How Do Transformers Work? -- The Learning Circuit 97

WOW CH, the company I worked for was ahead of its time.

I worked on a team responsible for electronic installations. I recall the contractor ground grid install, that ensured all grounds from multiple floors came to one central point. We were required to use a copper paste to make connects from the copper plate in a sealed box under the floor to any new racks that were installed. We call the paste "copper shite" because if you got in on your cloths no amount of washing removed it. I visited some of those sites years later and the paste was still malleable.

That ground grid integrated into the build electrical ground grid. That was in the early 80's. I never realized how progressive we were. 

I agree with most of what you said but about 4min in you start talking about appliances, and you say an isolation transformer will stop you from being fried. The problem is that you show an appliance (stove) and there are no Isolation transformers in major appliances. In my server room at my old ISP, we had 48volts DC all over the place. And yes some of the things had earth grounds, which we tied to a ground plate. This ground plate had one wire attached to a salted copper ground rod. This was not the same ground rod that the mains power used. And the only safe way not to get killed around live wires is to make sure that they were protected by GFI Breakers. New rules want ARC fault breakers, the problem is that fans ARC so please don't use them where your computers are plugged into. Get sparky to change them, or rip them out yourself, you can replace them with plain or GFI Breakers.

~~Cris

The power is determined by the inductive reactance of the primary coil. The reactance is given by the formula:

Xl = 2πfL f is the frequency of the AC current and L is the inductance of the primary coil.

To obtain a high reactance you can either use a high inductance coil and work at 60Hz or 50Hz, or raise the frequency and use a coil with a lower inductance. In the first case you will need a large transformer because a high inductance implies a coil with a large number of turns, just like in old arc welders. In the second case, the transformer is much smaller because the inductance is low, and the device is much smaller like new arc welders and phone chargers.

To cut all that power, UPS use high frequency.

That is why new arc welders are called inverters. The AC current is first rectified than inverted at high frequency.

True, it is the AD/DC regulation circuit that limits the output power.

DAB

I have not watched the video yet, but regarding the question

"The question is therefore, how do they cut all that power ?"

I find the following demo to help:

https://www.falstad.com/circuit/e-powerfactor1.html

The right side can be considered to be a transformer with no load connected to the secondary side.

The animation, and the text shows that to a first approximation, the transformer uses no power at all. Only when a load is attached, will power be transferred (into the load). It follows that if (say) the load has a very high resistance, then very little power will be transferred, despite the mains side having the capability to deliver huge amounts of power.

In a similar vein, the (pretty dangerous) project video here https://hackaday.com/2021/09/14/gorgeous-battery-welder-hits-the-spot/

makes (amongst many mistakes) one in particular at 3 min 48 seconds - he measures 11 Amps, and doesn't question if that's really true or not.. i.e. whether that's real or apparent power.

miles90 Alas, my purpose for responding was to address the idea that chargers limit or "cut" the power.  They don't... as you verified with your Samsung charger.  One wouldn't need a fuse resistor if they did.

I had an opportunity to work with some of the "new" inverter welders when they were first being launched.  1/10th the weight.  It's unreal.  And while we say we can't transform DC, we do.  Crazy, eh?  It just has to vary or pulse.

I sacrified my old Samsung charger to find out and the very first component is a fuse resistor. So these devices are protected and they are not meant to be repaired if an overcurrent or overvoltage condition causes the fuse to burn.

The second component is a rectifier. So these are UPS chargers because on top there is a small transformer.

I give you a hint. Look at an old arc welder and then look at a new one.

No, transformers transfer energy from one coil to another and if the voltage is multiplied by some factor than the current is divided by that same factor.

miles90  I believe that your question is intended to create discussion. I'll take that bait. 

How do chargers cut all that power?  They don't - at least, most of them don't.

As DAB has alluded to, it's really all about the design of the charger.  Many, many chargers have no circuit protection whatsoever - so they will attempt to deliver the power the load demands.  If we use the charger for what it was intended, all is well because we are controlling the connected load.  Connect the same charger to a greater load and it could be that the only thing limiting the power through it is (I^2 * R) losses and the thermal breakdown point of the components.

* I'm confident that I'm not the only one in the e14 community that has burned up a repurposed charger because I failed to pay attention to how much load I put on it.  My latest one was this week.

"It's just LEDs.  They don't draw that much power."

"Hey!  It's dark!"

"!@#$%  That sucker's HOT!!"

In power generation, we say "The load is what the load is." - because we can't change the load from the source side.  The connected load determines the current draw.  We try to maintain a constant "E", so it's all about the "R".  "I" is just what happens.  P=(E^2)/R

Your problem is just one of the many implementations of step down transformers.

When you step down the voltage, you also step down the current. Power = Voltage * Current.

The design of these wall warts is a science all to itself.

You can see many different approaches to building an effect transformer solution.

Basically, you can play with power, voltage, current, efficiency, cost and safety.

Each design is a compromise of these factors.

Cheap wall warts are cheap for a reason.

They can be very dangerous.

DAB