ### 12stephen12 Thread Starter New Member

Jan 4, 2009
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Hello...I just want to ask information about transformers because I want to make a power supply for my project..Do transformers have current rating???? What if my load is to high?? Will the transformer explode??? Can I make a regulated 12Vdc or 24V dc output from a 9V ac secondary transormer???

2. ### Audioguru Expert

Dec 20, 2007
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1,349
Of course a transformer has a current rating. If you draw more than the rating then the transformer will get hot and burn out or set your house on fire.
If your application allows the transformer to be overloaded then connect a suitably calculated fuse in series with its primary winding.

A 9V transformer has a peak voltage of 12.7V at its max current rating. A full-wave brige rectifier and filter capacitor will have an output voltage of 10.7V when the DC current is 0.7 times the transformer's AC current rating.

A fairly complicated circuit can convert 9VAC to 24VDC.

3. ### beenthere Retired Moderator

Apr 20, 2004
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You purchase a transformer based on the output voltage and current rating.

4. ### studiot AAC Fanatic!

Nov 9, 2007
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transformers are basically power devices.

Most transformers are specified by their power rating on load and the secondary voltage when providing that power.

Off load or with a lighter load the secondary voltage will be higher.

You obtain the curent rating by using the relationship power = voltage times current, ie you divide the power rating by the secondary voltage to obtain the secondary currrent rating.

Thus a 9 volt secondary, rated at 2.5 watts will output 2.5/9 amps or 320 milliamps.

Off load the secondary voltage may be as much as 11 - 12 volts.

You can obtain greater DC voltage at reduced current with some extra circuitry from a 9 volt secondary. You must keep within the constraints of the equation P = IV above. The simplest way would be to use a voltage doubler for 12 volts at half currrent or a voltage tripler for 24 volts at one third current.

However the cost of the extra components would likely outweigh the cost of an extra transformer.

5. ### 12stephen12 Thread Starter New Member

Jan 4, 2009
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I had a transformer which had two secondary output terminals 9Vac and 12V ac... Can I rectify them both "simultaenously" in order to have two different DC outputs???

6. ### leftyretro Active Member

Nov 25, 2008
394
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Yes, of course you can, if they are indeed separate windings. If it's a tapped winding then not as simple but possible using half wave rectifiers.

Lefty

7. ### 12stephen12 Thread Starter New Member

Jan 4, 2009
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Why is it that the fuse need to be placed on the primary winding??? Why not on the "secondary" winding???

8. ### 12stephen12 Thread Starter New Member

Jan 4, 2009
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I know that transformers have no electrical connection between the primary winding and secondary winding. So why place the fuse on the primary winding???? Anybody mind to enlighten me about this??

9. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
When you are dealing with line power, you want a fuse to be the first thing that the "hot" wire goes through. That way if there is a fault past the fuse, the fuse burns up instead of everything after the fuse.

You can also fuse the secondary winding(s) if you like. Whatever you do, make sure that the transformer body is grounded, so that if there is a fault, the fuse will blow right away.

10. ### 12stephen12 Thread Starter New Member

Jan 4, 2009
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If I am protecting the transformer from overloading, then I will put the fuse on the secondary winding. Not on the primary winding.. Am I correct????

11. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
You want a fuse on the primary to protect the entire project in case there is a serious fault with the transformer.

Primary windings can, and do, develop faults - like shorting to themselves, shorting to the transformer laminations, etc.

The fuse in the mains line protects the equipment from turning into a spectacular flaming pile of exploding debris.

Fusing the "hot" lead on the primary side is mandatory.

Fusing the secondary side is purely optional.

If you have a 120v to 12v transformer, and you want to limit the secondary current to 1A, you can use a 1/10A primary fuse. Both sides are then protected.

If you just had a 1A fuse on the secondary side, it would not protect the project if a fault in the primary developed.

12. ### Wendy Moderator

Mar 24, 2008
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I've designed with them all my life, but I'm going to ask a dumb question, just to keep in practice.

Take a transformer 12VAC @ 300ma. Is the 300ma max, and is the output voltage at or near zero volts, or can it provide 12VAC at 300ma as a max rating?

When I graduated from tech school my feeling was a linear curve, with 12V on one axis, and 300ma on the other, and a straight line between them (the first senario I proposed in other words).

13. ### eblc1388 AAC Fanatic!

Nov 28, 2008
1,543
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Think of transformer rating as the loading on a car, or the weight that can hung from a rope.

If it is "rated" for 300mA, then it means it can safely provide 300mA @12V forever.

That doesn't means it cannot provide 350mA @11.9V or even 400mA @11.5V, albeit for a short time.

However, there are consequences if one exceeds the rated rating of a transformer. The most noticeable effect is heating. Others common ones being large voltage drops and voltage distortion.

14. ### Wendy Moderator

Mar 24, 2008
21,838
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The reason I'm asking is I've noticed most transformers drop rather dramatically in voltage with a bit of loading. One of these days I'm going to have to try the experiment.

15. ### studiot AAC Fanatic!

Nov 9, 2007
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Here are some figures,from when I was evaluating a batch of power transformers back along.

This one showed a pretty regular (linear) voltage drop with secondary current. Most don't - I have plenty more measurements available.

I have chosen this one because it shows most of the characteristics we have been talking about.

It also has an electrostatic screen between primary and secondary, which should be earthed.

Note our mains voltage was 238 volts on that day.

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16. ### leftyretro Active Member

Nov 25, 2008
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A fault could be an internal transformer short. Fusing the primary input give the most protection for both the equipment and personnel.

Lefty