# Transistor Type for Transformer

Discussion in 'General Electronics Chat' started by cjdelphi, Feb 7, 2010.

1. ### cjdelphi Thread Starter New Member

Mar 26, 2009
272
2
I was reading and it talks about how voltage ac is stepped up or down via an isolated transformer, correct me if i'm wrong this was a few weeks back so if 1vac went in 5ac went out, you'd have to put 1 coil on the primary and on the secondary windings, 5 coils 1*5=v

10 coils on primary, 10 coils on secondary, 1v in, 1v out.... so if the same number of coils is used, it merely passes on the magnetic field...

Let's suppose 1 coil on primary, 2 coils on secondary, 5v in, does this mean 10v comes out? and if so...

5vac
100 coils on primary, 100 coils on secondary = 5vac
100 coils on primary, 200 coils on secondary = 10vac
1 coil on primary, 1 coil on secondary = 5vac

now if that's right?, to double the voltage why spend money on wrapping
it 100 times when you could do 1 coil on primary and 2 coils on secondary
doubling your voltage??... Which leads me to the ultimate question, how do you know how many volts / amps come out of the other end...

So could I use a Transformer as some kind of Resistor or Regulator?, for example you wish to work with 12v but you don't ever want to exceed
10ma at 12v

instead of 1000 windings, simply have say 20 windings on the Primary and 20 on the Secondary, that should give the same voltage out (12v) however the current should be low right?...

Could that be used to power LED's and keep them regulated, you could then use a 12v source directly onto the LED it'll always be low current keeping the LED regulated until the voltage drops below 3volts or so.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
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Are you using the word "coils" for "turns"?

3. ### cjdelphi Thread Starter New Member

Mar 26, 2009
272
2
Um, Yes Sir, sorry Sir, i'll not do that again.......

4. ### MikeML AAC Fanatic!

Oct 2, 2009
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1,073
When building a power transformer, the number of turns on the primary is dictated first by having the primary winding present the correct impedance to the AC line. This is what limits the current in the primary to a reasonable (and efficient) value.

Once the number of turns for the primary is determined, the turns ratio ns/np is what dictates the amount of step-up or step-down. So the number of turns on the secondary is dictated by the number of turns on the primary (already determined as above), and the desired secondary voltage.

Unless the transformer is overloaded, the secondary voltage is the primary voltage * ns/np.

Another thing to consider is that as voltage is stepped up, available current goes down, and vice versa. To a first approximation (ignoring losses), Pin = Pout, so Vp*Ip = Vs*Is. If you step down voltage from 120 to 12 (1/10) , and the secondary load draws 6A, the input current should be close to 0.6A (1/10)

5. ### Wendy Moderator

Mar 24, 2008
21,500
2,961
To expand on what was already said, there is a minimum number of turns you need for a transformer to be efficient, then you match your output turns to match the input numbers.

6. ### iulian28ti Member

Dec 4, 2009
42
0
Yeah, and there is also a relationship between the frequency and inductance.
The more turns, the bigger the inductance, lower frequencies possible.

7. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
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Where does the transistor come in? I'm puzzled

8. ### Wendy Moderator

Mar 24, 2008
21,500
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I suspect it is coming. Wait for it...

9. ### cjdelphi Thread Starter New Member

Mar 26, 2009
272
2
Well, i thought that was obvious a little 20c transistor is not going to be able to draw enough current now is it, so a more expensive one to drive a decent transformer..

Anyway i pulled out a flyback transformer from an old monitor, i have a similarly specced transistor to the one on the site, i might have a go....