So does that mean each end would be 4.5V and the center 0v?

It means that the ends of the transformers' secondary winding are equal in amplitude, but 180° out of phase with each other.

If you grounded one end of the secondary winding, the other end would show a voltage alternating above and below ground.

If you grounded the center tap, the ends would show a voltage alternating above and below ground, 1/2 the voltage measured before, and they would be 180° out of phase with each other.

Also, how do current rating work on a transformer. I see alot of them(on jameco anyways) listed with a VA Cap. How does that work into Max Current Output?

Roughly, divide the VA output by the voltage output. That will give you the current output. For resistive loads like lightbulbs and resistors, it's the same as Watts. For non-power-factor corrected loads (like some computer power supplies) VA can vary significantly from wattage rating. Google is your friend here.

From what I have read the lower the voltage out the higher current output you should have, since transformers don't waste any power.

Well, transformers exchange voltage for current, and vice versa. However, with standard transformers, how much current they can output is related to their physical size and weight. If you want a transformer that can output 3A @ 20V, you're probably looking at a transformer that weighs in the neighborhood of 6 to 8 lbs.

 

It means that the ends of the transformers' secondary winding are equal in amplitude, but 180° out of phase with each other.

If you grounded one end of the secondary winding, the other end would show a voltage alternating above and below ground.

If you grounded the center tap, the ends would show a voltage alternating above and below ground, 1/2 the voltage measured before, and they would be 180° out of phase with each other.



Roughly, divide the VA output by the voltage output. That will give you the current output. For resistive loads like lightbulbs and resistors, it's the same as Watts. For non-power-factor corrected loads (like some computer power supplies) VA can vary significantly from wattage rating. Google is your friend here.


Well, transformers exchange voltage for current, and vice versa. However, with standard transformers, how much current they can output is related to their physical size and weight. If you want a transformer that can output 3A @ 20V, you're probably looking at a transformer that weighs in the neighborhood of 6 to 8 lbs.

So If I used the ends of the secondary winding, and I recified it, would my ground technically be -4.5V or would it be 0V?

Second question is if you use the center tap pin do you "gain" current? An example would be a VA of 1.5. If you were using the ends of the widing on our previous example giving 9V. that would be 1.5 / 9 = 166mA and using center tap would be 333mA. Is this correct?

 

Sorry to do this I'm bumping this for my last question.

 

So If I used the ends of the secondary winding, and I recified it, would my ground technically be -4.5V or would it be 0V?

"Ground" is the 0v reference point. Most voltage readings are made using ground as a reference. Generally, it is a good idea to connect your reference point to earth ground, as that protects the user against exposure to possible high voltage levels, for example if the transformer developed a short between the primary and secondary windings.

Second question is if you use the center tap pin do you "gain" current? An example would be a VA of 1.5. If you were using the ends of the winding on our previous example giving 9V. that would be 1.5 / 9 = 166mA and using center tap would be 333mA. Is this correct?

You'd have twice the current, but half the voltage - so you're not really gaining anything.

A center-tapped transformer secondary makes a full-wave rectifier less complicated; only two diodes are needed to effect a full-wave rectifier.

Without the center tap, you must use a full-wave bridge rectifier, which consists of four rectifier diodes.

But rectifiers are much less expensive than more complex (and heavy) transformers with expensive copper windings.

 

"Ground" is the 0v reference point. Most voltage readings are made using ground as a reference. Generally, it is a good idea to connect your reference point to earth ground, as that protects the user against exposure to possible high voltage levels, for example if the transformer developed a short between the primary and secondary windings.

I'm sorry I'm a little confused could you elaborate a little more on this? If I have this right if you use the end pins on the coil then one end would be 4.5 and the other would be -4.5 and they would be alternating. Making a difference of 9V. Correct?

I don't think I've ever seen a DC circuit that was actually tied to earth ground. None of the wallwarts or VCRs I've taken apart have ever been earth grounded.


Second question. Is there cases where you recitfy the AC before the transformer? I was looking at an old VCR and a couple of walwarts and they both look like they rectify the AC and then send it to the transformer. What is the reason for this?

 

I'm sorry I'm a little confused could you elaborate a little more on this? If I have this right if you use the end pins on the coil then one end would be 4.5 and the other would be -4.5 and they would be alternating. Making a difference of 9V. Correct?

Basically, you'll wind up with 9VAC output from the transformer at the rated current no matter how you look at it.

I don't think I've ever seen a DC circuit that was actually tied to earth ground. None of the wallwarts or VCRs I've taken apart have ever been earth grounded.

I don't know where in the world you are.
Here in the USA, wall-warts typically are only supplied with Hot (120VAC) and Neutral. Appliances may be supplied with the same, or additionally with an earth ground. If the appliance does not have an earth ground, the chassis have to be double insulated from the mains voltages.

Second question. Is there cases where you recitfy the AC before the transformer?

Not usually. It wouldn't really make sense to do that.

I was looking at an old VCR and a couple of walwarts and they both look like they rectify the AC and then send it to the transformer. What is the reason for this?

You may be looking at a thermal fuse. It's not hard to confuse a thermal fuse with a rectifier diode.

 

I don't know where in the world you are.
Here in the USA, wall-warts typically are only supplied with Hot (120VAC) and Neutral. Appliances may be supplied with the same, or additionally with an earth ground. If the appliance does not have an earth ground, the chassis have to be double insulated from the mains voltages.

US. I think you may have misinterpreted what I was saying. I said none of the wall warts I have ever seen were ground to earth. Thats why I was confused about your statement.

You may be looking at a thermal fuse. It's not hard to confuse a thermal fuse with a rectifier diode

No it was a rectifier. The wall warr had 4 individual diodes and the VCR was a DIP package S1WB60. Which is a rectifier. Maybe I was following the path from where the 120VAC came in but I don't think so. Especially not on the wall wart. It was a really simple circuit.

 

US.

OK. It would be helpful to put that in your profile. State, too.

I think you may have misinterpreted what I was saying. I said none of the wall warts I have ever seen were ground to earth. Thats why I was confused about your statement.

OK.
But for ease of documentation/troubleshooting, you usually declare some point of a DC circuit to be the 0v reference.

No it was a rectifier. The wall warr had 4 individual diodes and the VCR was a DIP package S1WB60. Which is a rectifier. Maybe I was following the path from where the 120VAC came in but I don't think so. Especially not on the wall wart. It was a really simple circuit.

The S1Wx60 is a 600v 1A bridge rectifier in an IC.
Rectifiers would be connected to the secondary windings to convert the AC output into rippled DC.

 

I think some switchmode units have rectifirers at the primary side.

 

I think some switchmode units have rectifirers at the primary side.

The Wall Wart was switching, I assume the VCR also

OK.

But for ease of documentation/troubleshooting, you usually declare some point of a DC circuit to be the 0v reference.

If you are using a non-center tapped transformer where do you reference 0V then? Do you just say that the -4.5 is 0 and the +4.5 is 9 and call it good? Or how does that work if you don't actually have something thats 0v?

 

The 0 volt is your reference point. And all voltages is referenced in respect to that point.

 

The 0 volt is your reference point. And all voltages is referenced in respect to that point.

What if there is not a physcal "source" of 0V? As in the case with the +4.5 and -4.5. There is no point of 0V.

 

Oh yes you do have a point of 0 volt. Your center-tap is 0 volt

 

Oh yes you do have a point of 0 volt. Your center-tap is 0 volt

Not all transformers are Center-tap though. And it was my understanding that you don't even have to use a center-tap connection even if it is.

 

OK, I mis-spoke earlier.

If you're going to use the transformer's secondary center tap with two rectifier diodes, then the center tap becomes your "ground" 0v reference.

If you're going to just use the ends of the transformer secondary with a full-wave bridge, the transformer is isolated from the reference by at least one rectifier diode at any time.

 

OK, I mis-spoke earlier.

If you're going to use the transformer's secondary center tap with two rectifier diodes, then the center tap becomes your "ground" 0v reference.

If you're going to just use the ends of the transformer secondary with a full-wave bridge, the transformer is isolated from the reference by at least one rectifier diode at any time.

Ok, so then if you use just the ends, is your "ground" or cathode going to be -4.5V(in the case of 9V)?

 

Ok, so then if you use just the ends, is your "ground" or cathode going to be -4.5V(in the case of 9V)?

No, it will be away from ground by the Vf (forward voltage) of the rectifier diode, which depends upon the current flowing through the diode at the time.

 

No, it will be away from ground by the Vf (forward voltage) of the rectifier diode, which depends upon the current flowing through the diode at the time.

I'm totally confused.

Maybe I should just stick to center tapped transformers. That way I will have 0V and it will make sense.

 

Maybe this will help. Have a look at the attached.

See how the magenta (A) and cyan (B) traces vary between -1v and nearly 16v?

The -1v is the Vf of the rectifier bridge with the moderately heavy output load on the regulator. 12V across 24 Ohms (Rload) gives a load current of 1/2 Ampere. There is some additional current due to R1 & R2.

Don't worry about Rwire and Rinsul - they're just there to allow the simulation to run.

 

Similar simulation, only using a center-tapped secondary.

Note the voltage drop across the diodes; that's the difference between the voltage on the 2200uF filter capacitor (yellow plot) and the magenta/cyan plots.