Transformers with a dozen pins - what's the method to the madness?

[z.s.tar.gz]

I've been scrapping boards left and right looking for a transformer with 4 pins because I know off the top of my head what those pins go to.
The problem is that all the ones I find have at least 8 or 9 pins and my el-cheapo mulimeter doesn't help with finding what goes where.

1. Why do all these transformers have so many pins?
2. How can I find what is the primary and what is the secondary?
3. Is there usually more than one coil or more than one tap?

I just need to step up voltage for my coilgun.

[lrmall01]

My guess is that you are finding transformers which are used in switch mode power supplies.  They typically have more than a primary and secondary winding.  Depending upon the algorithm they can have multiple windings used for feedback loops.

[corrado33]

They could also have center taps, which does exactly as it sounds.  Taps the coil in the middle to get a different value than you'd get out of the full thing.

Ok that made sense in my mind.  Just google it.   

[z.s.tar.gz]

Is there any way to figure out how it's set up without buying any fancy expensive tools?
I tried with two multimeters but I only get 0 or not connected when trying to figure it out.

[lrmall01]

Do you have the range set on your multimeter really low?  Basically I would say the 0 reading is indicating the two pins are connecting.  The number of Ohms in a transformer coil will be small, or even less than 1.

[corrado33]

Is there any way to figure out how it's set up without buying any fancy expensive tools?
I tried with two multimeters but I only get 0 or not connected when trying to figure it out.

I'm just speculating here, but of course they're going to be connected or not connected.  Think about how it's designed.  It has two things wrapped with wire.  So, in the most simple transformer would have 4 pins.  2 pins for wire in and out for one side, and same for the other side.  

What I would do, is separate the wires into connected and not connected.  Then hook up a voltage source (however transformers are usually hooked up) to two of the wires that are connected  (how does this not short the battery, I dunno).  Then check the voltage on all of the opposing wires.  The voltage might be higher or lower, but you'll be able to figure out the number of turns.  Then you can move the wires around on the voltage source side and try it again, and you'll be able to figure out the number of turns on that side.

Does that make sense?  With that said... I've never hooked up a transformer before.    So I might be telling you something that's not good at all.  But I can't think of any other pins coming out of a transformer other than center taps.

So... I'd wait till someone confirms my idea  

[rbtying]

You'll need an AC current source for the coil, not DC. You can probably use a function generator with a current limited power supply, to avoid burning something out.

[Soeren]

Hi,

1. Why do all these transformers have so many pins?

Coil formers and their mounting parts are often made to cater for all eventualities, so several pins may not be used in a given transformer design. Sometimes transformers need several windings, ether to give several independent outputs, or to work together.in the same closed magnetics field.
Eg. power supply transformers for valve amps used to have a ~6.3V winding together with the 100+V winding(s), as this was always needed for the heaters.

2. How can I find what is the primary and what is the secondary?

If the transformers you have in mind are mains transformers with a fairly low secondary voltage (say 24V or less), it will be easy to measure with even the cheapest DVM - 200 Ohm range - lowest resistance pair will be the secondary and there will be no connection between primary and secondary.

Sometimes they're actually marked, or the PCB they were on were marked or could be traced to the mains input.

The wire gauge if can be seen will reveal it as well - the mains side is thin wire.
Some mains transformers will have several taps on the primary, to be used for 110V, 120V, 210V, 220V, 230V or something like that, often with a switch to set the voltage for the country the product goes to.

3. Is there usually more than one coil or more than one tap?

Yes.

I just need to step up voltage for my coilgun.

Roll your own if you cannot figure one out.

Some transformers (mainly those found in SMPSU's) will come apart if you boil them for around 15..20 minutes and can be pulled apart when the epoxy is softened (don't use the pot for food after cooking epoxy though). Use heavy gloves to avoid burns.
In some cores the primary and secondary are on different sections and the one with thin wire should be spared while the other is cut and rewound (few windings for the low voltage side).

If you have a function generator and a 'scope, it's easy to tell the wires from each other - lacking the 'scope, a DVM with even a crude AC range can be used as well.

[Soeren]

Hi,

You'll need an AC current source for the coil, not DC. You can probably use a function generator with a current limited power supply, to avoid burning something out.

While a function generator makes it easier to determine the relative inductance of each coil, no transformer have ever been harmed by an Ohm-meter.

[z.s.tar.gz]

I have an el-cheapo autoranging multimeter but I've tried it with an autoranging fluke as well to the same results.
Is there just something wrong with using an autoranging one?

[Soeren]

Hi,

I have an el-cheapo autoranging multimeter but I've tried it with an autoranging fluke as well to the same results.
Is there just something wrong with using an autoranging one?

Not unless you hit the point where it flutters back and forth range-shifting.

Personally, I'm not too crazy about rangeshifting and disable it most of the time on the ones that 's got it, as I usually know what I'm gonna measure in range-terms and autorange means that you have to spend more time reading the display.

Some transformers will have resistances lower than your meter leads, in which case you have to short the probes and zero the meter (either on a dial /button or mentally), to get a valid reading.

If you have access to an ESR-meter, it's even better, as they're made to measure lower resistance values than ordinary Ohm-meters.

A 'scope and a simple oscillator is still another option (and some 'scopes have "component testers" that can be used too).