voltage regulator wattage

[hardwareuser]

What causes voltage regulators and adapters to have wattage limits? Is it cuz if there are too many amps, then the insulation between the coils would fry?

 

[phisrow]

Less than perfect efficiency. All electronic devices, certain edge cases requiring liquid nitrogen excepted, convert some portion of the electrical energy into heat. Too much heat, and the system is damaged or destroyed. The more efficient they are, and the better their heatsinking, the higher the limit(generally).

 

[MrDudeMan]

phisrow basically hit the nail on the head. A simple voltage regulator can be made out of a Zener diode. Zener diodes work in reverse bias (the current is going 'against the grain' so to speak) and because the non-linear characteristics of diodes, the voltage changes very little even though there can be a massive difference in current (all of this is assuming there is enough voltage to turn 'on' the diode). Basically what I am saying is you can force a ton of current through a Zener diode while the voltage will remain constant but because the diode has some resistance it will absorb some power. When the power exceeds the rating, that usually means it can't handle the heat and it will literally burn.

To avoid confusion, that isn't how all diodes work. A regular PN juntion diode is designed to work in the forward bias mode (opposite of the Zener). This type of diode, however, has no voltage regulation characteristics. When the potential across the diode is higher than the internal voltage of the PN juntion, current starts to flow through the diode basically uninhibited. The voltage in the juntion is called Vgamma usually, and for most diodes it is .5-.7 volts. So if you connect a .4 volt battery across a diode with a Vgamma of .7 volts, no current will flow. If you connect a 1v source across a .7Vgamma diode (throw in a 1ohm resistor in the circuit and assume the diode has no resistance) then .3v/1ohm = .3A of current will flow. hope this helps...

 

[hardwareuser]

Bigsmooth, you're saying that step down transformers (220->110/120) use diodes to limit the voltage? It makes sense for a limit to be there if that's what you mean. How would it work with a step up voltage regulator and adaptors though? I'm quite sure they don't just use diodes.

 

[BrownTown]

not sure what you're saying in that last post hardwareuser, transformers use coupled magnetic fields to change voltages. Current limits on transformers are upheld by fuses which will blow aboover certain currents. But the reason for the wattage reating is the same, inefficientcy casues heat which can destroy the system.

 

[MrDudeMan]

Originally posted by: hardwareuser
Bigsmooth, you're saying that step down transformers (220->110/120) use diodes to limit the voltage? It makes sense for a limit to be there if that's what you mean. How would it work with a step up voltage regulator and adaptors though? I'm quite sure they don't just use diodes.

I didn't mean to make it sound like I was talking about transformers. I was strictly talking about voltage regulators. The whole point of a regulator is to keep the load voltage pretty close to what the design calls for regardless of fluctuations in the source.

Regulator

Vz is the regulating part of the circuit - the Zener diode. VL is the voltage across the load, RL, and it should always be pretty close to Vz. Ri will drop the rest of the voltage from the source, so if Vz is 5V and VPS is 25V, 20V will drop across Ri. This isn't exactly right because the Zener diode will also have a built-in resistance, but in an ideal situation this is correct.



To go from 220 -> 120 or any other step-down, you simply use a transformer, which is two coils of wire. Step-up transformers work the same way. To get a DC voltage out of AC, you use a transformer to step up/down the voltage, a half/full wave rectifier and a filter to get a DC voltage, and then a regulator to keep it where you want it. The biggest fluctuation you usually get is called the ripple voltage, and you reduce this big time by using a full wave rectifier instead of a half wave.

Half-Wave Rectifier w/ Filter - No Transformer or Regulator

This is an example of a half-wave rectified AC->DC converter. This, however, does NOT transform the voltage. The peak AC voltage is what the capacitor will charge to, and that is the voltage across R also. So if this was connected to the wall socket, that capacitor would charge to 169V (120V is the RMS, so multiply by sqrt(2) to get the peak). In this particular filter, the ripple would be large because only the positive half cycles of the AC signal are used. The ripple voltage in this circuit if it were a full-wave circuit would be literally half as much.


Fullwave Rectifier w/ Transformer - No Filter or Regulator

The left half of the picture is the path of the current during the positive half-cycle of the AC signal and the right side is the negative half-cycle. Notice the direction of the current (the arrows). However, the resistor in the middle of this full-wave rectifier would still get an AC signal, not DC. It needs a filter and regulator to be a useful DC powersupply.


Here is an example of what the AC sine wave looks like after being put through a half/full wave rectifier and also a filter.

Examples of rectified AC waves

 

[hardwareuser]

Bigsmooth, I know about AC->DC, thanks. My question still stands, what causes the wattage limit on transformers? Everybody else seems to be just saying what I was guessing in the first place.

 

[MrDudeMan]

Originally posted by: hardwareuser
Bigsmooth, I know about AC->DC, thanks. My question still stands, what causes the wattage limit on transformers? Everybody else seems to be just saying what I was guessing in the first place.

If you know about AC->DC then why did you think I meant diodes are used to transform voltage...?


The limitation on anything is how much power it can absorb without breaking. This has already been answered in this thread. If you want the real technical details, it is because electrons bounce off of atoms when the current is flowing and that causes a release of photons, which cause heat. The faster the electrons are flowing, the more heat is released and the rate at which the component can dissipate it determines the power rating. They aren't directly connected, but that is the cause and effect. It boils down to the resistance of the materials used, as I mentioned earlier.

 

[hardwareuser]

Basically what I am saying is you can force a ton of current through a Zener diode while the voltage will remain constant but because the diode has some resistance it will absorb some power.

You said that so I thought that's what you were saying.

Is it cuz if there are too many amps, then the insulation between the coils would fry?

This is what I said on my very first post. So that's all it's about? That's the part that I think would fail first due to heat.