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transformer secondary turns

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m3vuv:
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.

Leopoldo:
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...

andy3055:

--- Quote from: m3vuv 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.

--- End quote ---

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.

m3vuv:
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.

GeorgeOfTheJungle:

--- Quote from: m3vuv on December 19, 2019, 08:51:28 am ---not beeing rude,but was that the question i asked?

--- End quote ---

Yes you're being rude. Solve this: n*0.968*sqrt(2) = 30+(2*Vfm)

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