Calculating AC full wave rectified voltage - Page 1
Forum section: electronics / beginners

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 01:30:23 pm
content: A while back I bought two brand new transformers off of someone that accept 115v/230v dual primary and a center tapped secondary winding rated at 115v 56VA or obviously half that using the center tap.I thought as a learning experience, I would try to build a linear PSU out of one of them. (maybe a switching with the second one) I figured a LM317-HV regulator would work since it's rated at up to 60v, but I now believe I was wrong. I don't have a cap right now that is capable of high enough voltage to fully test the rectified voltage, but I tested at 20v AC in and I got 26.8v DC out which is over 20% increase in voltage. That suggest my full wave rectified voltage will be closer to 70v than below 60v.I Googled around, and I don't see a clean or consistent description on how to calculate what a full wave rectified voltage will be from a secondary winding. I want this so in case I decided to buy another transformer, I can get one that I will have a good idea of what the full rectified output voltage will be so I can properly plan the circuit components.The second question is: Is there away to even somewhat efficiently lower the DC voltage to a usable state before it reaches the voltage regulator of the transformer it putting out a higher voltage like this one?Thanks,Dave

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: July 01, 2020, 01:34:53 pm
content: Nominal output voltage multiplied by square root of 2 (1,44...) minus two diode drops if you are using a bridge rectifier.So, in your case:115V / 2 = 57,5V57,5 * 1,44 = 82,2 Volts82,2 Volts -1,2V = 81 Volts Peak.Edit: Recommended reading:https://www.electronicdesign.com/power-management/article/21801974/whats-all-this-powersupply-design-stuff-anyway

author: greenpossum
author_rank: Frequent Contributor
timestamp: July 01, 2020, 01:40:43 pm
content: The reason for sqrt(2) is because that's the ratio between the RMS voltage (what is normally stated for AC) and the peak of the wave. Under no load conditions the capacitor will charge up to sqrt(2) * Vrms - diode drop voltage as BreakingOhmsLaw has explained. As more current is drawn, the capacitor voltage will droop between peaks.

author: tunk
author_rank: Super Contributor
timestamp: July 01, 2020, 01:44:30 pm
content: Also remember that the stated secondary voltage is at full load;at lower loads (or no load) the voltage will be higher."center tapped secondary winding rated at 115v 56VA or obviouslyhalf that using the center tap.""but I tested at 20v AC"These two do not add up.

author: bdunham7
author_rank: Super Contributor
timestamp: July 01, 2020, 02:09:19 pm
content: There is no efficient and convenient way to drop the voltage as you want. A switch-mode buck converter is efficient but would be much added complexity. A dropping resistor only works for a constant load and is horribly inefficent.What you can do, if you already have all the parts on hand, is simply wire your primary as if it were going to be 230VAC but run it on 115VAC. Then use half of your bridge in a center-tapped full-wave configuration with a capacitor rated 63VDC or better and you'll have about 40VDC peak. This will drop quite rapidly under load at first, so your power supply will be limited to 0-32VDC or so. You can put a negative side on it as well this way if you want.https://www.daenotes.com/electronics/devices-circuits/center-tapped-full-wave-rectifier

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: July 01, 2020, 02:15:36 pm
content: Quote from: greenpossum on July 01, 2020, 01:40:43 pmBut make sure your capacitors are rated for well above that theoretical peak voltage, because thats whats going to be the voltage without load. Also, keep i mind that in most countries, the mains voltage is allowed a tolerance of +-10%, so you *could * end up with ~90V. You should add a good safety margin to that when selecting a filter cap.Also, keep in mind, that a 56VA transfomer is not goig to give more than 20-25 VA continuous power output.Lastly, do not make that cap too large. A good rule of thumb is 1000uF per amp of output power. Using more is going to fry your bridge rectifier, because it will only charge too large a cap when voltage is at peak and the inrush current can exceed the rectifiers ratings.

author: ratio
author_rank: Regular Contributor
timestamp: July 01, 2020, 03:22:47 pm
content: This might be of interest:http://www.hammondmfg.com/pdf/5c007.pdf

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 04:01:07 pm
content: Quote from: tunk on July 01, 2020, 01:44:30 pmI suppose that didn't come out clear.I was just saying that since I didn't have a capacitor rated for a high enough voltage. (all I had was a 35v 2200uF cap) I used my variac to only supply the transformer with 20vac so I could see approximately what the output rectified / filtered voltage would be ratio wise. So I could figure out approximately how much rectified / filtered vdc I would get with the full 117vac applied to the transformer.The provided RMS * 1.44 calculation is what I was really looking for.

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 04:01:47 pm
content: Quote from: ratio on July 01, 2020, 03:22:47 pmThanks, I archived a copy of that for future use.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2020, 04:01:54 pm
content: Besides the Hammond app note, I recommend downloading this freeware from Duncan Amplification:http://www.duncanamps.com/psud2/download.htmlNote that the software requires specification of the transformer by no-load voltage and equivalent secondary resistance, which can be calculated from the manufacturer's specified loaded voltage at spec current if you know the no-load voltage, or can be estimated if you have the efficiency (there is now a window in the software to do this calculation). Also, center-tap full wave transformers are specified by voltage each side of CT.The software will calculate output voltage, output ripple voltage, capacitor ripple current, transformer current, PRV, and other important parameters.The load can be specified as a resistance, current, or stepped current (to look for ringing in choke filters).

author: greenpossum
author_rank: Frequent Contributor
timestamp: July 01, 2020, 04:04:15 pm
content: >The provided RMS * 1.44 calculation is what I was really looking for.Actually it's closer to 1.414. Never trust a remembered number when it only takes a moment to check, in this case with a calculator.

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 04:28:48 pm
content: Quote from: TimFox on July 01, 2020, 04:01:54 pmThank you. I will take a look at this applications.Quote from: TimFox on July 01, 2020, 04:01:54 pmJust so I'm clear. When you have a center tapped transformer that says it's secondary center tapped winding produce 28v at say 56va. You are saying that both taps produce this 28v and 56va and that if I wired without the center tap I would get twice that voltage? I was thinking the spec was the full secondary winding, not each side. What doesn't make sense is if the center tap isn't in the middle which I've heard is common with transformers.

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 05:02:28 pm
content: Quote from: BreakingOhmsLaw on July 01, 2020, 02:15:36 pmI had been googling to figure out how to calculate how large of a capacitor would be required and I read about the negatives of having a cap that is too large. I found thisSmoothing Capacitor Calculator, and I may be using it incorrectly. I set it to output the capacitance required. I put it 30v and 8.3ms charge time.Based on that pages explination, the 8.3ms charge time was calculated ashalf a wave length, since we are taking about AC ripple: 0.51/60th as I'm in the US with 60hz AC0.5 / 1/60th = 0.0083 seconds or 8.3ms.It calculated the required capacitance as 276.67uF.I'm unclear how it determined the ripple voltage. It doesn't explain it.Obviously, I could be using this incorrectly, but it is showing approximately 30% of what you suggested. Is there something I'm missing / doing wrong, or the 1000uF per amp is just a wide rule of thumb?Also, I'm unclear how it calculates the ripple voltage to make this calculation. For the capacitance given, I'm just relying on what the form suggested the answer was.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2020, 05:15:20 pm
content: My remark about the voltage spec for center-tapped transformers refers to how you specify the voltage in the PSUD2 application.Manufacturers usually specify total voltage with center tap, but sometimes specify a 48 V CT as 24-0-24 V.Marketing always chooses the larger number, when given a choice. PSUD2 uses the full voltage (of course) with a 4-diode bridge, but the single-side voltage for a 2-diode FW CT rectifier.

author: floobydust
author_rank: Super Contributor
timestamp: July 01, 2020, 07:36:33 pm
content: That calculator is useless, it doesn't work with ripple voltage. The minimum voltage that the filter capacitor dips down to between cycles, kinda important. The math behind it (ripple voltage) is complicated and a theoretical answer- because you don't have an ideal, perfect transformer or diodes. This makes it not worth trying to calculate beyond the "1,000uF/amp" guideline.You must take in account high and low mains as it's not perfect 115VAC all the time. With a 50VA part, I find they have about 15% regulation, so at no load expect the raw DC to jump up that much, and with high line it can be too much for some voltage regulator IC's. Note also theLM317HVgives much less output current at high vin-vout above around 20V. You might consider adding a tap switch.A full-wave bridge is the most efficient utilization of a transformer. A full-wave center-tapped rectifier is only using one half a secondary winding at a time. Although only one diode verses two conducting.

author: TimFox
author_rank: Super Contributor
timestamp: July 01, 2020, 07:44:14 pm
content: That's why I like the PSUD2 simulator. For example, with a single filter capacitor, in the output row for V (C) it gives max, min, difference (i.e., peak-to-peak value), mean, and rms values for the voltage across the capacitor.Yes, you must be careful that the minimum value is sufficient to keep the regulator from dropping out.You can also watch the voltage build up, or monitor it after a sensible delay for asymptotic results, by changing the simulation time parameters.High and low mains, etc. is done with multiple simulations with different values of no-load secondary voltage (the simulation ignores the primary).

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 01, 2020, 09:08:53 pm
content: Quote from: floobydust on July 01, 2020, 07:36:33 pmThanks for the clarification of the 1000uF per amp.Adding a tap switch? I'm guessing that is for switching the taps on the secondary to get different voltage values? I thought about that, but being my first PSU I didn't want to add that much extra complexity to my first build. I will poke around and read up on it and see if it's something i can add very easily. I was not aware of that concerning the LM317-HV. I will see what I can figure out and how to proceed.As for the rectifier; to keep this first build simple. I did not intend to use the center tap. I'm using aGBU-1010 full wave bridge rectifierthat I have on hand.

author: floobydust
author_rank: Super Contributor
timestamp: July 01, 2020, 09:26:02 pm
content: Your transformer is 115V/230V dual primary and 115VCT 56VA secondary? It's just too high voltage for a bench supply IMHO. I think it's better used as an isolation transformer.LM317 works well with a 12VAC to 25VAC transformer. I found around 18VAC is best to get the higher output current before the IC limits SOA.