hmm too many leads on a transformer?

[daviddoria]

we made little "camera flash" circuits in my intro to electronics course, but what i learned from that is that you can take a 1.5v battery and make it into like 200v with this little tiny transformer. however, i was going to take the transformer and capacitor off the camera and make my own "ZAP" circuit, (zap = just discarging the cap), however, i noticed the transformer has 4 leads on one side, and three leads on the other. shouldn't there only be 4 leads total (2 on the primary and 2 on the secondary)?

thanks
david

 

[John Sorensen]

Transformers can have multiple coils on a single core, and multiple taps to a coil. Use your ohmmeter to figure out which ones are on the same coil, and maybe even determine the tap positions if the resistances are high enough.

j.

 

[daviddoria]

so is there a way to tell the (duno the name, the word that is used for how much it converts the voltage) with just a volt meter and an ohm meter? i can use the continuity checker to see which are the same coil, but then can i use any 2 coils to make it transform?

 

[Russlk]

All the coils interact thru the magnetic field, so you can use any two for the transformer. The 3 wires on one side may be a center tapped winding. The ohmmeter can confirm that. The 4 wires could be taps on one primary winding, or could be two seperate windings. Again, the ohmmeter will tell.

 

[daviddoria]

again, what am i looking for on the ohm meter?

 

[Jerran]

Your looking at the resistance in the ohm meter. On the three lead side measure the resistance between each line. Two should be about the same with one pair having the highest restance of all.

example
wire 1 to wire 2 three ohms, wire 1 to wire 3 seven ohms, wire 2 to wire 3 four ohms.

This would show the ratio of windings between any 2 points, as in above. 1 to 2 would be aproximately half the amount of windings between 1 and 3.

As for the four winding side a simple check for contenuity will tell you if its 1 coil with 3 taps (you will be able to measure resistance between any of the 4 leads), or if its two seperate coils(each pair will show some resistance but you will read infinite between the leads of 2 seperate coils).

 

[Jerran]

Your applications is probably going to need a step up transformer, Id imagine your using a source of 5-9 volts correct? If you are wanting more voltage on the secondary then your source you need to use a transformer configuration that has more windings on the secondary then the primary.

Transformer formulas:
Np = # turns on primary
Ns = # turns on secondary
Ep = voltage of primary
Es = voltage of secondary
Ip = current of primary
Is = current of secondary

Ep x Ns = Es x Np
Ep x Ip = Es x Is
Np x Ip = Ns x Is

 

[daviddoria]

thanks for all the info...

i found one with only 2 leads on each side. however, with a continuity checker, i see that

a--- -----c
0 0
0 0
b--- -----d

a and b are continuous... however c and d are not...

whats up with that??

 

[Jerran]

Sounds like one sides blown. A simple test would be to place a resistor across CD(100-1K should be good enough) then put a square waved DC current thru AB. You should be able to measure a voltage(or current, just dont try to measure current on a transformer without a load applied) across the resistor.

BTW can you read anything from a to c or a to d? If so it may be a autotransformer.

 

[daviddoria]

what do you mean "read anything". they are not continuous..
however, i'm relatively certain its not broken cause i have several and they are all like this...

however i am also confused. if i measure (see diagram)

http://bandtank.com/x.jpg

across the first coil with a voltmeter, i get 0v... but if i measure across the resistor i get 9v... (its a 9v source not 10 as it says in the diagram).

if i measure across the 2nd resistor, i get 0...

anyone?

 

[Jerran]

Is the voltage source pulsed?

For a transformer to work you need to have a colapsing magnetic field. The magnetic field cuts across the coil in the secondary and produces a current. With a constant DC current the coil is going to act like a regular conductor, having a much lower resistance then the 1K resistor it shouldnt show much(if any) voltage drop.

 

[daviddoria]

right right... but i'm saying i'm not even seeing continuity in one of the coils....and i would think it was just broken but i have several like this...

 

[gerty]

You said you were using a 'continuity checker' ..It could be that the winding resistance is too high for it to check..Typically a continuity tester is only capable of testing 100-200 ohms and your winding may be considerably higher ,depending on number of turns, wire size etc.....
I would definatly reccomend using a ohmmeter for such tests

 

[daviddoria]

ahhh excellent. indeed you were correct.

however, one coil says .3 ohms, while the other says 600.....
and this is 1.5v circuit, so then it is 1.5*600/.3 = 3000v. correct?
however its only a 330v capacitor, but it hasn't blown up yet....
did i do some math wrong? or maybe the measurement wasn't very accurate?

 

[jem]

Hi:

Let me take a wild guess as to the type of transformer you have. I believe you have one with two primary windings, and one center-tapped secondary winding. Typically, this type of "flash" circuit uses a single transistor Armstrong-type oscillator. The main primary winding is connected to the supply, and the collector of the transistor, and the other primary winding provides positive feedback to make the circuit oscillate. The secondary winding is center-tapped to get full wave rectification using two diodes instead of a bridge rectifier. The rectified output then charges the main flash capacitor through a current limiting resistor.

If that is the case, you should find continuity between the two pairs of the side with four windings, but not between the pairs themselves. On the side with the three connections, there should be continuity between all three wires. Generally the secondary is made of thinner wires (and more turns). Therefore, the DC resistance will be higher, but not in proportion to the turns ratio.

Jem