Center Tapped Transformer Questions - Page 1
Forum section: electronics / beginners

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 21, 2013, 11:30:14 pm
content: Hello everyone.I have been trying to use a door bell transformer as a small power supply for some led lights and a fan on my bench. It is a 24V winding with a center tap that is not really in the center, thus giving 8v and 16v sharing a wire.I put full wave rectifiers on both and cannot seem to get 8 and 16 volts on the sides like I'd expect.Any help is appreciated.

author: mariush
author_rank: Super Contributor
timestamp: August 21, 2013, 11:59:18 pm
content: Your transformer is 24v AC rms... but you don't know how much current it can provide. If you try to take too much current, it's natural you may not be able to get the voltages you want.Full wave rectifiers have 2 diodes that are always active, therefore it's a 0.6-1v drop per diode. So a bridge rectifier loses a maximum of 2v after rectifying the voltage. Let's just round it up to about 1.2 volts.The bridge rectifier converts the AC to DC with a peak of voltage x 1.414 , so you'll have 8x1.414 = 11.3v , and 16x1.414 = 22.6v.After considering the losses on bridge rectifiers, you'll have DC with peaks of about 11.3v - 1.2v = 10.1v and 22.6v-1.2v = 21.4vNow these are peaks, from time to time the voltage will still almost drop down to 0, so you need a capacitor to store energy and fill the gaps. An ideal capacitor will create a perfectly flat smooth DC voltage, but you don't have ideal capacitors with infinite capacitance.So you have to be tolerate a certain voltage drop. Let's say you're willing to go with 1v drop. The capacitance you need can be estimated with formula C = ~ 0.75 x current (amps) / 2 x freq x VrippleVripple is 1v , frequency is 60 Hz in US, current let's say 0.5 A ... so C = 0.75 x 0.5A / 120 x 1v = 0.003125 F or about 3125uF.. let's go with 3300uFSo if you use a 3300uF capacitor on the 10.1v side, when you draw up to 0.5a from that winding, you'll see the voltage vary between 9 and 10v. You can repeat the same procedure for the other winding.My guess is that you didn't use a capacitor at all, or it's very undersized, and the transformer is probably very small, maybe 10-15VA (so it can probably do only 0.3-0.5A on 8v and 16v.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 22, 2013, 12:29:00 am
content: I do what is shown in the picture attached (with capacitors as well).The 16V side actually shows 26VDC on the output and the 8VDC side I didnt get to measure because I had to shut it off because the 16VDC capacitor got very hot.Something is not right.

author: Paul Price
author_rank: Super Contributor
timestamp: August 22, 2013, 12:41:58 am
content: You probably have the all-too-warm capacitor connected backwards or else maybe your rectifier diodes?What you presented as a schematic is not so much a schematic as a block diagram, clarify please.

author: mariush
author_rank: Super Contributor
timestamp: August 22, 2013, 01:07:10 am
content: EpicIntelGamer, a bridge rectifier does not convert the AC voltage into exactly the same DC voltage.The AC voltage your multimeter shows is the RMS value.http://en.wikipedia.org/wiki/Rectifier#Full-wave_rectificationThe bridge rectifier converts the sinusoidal wave on the left into that wave on the right, which is always positive but with bumps. The peak of those bumbs is that Vpeak in the formulaSo if you have 16v AC measured with the multimeter, as the wikipedia page says, you have a DC voltage with bumps that have a peak voltage of Vrms x 1.414 (square root of 2) = 16x1.414 = 22.62vBut this is with ideal diodes in the bridge rectifier, the kind that don't have a voltage drop on them. But normal diodes have about 0.6-1v drop on each diode, and in a bridge rectifier two of those diodes work all the time, so you decrease the peak voltage to take that in consideration.So now you have a DC voltage with bumps with a peak voltage of 22.6 - 1.2v (2x0.6v) = 20v.So as you can see, after the bridge rectifier, you don't have 16v DC, you have up to 20v DC. It's only normal that the capacitor will overheat if it's only rated for 16v. You need a capacitor that will be rated for at least 25v.The capacitor is there to fill in those gaps in the output. I gave you the formula in my previous post - the bigger the capacitance, the smoother the voltage will be. But up to a point, for example it's pointless to use 10.000 uF or something like that, and in some cases it can cause problems.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 22, 2013, 01:14:02 am
content: Someone else might chime-in to correct me but I think you will have to do something like the attached to get a +16V and +8V output. If there's a way to use a bridge rectifier, I can't think of it.I made the capacitors a bit too small to illustrate that each rail only gets a half cycle to charge the caps. The transformer looks weird due to how the simulator works (it doesn't support asymmetric center-tapped transformers) but it is basically what you have.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 22, 2013, 01:15:28 am
content: Quote from: mariush on August 22, 2013, 01:07:10 amThe thing is, with no load, a 1000uf 25V cap, it shows 26VDC off of that tap marked as 16VAC. And the 8VAC winding must be putting out quite a bit more to heat that other 330uf 16v cap (its not backwards).

author: Monkeh
author_rank: Super Contributor
timestamp: August 22, 2013, 01:23:36 am
content: Try measuring the no-load AC voltage. You might be surprised..

author: mariush
author_rank: Super Contributor
timestamp: August 22, 2013, 02:09:25 am
content: Quote from: EpicIntelGamer on August 22, 2013, 01:15:28 amThat's normal. Just like with a lot of electronic parts, transformers also have a certain percent of "tolerance".Depending on the construction (what type of transformer, EL, toroidal, etc) and how big it is (how many VA it can provide), a transformer can be as much +20-30% at low loads. When you're using a lot of power from the transformer, the output voltage goes down, much closer to the actual value listed on the transformer, or it can even drop below the specified value by a few percent points.Larger transformers (like let's say the ones in audio amplifiers, that can do 200-500w) usually have less variation, usually around only 3-5% extra at low loads.So if your transformer is a 24v AC with let's say 20-30VA (about 1A) , at only 10-50mA the transformer could very well output 24v AC rms +20% or about 29v.So then your 16v AC becomes 19.2v, the bridge rectifier converts it to about 27v DC, the diodes in the rectifier will drop about 1v and you have now 26v, which is above the 25v rating of your capacitor.The voltage rating on capacitors is the maximum that capacitor can tolerate. There's nothing stopping you from using capacitors with higher voltage rating. Use 35, 50v if you have to.Use 25v or more for the 8v part as well.Transformers are not designed so that they would output an exact voltage, if you want for example 5v, you don't go shopping for a transformer that would output exactly 5v you want. Transformers give you something a bit higher than what you need, then you smooth it out somehow to the exact value you want. That's what linear regulators are for, or other methods of regulating voltage (zener diodes, switching regulators, etc).Don't get frustrated by this, it's good, you're learning.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 22, 2013, 03:02:12 am
content: Quote from: mariush on August 22, 2013, 02:09:25 amI do know they are made with some headroom but I guess It didn't really dawn on me that that and the gain from rectification would give me such a high voltage.I ended up using another transformer from another project but I'll probably use the transformer described in this thread for that other project now that I realize its not faulty or anything.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 22, 2013, 03:50:20 pm
content: What did you have to do to get it to work with bridge rectifiers?

author: SeanB
author_rank: Super Contributor
timestamp: August 22, 2013, 06:17:19 pm
content: As it is a doorbell transformer you will normally find the load regulation is very poor, and the voltage under no load can be quite high, up to 30% is quite common, so the 8V will give 10VAC, the 16V will give 20VAC. This is because they are designed for long periods at no power draw, so the wire is often undersized so it will cost less. A load is not a long term item, if there is a load of 1A every hour for 5 seconds then the windings will not heat up much. Drawing the rated current from them for more than a minute is not a recommended thing, as they will often burn out at around the 1 hour mark as they heat up.

author: megajocke
author_rank: Regular Contributor
timestamp: August 23, 2013, 05:28:23 am
content: I've seen even worse: a 12 V doorbell transformer giving 22 V under no load! It is designed with high leakage inductance, probably to become short-circuit proof.