Author Topic: transformer secondary turns  (Read 1867 times)

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Offline m3vuvTopic starter

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transformer secondary turns
« on: December 18, 2019, 10:07:22 pm »
Hi all,im rewinding the secondarys on a microwave oven transformer,at the moment with just one turn secondary winding i get 0.968v,so im after 30v,if i divide 30 by 0.968 ,it works out at 20 point something turns,i notice when the output on a transfowmer is full wave rectified and smoothed the voltage increases,so i assume 29 turns will give way over 30v with those turns,how do i know/calculate how to reduce the turns to compensate to get 30v?,cheers M3vuv.
 

Offline Leopoldo

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Re: transformer secondary turns
« Reply #1 on: December 18, 2019, 11:00:46 pm »
I'm assuming what is not clear to you is the difference between RMS voltage and peak-to-peak voltage.

Let's dig in a bit of theory, then! ;)
When you probe in "AC mode" a sine wave (such as the output of a transformer) on your multimeter, what you are reading is the RMS voltage; in other words, this is the voltage that is comparable to a DC equivalent, in terms of power delivered to a resistive load (such as light bulbs or heating appliances). For example, 230V (or 115V, depending on where you live) is the RMS voltage of the power grid.

On the other hand, the peak-to-peak voltage is the voltage you literally would see between peaks on an oscilloscope, were you to probe the signal (don't do it on the power grid directly, though...). It's the voltage measured between peaks of the sinusoidal voltage wave, and it's also closely related to the voltage you get when you rectify it. The peak-to-peak voltage is twice as high as the same rectified voltage (assuming capacitance on the output and no load).

RMS and peak-to-peak voltages are related by this equivalence (assuming a perfect sine wave): Vptp = 2 * sqrt(2) * Vrms = 2 * 1.414 * Vrms

In summary:
Vptp = 2 * sqrt(2) * Vrms = 2 * 1.414 * Vrms
Vrectif = 0.5 * Vptp

Hence:
Vrectif = 1.414 * Vrms

I'll leave to you the calculations as an exercise, though you should expect about 20 turns for a DC output of 30V.
Finally, if the voltages don't quite match between theory and circuit, and are a couple of volts off, remember that bridge rectifiers have a forward voltage drop...
 

Offline andy3055

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Re: transformer secondary turns
« Reply #2 on: December 19, 2019, 12:53:10 am »
Hi all,im rewinding the secondarys on a microwave oven transformer,at the moment with just one turn secondary winding i get 0.968v,so im after 30v,if i divide 30 by 0.968 ,it works out at 20 point something turns,i notice when the output on a transfowmer is full wave rectified and smoothed the voltage increases,so i assume 29 turns will give way over 30v with those turns,how do i know/calculate how to reduce the turns to compensate to get 30v?,cheers M3vuv.

I believe you are after 30 Volts DC after rectification. If that is the case, you can use a regulator for your required output. For the regulator to work properly, you will anyway need a higher input than what you want as output and that number differs for each regulator (check the spec sheet).

In other words, check the specs for the regulator first and decide on the output voltage of the secondary. You can always leave taps while winding so that you can select the best point later on.
 

Offline m3vuvTopic starter

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Re: transformer secondary turns
« Reply #3 on: December 19, 2019, 08:51:28 am »
not beeing rude,but was that the question i asked?,no it wasnt,i already know what you explained!,please stick to the question as asked!,regards.
 

Offline GeorgeOfTheJungle

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Re: transformer secondary turns
« Reply #4 on: December 19, 2019, 09:26:36 am »
not beeing rude,but was that the question i asked?

Yes you're being rude. Solve this: n*0.968*sqrt(2) = 30+(2*Vfm)
« Last Edit: December 20, 2019, 06:41:37 pm by GeorgeOfTheJungle »
The further a society drifts from truth, the more it will hate those who speak it.
 
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Offline gnif

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Re: transformer secondary turns
« Reply #5 on: December 19, 2019, 09:42:08 am »
not beeing rude,but was that the question i asked?,no it wasnt,i already know what you explained!,please stick to the question as asked!,regards.

@m3vuv you were given the math required to answer your question, and the reason why it needs to have a higher voltage. In short, your question was directly answered.
 
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Offline gbaddeley

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Re: transformer secondary turns
« Reply #6 on: December 19, 2019, 12:25:10 pm »
I guess you are aware that MOTs are not designed for continuous use and may get quite hot after a few minutes, even with no load. YMMV
Glenn
 

Offline Zero999

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Re: transformer secondary turns
« Reply #7 on: December 19, 2019, 12:47:34 pm »
I've noticed that when people say things like "not being rude" it generally means they know they've being rude and need to justify it somehow. . .

The question wasn't even that clear.  Does the original poster want 30VDC, after the rectifier or, 30VAC before?

The transformer's voltage will drop and the ripple on the DC side will increase, when loaded. The unloaded secondary voltage required will depend on what voltage drop and ripple are acceptable.
 
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Offline m3vuvTopic starter

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Re: transformer secondary turns
« Reply #8 on: December 19, 2019, 04:47:28 pm »
i simply wanted to know,if i have say 30v output from the transformer unloaded unrectified,what would it be when smoothed and rectified unloaded,ie a percentage?,btw maths is my worst subject,cheers.
 

Offline IanB

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Re: transformer secondary turns
« Reply #9 on: December 19, 2019, 05:11:12 pm »
Hi all,im rewinding the secondarys on a microwave oven transformer,at the moment with just one turn secondary winding i get 0.968v,so im after 30v,if i divide 30 by 0.968 ,it works out at 20 point something turns...

I'm surprised nobody commented on this. If you divide 30 by 0.968 it does not work out at 20 point something. How could it be less than 30?

btw maths is my worst subject

Then you should really try to work on your maths skills  :)
 

Offline andy3055

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Re: transformer secondary turns
« Reply #10 on: December 19, 2019, 06:12:24 pm »
not beeing rude,but was that the question i asked?,no it wasnt,i already know what you explained!,please stick to the question as asked!,regards.

You seem to be fuming over nothing. If you had given more info as to if it is AC or Dc volts you are looking for, my comment would have been different.

You ask for advise but you respond with rudeness expecting the other person to treat you nice! Please tone down your language and you will get more help around here. As for me, this is the EOL. Have a nice day  :clap:
 

Offline Zero999

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Re: transformer secondary turns
« Reply #11 on: December 19, 2019, 06:25:14 pm »
i simply wanted to know,if i have say 30v output from the transformer unloaded unrectified,what would it be when smoothed and rectified unloaded,ie a percentage?,btw maths is my worst subject,cheers.
30√2 - 1.4 = 41V
Here's an explanation.
https://www.instructables.com/community/Rectifying-ac-increases-voltage/
« Last Edit: December 19, 2019, 11:18:25 pm by Zero999 »
 
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Offline m3vuvTopic starter

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Re: transformer secondary turns
« Reply #12 on: December 19, 2019, 08:02:25 pm »
Cheers zerro,thats exactly what i was looking for,cheers 73 m3vuv  Paul.
 

Offline Zero999

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Re: transformer secondary turns
« Reply #13 on: December 19, 2019, 11:19:17 pm »
Cheers zerro,thats exactly what i was looking for,cheers 73 m3vuv  Paul.
Good, but I'm disappointed no one spotted my error: I did √30, rather than 30√2. Post edited, before anyone notices. :palm:
 


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