How does the n turns ratio works in Multisim 10?

[Arnie_8446]

Hello,

I removed my previous version of Multisim 9 Student Edition and installed Multisim 10 Student Edition.  I am having issues getting the transformer TS_MISC_VIRTUAL turns ratio to work.  In Multisim 9 Student Edition, I used to enter the transformer ratio in the "primary-to-secondary turns ratio" window and it worked fine.  Now in Multisim 10, the info help states "The relationship is given by V1/V2 = n, where n is the ratio of the primary turns to the secondary turns. The parameter n can be adjusted by editing the transformer's model."  I go into the "edit model" and changed the default value of n = 5 to 2 and it does not affect the secondary output voltage of a center-tapped half-wave rectifier.  I have the AC voltage-source set to 200 Vpk and I keep measuring a secondary output voltage of 20 Vpk.  In fact, if we do the math V1/V2 = n, 200 Vpk/20 Vpk = 10 not 5 or 2.  It does not matter what n value in the "edit model" I change it to, it shows the same output regardless of the n value.  So, is this the expected behavior?  Is there any way to change the turn’s ratio of the transformer?  Is this a bug with the software?  If is not, what is the appropriate procedure to change the turns ratio of the transformer that will work?

Thanks!

[lacy]

The reason this won't work is because the line in the model file that you editted was a comment line. With this transformer in order to alter the turns ratio you have to modify other variables. I believe these variable are E1, E2, F1, and F2. I don't remenber exactly how to adjust these at the moment (I would have to look it up). Suffice it to say that editiing this transformer in this manner is not the easiest way to do this.

A better option for you would be to goto BASIC>BASIC_VIRTUAL and select the TS_VIRTUAL transformer from there. When you click on it it will come up with a dialog box that will allow you to adjust all the parameters to your liking.

I hope this helps some.

[lacy]

One other thing. When thinking about the center tapped transformers in Multisim it can get quite confusing.  The voltage across the secondary divides evenly between top and bottom. To figure the turns ratio the way Multisim does it you have to do your formula 200/20=10 and since it divides equally between the 2 halves of the secondary you have to divide this by 2 to get a turns ratio of 5.

I know this is really hard to grasp because it took me a long time to understand how the center taps worked in Multisim. Always remember to figure your turns ratio first and then divide this by 2 and it will get you close to what you need across the secondary. You may have to tweak it up or down from this some to get it exactly where you want it, but it will get you close.

I hope I explained this to where you will understand it and hope it helps.

[Arnie_8446]

Hi Lacy,

I followed your recommendations and now I am obtaining the expected results from the virtual center-tapped transformer.

Thank you very much!
Arnie

[lacy]

You are correct in you calculations for the transformer, but you are using a peak voltage and I believe this is why it is throwing the calculation off by a factor of 1.4. If you use the AC Power Source under the Sources section then the turns ratio formula is the way it should be. I will illustrate this with the following formula: 200V/1.4 (Peak-Peak)=142.8 Vrms  142.8 Vrms/20V=7.14. This is why I had to divide by a factor of approx 1.4 to get the output voltage to where it was needed. So the turns ratio is based upon the RMS voltage and not the Peak Voltage and this is the way it should be.

I did some more test to find out why I needed to do this division scheme because I knew that something didin't sound right about it after I made the post.

I hope I haven't confused you in any way and if you have any questions about this please let me know.

[Arnie_8446]

Hello Lacy,

Right, I agree that RMS should be used.  However, Vpk will also provide the same results as the RMS.  We just convert RMS to Vpk.  On a site note, I am assuming your logic behind of the 1.4 factor is to take into consideration the diode voltage drop.  As you know, usually that will be part of the V(out) = V(sec)/2 – 0.7 Volts formula.  But I think I understand how that will affect the calculation of the center-tapped transformer n turns in Multisim and the key point of n/2 for  center-tapped transformer.

Regards,

Arnie

[lacy]

This is kinda hard to explain but I will try. There are two explainations for what I am seeing and I will attempt to outline them as best I can. I will also post a circuit to show this so maybe that will help in the understanding of my explainations.

1). In your original formulat 200Vpk/20V=10 is correct. The problem is that it gives only 14V output on the secondary. This is the RMS value.of the peak voltage of 20V. If you were wanting exactly 20V on the secondary then you would have to calculate the seconday using the peak of the desired output. To do that you would have 20V*1.4=28. Plug that into the formula to give 200Vpk/28Vpk=7.14 turns ratio. In other words if your using a Pk input you have to calculate both sides with the peak value to arive at the desired output..

2) Since Multisim is using RMS values in its transformer calculations to get the turns ration you could use 200Vpk/1.4=142.8. Plug that into the formula gives 142.8/20V=7.14. The same as above but calcualted using the RMS value on the primary.

It appears based upon this that Multisim is using the RMS values to calcuate the turns ratio and by using a Pk voltage it throws this off by a factor of 1.4 when measuring the secondary. To arrive at the desired output you are going to have to use one of the above procedures to get the turns ratio correct for a peak input.

You can take a look at the circuits that I posted an you can see clearly what I am referring to.

It is complicated to explain correctly. I can see it in my head but can't really put it into words.

I hope this helps.

[lacy]

I am going to post an addition circuit. This time I replaced the AC Pk Voltage Source with the AC Power Source which is in RMS. From this you can see that the formula 200V/20V=10 actually works out correctly and the output on the secondary is what you would expect from a 10:1 transformer.

I hope these explanations and circuits help you understand the way Multisim is doing its tranformers.

[Arnie_8446]

Hey Lacy,

Now I see what you're doing, which actually makes sense and it is simpler than the way I am doing.  I was actually looking for a Vsec output waveform of 20 Vpk or 40 Vpk-to-pk.  So, I am dividing  169.68 Vpk (120 Vrms)  by 20 Vpk =  8.484, and then divide this result by 2.  That provided me with a setting of 4.242 for the center tapped transformer with a Vsec output of 20 Vpk.  Attached is the test circuit, thus you can see what I was trying to do with the use of the scope.  At this point, I was not too concerned with the rms or average reading of a DMM.  I was just trying to see if I could get the correct Vsec sinewave amplitude that I was looking from a center-tapped full-wave rectifier.   However, you've definitely stirred me in the right direction to figure out how to accomplish this with Multisim.

Thanks,
Arnie

[lacy]

O.K. we have both learned soemthing here and that is always a good thing. I was thinking that you wanted the output at the trasformer secondary to read exactly 20V, but since you just interested in the peak voltage then I believe you have accomplished it and the turns ratio calculations are right for your particular application.

Congratulation on a job well done.