transformer secondary turns - Page 1
Forum section: electronics / beginners

author: m3vuv
author_rank: Super Contributor
timestamp: December 18, 2019, 10:07:22 pm
content: 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.

author: Leopoldo
author_rank: Contributor
timestamp: December 18, 2019, 11:00:46 pm
content: 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 theRMSvoltage 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 * VrmsIn summary:Vptp = 2 * sqrt(2) * Vrms = 2 * 1.414 * VrmsVrectif = 0.5 * VptpHence:Vrectif = 1.414 * VrmsI'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...

author: andy3055
author_rank: Super Contributor
timestamp: December 19, 2019, 12:53:10 am
content: Quote from: m3vuv on December 18, 2019, 10:07:22 pmI 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.

author: m3vuv
author_rank: Super Contributor
timestamp: December 19, 2019, 08:51:28 am
content: 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.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: December 19, 2019, 09:26:36 am
content: Quote from: m3vuv on December 19, 2019, 08:51:28 amYesyou're being rude. Solve this: n*0.968*sqrt(2) = 30+(2*Vfm)

author: gnif
author_rank: Unknown
timestamp: December 19, 2019, 09:42:08 am
content: Quote from: m3vuv on December 19, 2019, 08:51:28 am@m3vuvyou 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.

author: gbaddeley
author_rank: Regular Contributor
timestamp: December 19, 2019, 12:25:10 pm
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2019, 12:47:34 pm
content: 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.

author: m3vuv
author_rank: Super Contributor
timestamp: December 19, 2019, 04:47:28 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: December 19, 2019, 05:11:12 pm
content: Quote from: m3vuv on December 18, 2019, 10:07:22 pmI'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?Quote from: m3vuv on December 19, 2019, 04:47:28 pmThen you should really try to work on your maths skills

author: andy3055
author_rank: Super Contributor
timestamp: December 19, 2019, 06:12:24 pm
content: Quote from: m3vuv on December 19, 2019, 08:51:28 amYou 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

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2019, 06:25:14 pm
content: Quote from: m3vuv on December 19, 2019, 04:47:28 pm30√2 - 1.4 = 41VHere's an explanation.https://www.instructables.com/community/Rectifying-ac-increases-voltage/

author: m3vuv
author_rank: Super Contributor
timestamp: December 19, 2019, 08:02:25 pm
content: Cheers zerro,thats exactly what i was looking for,cheers 73 m3vuv Paul.

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2019, 11:19:17 pm
content: Quote from: m3vuv on December 19, 2019, 08:02:25 pmGood, but I'm disappointed no one spotted my error: I did √30, rather than 30√2. Post edited, before anyone notices.