Could a variable capacitor divider replace a Variac?

In summary, a capacitor divider can be used as a lossless voltage step-down device for AC signals. This makes it a cheaper alternative to a variac, as it is continuously variable and has the ability to vary the impedance of a circuit. However, using a variable capacitor for high current circuits may require large values, making it more practical for low current circuits. Additionally, the impedance of the capacitors is frequency dependent, so using non-sinusoidal signals may result in distorted signals at the load. A variac, on the other hand, can supply unlimited current at a fixed voltage and can be useful for testing and developing switching power supplies.
  • #1
Omegatron
68
2
Hmmm... You can definitely drop down the voltage, and ideal capacitors don't dissipate any power. So it seems, at first glance, that you could use a capacitor divider as a lossless voltage step-down device for AC.

So you could use a cheaper variable cap divider as a replacement for a Variac? And it would be continuously variable, too, whereas the Variac is only variable in discrete windings.

There has to be something wrong with this. What am I not thinking of?
 
Engineering news on Phys.org
  • #2
Omegatron said:
Hmmm... You can definitely drop down the voltage, and ideal capacitors don't dissipate any power. So it seems, at first glance, that you could use a capacitor divider as a lossless voltage step-down device for AC.

So you could use a cheaper variable cap divider as a replacement for a Variac? And it would be continuously variable, too, whereas the Variac is only variable in discrete windings.

There has to be something wrong with this. What am I not thinking of?

With a constant frequency (50-60hz) a variable capacitor can be used to vary the impedance of a circuit so it need not be part of a divider circuit. There is nothing that is loss less and it might be difficult to make one to handle large currents. Offhand, I don’t think it would be too difficult to power an LED directly from the mains and alter its intensity with only a variable capacitor in the circuit.

...
 
  • #3
Capacitors can be and are used as voltage dividers not only with AC but with DC voltages. The divider probe of an oscilloscope uses a capacitor voltage divider.
 
  • #4
SGT said:
Capacitors can be and are used as voltage dividers not only with AC but with DC voltages. The divider probe of an oscilloscope uses a capacitor voltage divider.

Yeah but at DC the caps are high impedance, so you can't get any useful power transfer. At low freq AC I guess there would be some impedance seen, too. But caps don't dissipate power; they just slosh it around, so it seems like it would be lossless step-down for ideal components.
 
  • #5
Omegatron said:
Yeah but at DC the caps are high impedance, so you can't get any useful power transfer. At low freq AC I guess there would be some impedance seen, too. But caps don't dissipate power; they just slosh it around, so it seems like it would be lossless step-down for ideal components.
Only if the load is high impedance, like a scope input.
Since the AC impedance of a capacitor is frequency dependent, if you use a capacitor voltage divider with anything but a sinusoidal signal, each frequency of a composite signal will see a different impedance and the signal would arrive distorted to the load.
 
  • #6
The full answer is this.

Yes you can do exactly what you want to do.

There is a difference between a variac and voltage divider. A variac
is a transformer and (if idealized) can supply unlimited current at a fixed
voltage.

The variable capacitor will be an adjustable series impedance and as such,
the voltage will fluctuate will the load.

As a practical matter, to deliver lots of current the caps would have to
have large values. But for dividers powering low current circuits this is a cost
effective alternative to a transformer.
 
  • #7
SGT said:
Only if the load is high impedance, like a scope input.
Since the AC impedance of a capacitor is frequency dependent, if you use a capacitor voltage divider with anything but a sinusoidal signal, each frequency of a composite signal will see a different impedance and the signal would arrive distorted to the load.

But the impedance of both capacitors would change at the same time, so the level of each frequency component wouldn't change. Do you mean phase distortion? Power in power lines is by far the fundamental, though, and I doubt phase lagging the higher frequency components would have very much effect on the power you could deliver at all.

Antiphon said:
The variable capacitor will be an adjustable series impedance and as such,
the voltage will fluctuate will the load.

But if the caps are small compared to the load, a change in the load impedance wouldn't affect it much.

As a practical matter, to deliver lots of current the caps would have to
have large values. But for dividers powering low current circuits this is a cost
effective alternative to a transformer.

Yeah.

Hmm... I know I could figure this all out quite easily just by doing that math, but I don't feel like it. :-)
 
  • #8
Omegatron said:
But the impedance of both capacitors would change at the same time, so the level of each frequency component wouldn't change. Do you mean phase distortion? Power in power lines is by far the fundamental, though, and I doubt phase lagging the higher frequency components would have very much effect on the power you could deliver at all.
Don't forget that your load will be in parallel with one of the capacitors, so you will have phase lag. If you have a sinusoid, this phase lag is not important, but if you have any other waveform, each sinusoidal component will have a different phase lag and the signal will be distorted.
Also, remember that your load does not change, so the voltage divider will be between a variable capacitor and the parallel of another variable capacitor and a fixed load.
 
  • #9
One other handy property of a variac... Most variacs are built as autotransformers, with the input AC voltage brought in with one connection at the bottom end of the winding, and the other connection brought in about 75% of the way to the other end of the winding. The variable tap wiper can sweep all the way up to the top of the winding, so Vout can be higher than Vin if you run the wiper all the way to the top of the windings. This is very helpful for things like switching power supply development and testing, since you can vary the AC Mains input to your supply under test all the way from high line to low line.

Also keep in mind that since variacs are generally autotransformers, they are not isolated. You should use a series isolation transformer with the variac if you are going to be working with exposed AC Mains voltages.
 
  • #10
SGT said:
Don't forget that your load will be in parallel with one of the capacitors, so you will have phase lag. If you have a sinusoid, this phase lag is not important, but if you have any other waveform, each sinusoidal component will have a different phase lag and the signal will be distorted.

Yeah, but not a problem for power supplies.

Also, remember that your load does not change, so the voltage divider will be between a variable capacitor and the parallel of another variable capacitor and a fixed load.

The load can change, so this is an important consideration. <s>Somehow there would have to be a balance between low-impedance caps for output voltage steadiness and large-value caps for high current capability.</s>

Silly me. Large value caps are low impedance. So the bigger the caps, the better.

berkeman said:
This is very helpful for things like switching power supply development and testing, since you can vary the AC Mains input to your supply under test all the way from high line to low line.

Yeah, I use them at work. I was looking at it and thinking "man, those are expensive, I wonder if there's a cheaper way to get the same effect."

Also keep in mind that since variacs are generally autotransformers, they are not isolated. You should use a series isolation transformer with the variac if you are going to be working with exposed AC Mains voltages.

So just use a fixed step-up transformer before the cap divider and get the increased voltage and the isolation at the same time.
 
Last edited:
  • #11
>So just use a fixed step-up transformer before the cap divider and get
>the increased voltage and the isolation at the same time.

Interesting idea, Omega. I think that the main problem with using caps instead of magnetics will be the volumetric energy density. With effective load capacitances of hundreds of uF for most power supply inputs, your top divider capacitor element will need to be on that order. But most variable caps that I've seen are in the pF-nF range, and I can't imagine how you'd make one in the many-uF range. I guess if you could make the mechanicals tight and reliable, and immersed the moving plates in water or something with a high dielectric constant, you might be able to get there. Have you calculated how big physically the variable cap portion would have to be?
 
  • #12
berkeman said:
Interesting idea, Omega. I think that the main problem with using caps instead of magnetics will be the volumetric energy density. With effective load capacitances of hundreds of uF for most power supply inputs, your top divider capacitor element will need to be on that order. But most variable caps that I've seen are in the pF-nF range, and I can't imagine how you'd make one in the many-uF range. I guess if you could make the mechanicals tight and reliable, and immersed the moving plates in water or something with a high dielectric constant, you might be able to get there. Have you calculated how big physically the variable cap portion would have to be?

Of course I haven't. :-) It was just an idea. I guess it would be just as expensive as a Variac, or more. You could use a string of caps and have only discrete output values...

You mean oil, right? Water would conduct.
 
  • #13
Omegatron said:
You mean oil, right? Water would conduct.
I was thinking DI water, so it shouldn't conduct. Oil would probably be okay too, and I guess there'd be less of a worry about corrosion...
 

1. What is a variable capacitor divider?

A variable capacitor divider is a type of electronic circuit used to vary the output voltage of a circuit. It consists of a variable capacitor and a fixed resistor in series, with the output voltage taken from the junction between the two components.

2. How does a variable capacitor divider work?

A variable capacitor divider works by adjusting the capacitance of the variable capacitor, which in turn changes the voltage division ratio between the variable capacitor and the fixed resistor. This results in a change in the output voltage of the circuit.

3. Can a variable capacitor divider replace a Variac?

It depends on the specific application and requirements. While both can be used to vary the output voltage of a circuit, a Variac is typically used for higher power applications and can handle larger voltage and current ranges. A variable capacitor divider is usually used for lower power applications and has a more limited voltage and current range.

4. What are the advantages of using a variable capacitor divider over a Variac?

A variable capacitor divider is typically smaller, lighter, and less expensive than a Variac. It also has no moving parts, making it more durable and reliable. Additionally, a variable capacitor divider can provide a more precise and stable output voltage compared to a Variac.

5. Are there any limitations to using a variable capacitor divider instead of a Variac?

Yes, as mentioned before, a variable capacitor divider has a more limited voltage and current range compared to a Variac. It is also not suitable for high power applications and may not be able to handle large fluctuations in input voltage. Additionally, it may require more complex circuitry to achieve the desired voltage range compared to a Variac.

Similar threads

Replies
10
Views
1K
  • Electrical Engineering
Replies
5
Views
940
  • Electrical Engineering
Replies
9
Views
3K
  • Electrical Engineering
Replies
3
Views
759
Replies
3
Views
2K
  • Electrical Engineering
Replies
2
Views
1K
Replies
107
Views
9K
Replies
7
Views
2K
Replies
15
Views
1K
  • Electrical Engineering
Replies
1
Views
1K
Back
Top