Well , a transformer can regulate the voltage , increasing and decreasing it .

And , if the voltage increase , the current too .

So... a transformer regulates current too right?

Sorry if its a very basic question , but after lot of searching in google im getting more and more confused ...


  • \$\begingroup\$ For an ideal transformer, the input and output powers are the same. So power = I * V should be equal in both sides. If voltage doubles, the current is halved. So, yes, a transformer changes both voltage and current. However, I am not sure regulate is the right wording. \$\endgroup\$
    – muyustan
    Commented Feb 26, 2020 at 17:21
  • 1
    \$\begingroup\$ Regulation is the wrong word. Convert to a higher or lower level is the correct way to describe what a transformer does. Regulation implies keeping at a certain level and a transformer doesn't do that so, do you have a question? \$\endgroup\$
    – Andy aka
    Commented Feb 26, 2020 at 17:22
  • \$\begingroup\$ @Andyaka , Thank you for the reply , after all the comments now I understand a little better what a transformer can do , However, what is the name of the device that allows us to regulate a current or voltage? I want to learn more about it. Also , if a transformer cannot keep its voltage or current at the same level and need another device to regulate, then a transformer only works in AC circuits, right? \$\endgroup\$
    – Luis Anaya
    Commented Feb 26, 2020 at 17:38
  • 1
    \$\begingroup\$ @muyustan , really straight to the point and very clear reply , thank you very much muyustan \$\endgroup\$
    – Luis Anaya
    Commented Feb 26, 2020 at 17:38
  • \$\begingroup\$ When you start your car, it idles and consumes gas, but doesn't go anywhere yet. No work is delivered to its wheels. So too, a transformer consumes some idling current from its source, even if no load is connected. Once you connect a load, current drawn from the source increases from idling current. \$\endgroup\$
    – glen_geek
    Commented Feb 26, 2020 at 17:39

5 Answers 5


a transformer can regulate the voltage , increasing and decreasing it .

That's not how we usually describe it.

When we say some circuit regulates a voltage, we mean that it produces the same output voltage, even if the input voltage or the output current changes. Insensitivity to input voltage is called line regulation, and insensitivity to output current is called load regulation.

A transformer doesn't do that. If you change its input voltage, its output voltage will change proportionally (so there's no line regulation at all). If you change its output current, whether the output voltage changes depends on whatever's driving the primary side of the transformer, and how well regulated it is. The transformer doesn't improve the load regulation if you have a poor (high output impedance) source powering its primary.


Varying the transformer voltage only adjusts the current for a fixed load but it's not regulating the current.

Typically we like that fact that a transformer outputs a fairly constant voltage for varying loads. This is how the supply to your house works. You and your neighbours are merrily switching stuff on and off through the day and night and you expect the local distribution transformer to maintain constant voltage without any regulation of current. If it were not so there would be much smoke and wailing and gnashing of teeth on your street.


A Transformer Cannot Regulate Voltage or Current

Well, a transformer can regulate the voltage, increasing and decreasing it.

To regulate something, is to keep something a constant (does not change), even if the operating conditions have changed. For example, a voltage regulator keeps its output voltage unchanged, no matter how the input voltage or load resistance changes.

A 7805 regulator circuit

In this 7805 regulator circuit, the voltage from the battery can be 30 V or 10 V. You can also replace the resistor to another value. If the regulator is ideal, the output is

$$ V_\text{out} = 5 \text{V} $$

But a transformer only step-up or step-down an AC voltage by a fixed ratio, it doesn't regulate the voltage/current. If the input voltage is increased, the output voltage increases as well.

A 220-240 VAC source is stepped-up by a 1:2 transformer, with a 100-ohm load

For example, in this 1T:2T transformer circuit, the output voltage is \$ V_\text{out} = 2 \times V_{in} \$. If the transformer is ideal, an input voltage of 220 VAC gives an output voltage of \$ 2 \text{V} \times 220 \text{V} = 440 \text{VAC} \$, and an output current of \$ \frac{440 \text{V}}{100 \text{V}} = 4.4 \text{A} \$. But if voltage goes up to 240 V, T1 gives an output voltage of \$ 2 \text{V} \times 240 \text{V} = 480 \text{VAC} \$, and an output current of \$ \frac{480 \text{V}}{100 \text{V}} = 4.8 \text{A} \$.

As you see, an ordinary transformer doesn't regulate anything. It only step-up or step-down the voltage.

Real transformer is even worse, unlike an ideal transformer, its introduces additional impedance. When the secondary side of a transformer is open-circuit, the voltage is usually higher than expected, as the load current increases, the voltage drops.

A Transformer Changes Resistance/Impedance

And, if the voltage increase, the current too.

A transformer can't regulate current, but it can increase or decrease the current. Therefore, we can say a transformer is a device that changes resistance/impedance.

220 VAC source connected to a 100-ohm resistor

For example, if we connect a \$ 100 \Omega \$ resistor to a 220 VAC source, the current flows out from the power source is \$ \frac{220 \text{VAC}}{100 \Omega} = 2.2 A \$.

But if we add a transformer between the 220 VAC source and the resistor, the current flows at the secondary side of the transformer becomes \$ \frac{440 \text{VAC}}{100 \Omega} = 4.4 A \$, and it consumes \$ 440 \text{VAC} \times 4.4 \text{A} = 1936 \text{W}\$ of power.

220 VAC source is connected to a 1:2 step-up transformer with 100-ohm resistor load

A transformer is not a perpetual motion machine, due to the conservation of energy, it means the power that goes into the primary side of the transformer must also be 1936 W. So, the current flows out from the power source becomes \$ \frac{1936 \text{W}}{220 \text{VAC}} = 8.8 \text{A} \$.

Now imagine the resistor is in a blackbox.

220 VAC source connected to a 100-ohm resistor, the resistor is in a blackbox

We cannot see what's inside the blackbox, all we can see is that the blackbox consumes 2.2 A of current. So we say the equivalent resistance of the blackbox is \$ \frac{220 \text{VAC}}{2.2 \text{A}} = 100 \Omega \$.

Now we add a step-up transformer to the blackbox, with the same \$ 100 \Omega \$ resistor.

220 VAC source connected to a 1:2 step-up transformer with a 100-ohm load, the transformer and the load is in the blackbox

It's still a blackbox, and we don't know what is inside the box. We know that the blackbox uses 8.8 A of current. So we say the equivalent resistance of the blackbox is \$ \frac{220 \text{VAC}}{8.8 \text{A}} = 25 \Omega \$.

Here's the point: We only have a \$ 100 \Omega \$ resistor, but with a 1T:2T transformer looks like a \$ 25 \Omega \$ resistor. This is an important application of transformers: transform resistance/impedance for impedance matching.

The impedance equation of an ideal transformer is,

$$ \frac{Z_p}{Z_s} = \frac{N_p^2}{N_{s}^2} $$

\$ Z_p \$ is impedance looking into the primary side, \$ Z_s \$ is the looking into the secondary side, \$ N_p \$ is the number of turns of the primary side, and \$ N_s \$ is the the number of turns of the secondary side.

In other words, the impedance ratio is the square of the voltage/turn ratio.

If we have a 1T:2T transformer,

$$ \frac{Z_p}{Z_s} = \frac{1^2}{2^2} $$

And we know there's a \$ 100 \Omega \$ resistor at the secondary side, then impedance looking into the primary becomes

$$ Z_p = \frac{100 \Omega \times 1}{4} = 25 \Omega $$

Example: Audio Transformer in Vacuum Tube Amplifier

This is not meant to be realistic. Real vacuum tube amplifiers are more complex than this, but this is the main idea.

In vacuum tube amplifiers, the output impedance of the tube itself can be as high as \$ 700 \Omega \$, there's nothing we can do about it, yet, the impedance of the speaker is often \$ 8 \Omega \$.

A 700-ohm tube connected to a 8-ohm speaker

If the output voltage of the amplifier is 200 VAC, and we connect the vacuum tube directly to the speaker, the speaker will probably be destroyed because it cannot handle high voltage. But even if the speaker is okay, the current delivered is only \$ \frac{200 \text{VAC}}{700 \Omega +8 \Omega} = 0.28 \text{A} \$, and the power goes into the speaker is only \$ P = I^{2}R = 0.28^2 \text{A} \times 8 \Omega = 0.63 \text{W} \$, it's an useless amplifier.

But if we add a 10:1 output transformer,

A 700-ohm tube connected to a 10:1 step-down transformer, with a 8-ohm speaker load

First, there is no high voltage on the speaker, also, if we use the impedance equation,

$$ \frac{Z_p}{Z_s} = \frac{10^2}{1} $$

We find,

$$ Z_p = \frac{8 \Omega \times 10^2}{1} = 800 \Omega $$

Now the \$ 8 \Omega \$ speaker, when combined with the output transformer, looks like a \$ 800 \Omega \$ speaker to the tube, the impedance is matched.

The current delivered is \$ \frac{200 \text{VAC}}{700 \Omega + 800 \Omega} = 0.13 \text{A} \$, and the power goes into the speaker is \$ P = I^{2}R = 0.13^2 \text{A} \times 800 \Omega = 13.5 \text{W} \$. Now it's an usable amplifier for driving loudspeakers.


What you're groping around for is a current transformer. Yes; that is a thing. And it's at the heart of how discharge lighting works.

Arc Discharge lighting (neon, fluorescent, metal halide, mercury vapor and the sodium lights) is a sort of "arc light" going through a special mix of gases. This technology is much older than incandescent lights, and Tesla was playing with it quite a lot. The lamp has basically infinity ohms, until the arc strikes. Then, its resistance heads straight down to zero - it is a dead short. So the power supply must limit current to the lamp. That is what the ballast does. It (was) a transformer wound in current mode, so given a certain input voltage, it gave a certain output current. I think the only reason incandescent took off is Edison insisted on DC power - obviously you can't run a constant-current transformer on DC.

enter image description here

So the transformer you are looking for is commercially available inside fluorescent ballasts or as replacement transformers for HID (metal halide, mercury and sodium) lights. The mainstream manufacturers still make them with actual transformers, but amusingly, the (marijuana) grow light industry makes electronic ballasts!


a transformer is a feedback system, where the core implements a comparison of flux from the primary current with the flux from the secondary current.

If this comparison changes, then the primary voltage will slightly vary, in the direction that acts against the source impedance of the external energy to restore the primary/secondary voltage ratio.

Again, the transformer "regulates" by using feedback ( fluxes) inside the core.


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