Full wave and bridge rectifiers - Page 1
Forum section: electronics / beginners

author: GiMS_Pang
author_rank: Newbie
timestamp: May 01, 2022, 01:47:32 pm
content: Hello. I'm self taught, so I have plenty of gaps in my knowledge. I am trying to understand what type of transformer is needed for certain linear power supplies. In the attached picture, there are two supplies, one using a bridge rectifier and the other a full wave rectifier. Suppose the transformers are identical and they can provide, on the secondary, 24V 1VA. As far as I understand it, on the top circuit, T1 would supply the full 24V and 1VA. In the second circuit, the voltage would be half (before diodes' voltage drops). But what about current? Would T2 provide 12V 1A? 2A? 0.5A? Or what?Unfortunately, this is a very basic circuit and any info I could find on search engines, YouTube or specialistic websites only covers the basics (half voltage on the second circuit, excluding diodes' voltage drop). Info that requires deeper knowledge is reserved to more technical articles, which won't cover such a basic circuit, so I came up empty handed. Anyone can, please, help me out? I can't simulate the circuit (I have yet to find an open source simulation software that works on my Linux system) and I can't build and test it (no equipment, no components, little expertise and, above all, very poor manual skills, meaning if I solder it, there would be all pins on all components connected together, if the solder actually connects properly).It's very frustrating for me, because when I was a child (many decades ago) through simulations in my head, elementary calculations and much more courage than I have now, I successfully built tons of linear power suppliers, but they all used the bridge like in the top circuit. I have recently come back to study electronics, but my knowledge is very patchy and not very extensive.Thank you in advance for your help.PS: I got many more of such questions, but this is the most basic one. More to come at a later stage.

author: TimFox
author_rank: Super Contributor
timestamp: May 01, 2022, 02:18:20 pm
content: To simulate traditional power supply rectifiers, I recommend PSUD2 freeware from Duncan Labshttps://www.duncanamps.com/psud2/Note that the transformers are modeled as an open-circuit secondary voltage in series with an equivalent secondary resistance, while transformer data sheets normally specify output voltage at a specified load current (which, of course, is lower than the open-circuit voltage).The Spice-based simulation will give you output voltage ripple, rectifier current, transformer current, capacitor ripple currents, and initial start-up.The load can be specified as a resistance, a constant current, or a stepped current.

author: GiMS_Pang
author_rank: Newbie
timestamp: May 01, 2022, 02:22:35 pm
content: Thanks for your suggestions. It sounds like the software is only available for Mac or Win and not open source, so I can't run it in my Linux system.Any idea about the behaviour of the two different topologies of circuit? Do you know how the full wave rectifier compares to the bridge rectifier as far as current capabilities are concerned?If I could, I'd just build the circuit and see for myself.Thank you.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 01, 2022, 02:24:22 pm
content: Center taped transformers are half the full end to end voltage to center tap from each end,Consider the tap in this case to be the return or ground,Phases are opposite on each full end to center.So you only need to rectify the positive from each end to make a positive wave. Think of it as inverting the negative part of the incoming sine and adding it to the positive part of the sine.This sort of arrangement was common before solid state diodes were much cheaper than tubes. You only needed one dual diode tube for the PS. Besides the power supplies were in hundred of volts and would require high volt diodes. These diodes do and did exist, but apparently tubes were more practical.At present center tapped transformers are more expensive and less common.

author: GiMS_Pang
author_rank: Newbie
timestamp: May 01, 2022, 02:27:46 pm
content: Quote from: Wallace Gasiewicz on May 01, 2022, 02:24:22 pmThanks. From what you wrote, I'm still not sure if using the centre tap as return would give me twice the current (two half windings in sort-of-parallel configuration). Can you confirm either way? I'm only after that specific bit of information, as I can't move on without it. Thanks.

author: bdunham7
author_rank: Super Contributor
timestamp: May 01, 2022, 02:28:05 pm
content: The bottom circuit will indeed have about half the voltage, but while the current output will be more, it won't be double. So perhaps 1.5 amps?There is no simple rule for figuring this out because there are several factors that go into this "RMS to average" ratio in linear power supplies. So any simple explanations are going to be just approximations. In the case of the lower circuit, each half of the transformer is conducting for half a cycle, so you can run more current through it for that half cycle without overheating it. However, since P = I2R, you can't run double the current for half the time, because double the current would be 4X the power for half the time and that means 2X the power. If you did a bit of math you'd figure out that if you drew the square root of 2 amps or 1.414 A, then the power during each cycle would be 2X, and 2X the power half the time is 1X, so that would generate the same heat as the top circuit.If the output were a simple resistor, no capacitor, this would be pretty close to the correct answer. The capacitor filter complicates things a bit--with a capacitor filter the RMS AC current in the transformer will be higher than the DC average current even in the top circuit. So when you build a typical linear PSU, you have to use a transformer with a larger current rating then your desired DC output.

author: GiMS_Pang
author_rank: Newbie
timestamp: May 01, 2022, 02:30:03 pm
content: Quote from: bdunham7 on May 01, 2022, 02:28:05 pmThis makes sense. Thank you.

author: themadhippy
author_rank: Super Contributor
timestamp: May 01, 2022, 02:31:12 pm
content: Quote12v at 1VA gets you around 0.084A

author: bdunham7
author_rank: Super Contributor
timestamp: May 01, 2022, 02:32:20 pm
content: Quote from: GiMS_Pang on May 01, 2022, 02:22:35 pmYou need to do something about that. Unfortunately there are many useful Windows-only programs. Does your Linux flavor not support some version of Wine and PlayOnLinux?

author: GiMS_Pang
author_rank: Newbie
timestamp: May 01, 2022, 02:38:43 pm
content: Quote from: bdunham7 on May 01, 2022, 02:32:20 pmI am spending very little time for electronics, as I know too little about it. I wish I could spend more time, as I like it a lot. My system needs to be as stable as possible and emulated win software and/or the emulator themselves don't help much in that field. I am also only using open source software, so what they actually do is known and I could check it myself, if really needed. I am trying to learn Eeshema, which is supposed to have a simulation tool based on NgSpice. If and when I figure out how it works, I may have a useful program for my system. Thank you.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 01, 2022, 02:46:33 pm
content: First, you're mixing up A and VA.So I'll assume your transformer is:* 24 V (This is defined by input voltage and number of turns for the windings)* 1A (This is determined by wire thickness and ohmic heating).* 24VA (Determined by the amount of iron in your transformer, Mains frequency)In the top example, capacitor C1 will be charged to (approx) the peak of the 24Vac, minus two diode drops, which is (24*1.41) - (2*0.6) = 32.6VIf R1 was a 32 Ohm resistor, it would have 1A of current, but dissipate 32Watt, and the transformer can only deliver 24VA and it will be overloaded. (Even worse, because the diodes also dissipate some power).The current through the load should stay below approx 1/1.41 = 707mA.In the second example, one diode is always blocking, and there is only one diode in series with the output, so the voltage you loose here is a bit less.The voltage on C2 will be approx 12*1.41 - 0.6 = 16.3VThe safest way is to just stick to the 1A limit for the transformer, which would be 16V& 1A through the load (So 16VA for the transformer), but that is very conservative.In TR2, one winding is delivering all the power, while the other is doing nothing, and thus cooling down from previous effort. But Ohmic heating in a resistor is squared with the current through that resistor, and the result is that you can only draw sqrt(2) times more current out of the wire if you do it for half the time.This results in a max current draw through R2 of 1.41A.And this is also 16.3V * 1.41A = 24Watt, which is (no coincidence) the same as the 24VA rating of the transformer.In reality these are all back-of-an-envelope approximations. In reality the capacitors will draw higher peak currents from the transformers, and their voltage will sag during the constant load of the resistors.In practice, it's quite easy to determine if a transformer is overloaded. Transformers should stay relatively cool, and when they are overloaded they get hot, but that takes some time. So if you make a setup and draw your max load from the transformer for half an hour, and you can then still hold it in your hand without burning it, it's below approx 170c and it's all right.a lot of transformers have built-in temperature fuses, which interrupt the current if it gets too hot. Here in the EU this is mandatory for certain equipment such as transformers built into furniture or above ceilings.(Gosh, 9 other answers while I typed that, everybody must be bored on this sunday

author: ledtester
author_rank: Super Contributor
timestamp: May 01, 2022, 02:55:45 pm
content: Quote from: GiMS_Pang on May 01, 2022, 02:22:35 pmIt seems to work fine under Wine. Just run:Code:wine psud2_setup.exeto install it. On my system the executable was installed in the~/.wine/drive_c/Program Files (x86)/PSU Designer II/directory.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 01, 2022, 02:59:03 pm
content: Also, you've got a 240Vac and a GND for the primary winding, but what is the 0Vac wire doing?KiCad with it's Eeschema and ngSpice work quite good.The main problem with ngSpice in KiCad is the limited availability of spice models. A lot of them are incompatible or down right encrypted, or maimed in some way so they can only be used with the software from a certain manufacturer. And it's quite annoying to spend a few hours browsing the 'net just to find a model of a NE555 or a simple opamp that works in ngSpice. On top of that there are copyright limitations which prevent working libraries being integrated directly in open source software.ngSpice itself is quite robust and it's integration into KiCad is also quite workable.There are some complete examples on the KiCad user forum:https://forum.kicad.info/t/simulation-examples-for-kicad-eeschema-ngspice/34443