# Finding turns ratio of coil

Discussion in 'General Electronics Chat' started by vandaycalta, Mar 29, 2016.

1. ### vandaycalta Thread Starter Member

Mar 22, 2016
53
4
Hello all and thanks for reading my latest posting.

I want to find the turns ratio of a coil which has primary and secondary windings.
Can someone please describe to me how I can do this using Oscilloscope, function generator, and I have 2 multimeters available.
I know I can do this by comparing the AC voltage in the primary to the AC voltage in the secondary but do I need to think about the res freq for doing this?
What voltage should I use? I was thinking of 1 volt peak AC on the primary as I know this is a step up coil to make it easier.
What freq should I use?
I notice that the 1 volt AC on the oscilloscope isnt even close to 1 volt AC I measure on my multimeters, so which one should I use to set my voltage, the funct. gen. or the multimeter?

I do not have a schematic for this as this is just a question I have and didnt think one would benefit the question any.

Thanks again for all your help.

Val.

2. ### crutschow Expert

Mar 14, 2008
13,530
3,393
What "1 volt" are your measuring on your oscilloscope? You do realize a 1V sinewave on a meter corresponds to 2.8Vpp on an oscilloscope. Regardless, just use the same meter to measure the input and output voltages, and the absolute accuracy of the meter won't affect your measurement.

The type of coil will determine the frequency you use for the measurement. What type is it?

3. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,218
Reverse engineering a transformer takes a bit of trial and error at first. I normally will get a quick estimate of the turns ratio and load the secondary with the correct resistance, relative to the output impedance of the signal generator. Then I will change the frequency to get a idea of the frequency range that the transformer works well at. Once I get a estimate of the frequency boundaries I go to the center and refine the turns ratio measurement. Frequency first, everything else second.

4. ### vandaycalta Thread Starter Member

Mar 22, 2016
53
4

I was not aware that a 1v Sinewave on a meter was 2.8v on a oscilloscope. Thanks for that one!
I was just figuring that a 1volt peak(not p-p) sinewave at 1k frequency was exactally that as measured at the probe tip....that shows my logic and/ vs. my lack of knowledge.
The type of coil is an ignition coil e-core used with a HEI(high energy ignition). The primary resistance is .4 ohms and the secondary is 8.5k ohms.

Val

5. ### vandaycalta Thread Starter Member

Mar 22, 2016
53
4
Am I correct to say you are looking for the resonant freq?

6. ### Lestraveled Well-Known Member

May 19, 2014
1,957
1,218
Some transformers are resonate a one frequency, such as IF transformers. Most other transformers will have a frequency range that is "flat", or it will exhibit the same parameters over that range.

7. ### vandaycalta Thread Starter Member

Mar 22, 2016
53
4
Yes, am I correct that the voltage will be the same at the resonant frequency?
Ive reread this a few times and it makes real sense. Thank you Lestraveled for you kind help in helping me understand this.