# Variable voltage control

#### AccretionDisk

Joined Sep 27, 2019
7
I’m new to circuits and electrical design but I’m trying to learn more through practical application.

I’m working on a project where I need to convert 110 volts AC down to 40-60 volts AC and adjustable. It can be a greater range as long as I can put limits on the upper and lower voltages. The application would be for controlling the tint of a polymer, so no surge when starting.

Are there any resources or parts you would be able to point me towards? I’m not asking for a circuit design just something that would help me understand the process for this and how to apply it.

Joined Jan 15, 2015
4,883
While I am not aware of your current requirements if you want 40 to 60 VAC from a 120 VAC source I would think about a motor driven Variac with limit stops adjusted for 40 to 60 VAC. Just a simple Variac manually adjusted with mechanical stops would also work. Depends on how much current and how much automation you want as well as any budget constraints?

Ron

#### dl324

Joined Mar 30, 2015
9,577
Welcome to AAC!
I’m working on a project where I need to convert 110 volts AC down to 40-60 volts AC and adjustable. It can be a greater range as long as I can put limits on the upper and lower voltages.
Varying AC voltage is more complicated than you think.

Using an autotransformer or variable transformer, also known as a Variac, is probably the easiest way. But using a typical variac output directly doesn't provide line isolation and a failure can give voltages higher than line voltage.

An isolated variac or a variac driving a 1:1 transformer would be safer.

https://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers

Joined Jan 15, 2015
4,883
An isolated variac or a variac driving a 1:1 transformer would be safer.
Yes, I should have included that. I would add a 1:1 transformer just for mains isolation to reduce shock hazard.

Ron

#### AccretionDisk

Joined Sep 27, 2019
7
Welcome to AAC!
Varying AC voltage is more complicated than you think.

Using an autotransformer or variable transformer, also known as a Variac, is probably the easiest way. But using a typical variac output directly doesn't provide line isolation and a failure can give voltages higher than line voltage.

An isolated variac or a variac driving a 1:1 transformer would be safer.

https://en.wikipedia.org/wiki/Autotransformer#Variable_autotransformers
If varying AC voltage is more difficult would it be simpler to vary DC input with a fixed output ratio? 12V Dc in 36VAC out and then change the DC input to say 15V and get 45VAC out?

as far as application I would like it as small as possible maybe a 3”x5”x5”, so I don’t think a variac would work well

#### AlbertHall

Joined Jun 4, 2014
9,009
Does the AC need to be a good sine wave or would a square wave do?
How much current do you need?

#### AccretionDisk

Joined Sep 27, 2019
7
Does the AC need to be a good sine wave or would a square wave do?
How much current do you need?
both would work 100-150 watts MAX past that I would use multiple modules.
Basically I want to use a polymer dispersed liquid crystal film (single pixel lcd screen basically) but have it be dimmable and it works by polarizing the film which causes the crystals to align with the current and allow light through, different voltages will make the crystals align more and let more light through this means the voltage doesn’t have to be exact either.

this is just a personal project and I’ve never worked with this material before so it’s learning process on both the material and electrical side lol

#### dl324

Joined Mar 30, 2015
9,577
If varying AC voltage is more difficult would it be simpler to vary DC input with a fixed output ratio? 12V Dc in 36VAC out and then change the DC input to say 15V and get 45VAC out?
It depends. You've been asked twice how much current you need. I'll make it three.

Do you require the AC voltage to be a clean sine wave? If so, how much distortion can you tolerate?

#### be80be

Joined Jul 5, 2008
1,971

#### AccretionDisk

Joined Sep 27, 2019
7
It depends. You've been asked twice how much current you need. I'll make it three.

Do you require the AC voltage to be a clean sine wave? If so, how much distortion can you tolerate?
my apologies I thought I had provided that, 2.5 amps.

#### AccretionDisk

Joined Sep 27, 2019
7
exactly this but I’m using the film to be applied on existing glass and it should be able to be dimmed instead of just on/off I’m just trying to work out the power supply right now.

#### AccretionDisk

Joined Sep 27, 2019
7
my apologies I thought I had provided that, 2.5 amps.
And it does not need to be a sine wave square is fine. I’ve never worked with this material so I’m not sure about the distortion but I willing to implement something and then refine from there, I don’t believe distortion should affect it however.

Joined Jan 15, 2015
4,883
What I have seen of this is that a current is applied for a short time causing the tint to darken or lighten. When a desired tint is reached the current is removed. What I don't know is if for example a DC current is applied to for example darken the tint and the polarity reversed to lighten the tint? Not sure how the effect of lighten the tint or darken the tint would be done using an AC current? Do you have any links to where you came up with the 40 to 60 VAC? I remember reading somewhere that the required power applied was only something like 3 to 5 watts per square meter of surface glass but again that's about all I remember. It was pretty cool stuff.

Ron

#### AccretionDisk

Joined Sep 27, 2019
7
And it does not need to be a sine wave square is fine. I’ve never worked with this material so I’m not sure about the distortion but I willing to implement something and then refine from there, I don’t believe distortion should affect it however.
What I have seen of this is that a current is applied for a short time causing the tint to darken or lighten. When a desired tint is reached the current is removed. What I don't know is if for example a DC current is applied to for example darken the tint and the polarity reversed to lighten the tint? Not sure how the effect of lighten the tint or darken the tint would be done using an AC current? Do you have any links to where you came up with the 40 to 60 VAC? I remember reading somewhere that the required power applied was only something like 3 to 5 watts per square meter of surface glass but again that's about all I remember. It was pretty cool stuff.

Ron

This is where I’m getting that figure under the product parameters https://m.alibaba.com/product/1967579754/3m-Frosted-Color-Rim-Smart-Glass.html?__detailProductImg=//sc01.alicdn.com/kf/Hed6c213d7a144c3c8d209564ed061fd1U/3m-Frosted-Color-Rim-Smart-Glass-Pdlc.jpg_140x140xz.jpg

it is only 3-5 watts and about 0.1 amps per square meter but my application is closer to 25 square meters all told in this case. Initial test will be I believe .25 square meters.

as far as the switching it is a bunch of suspended crystals and when unpowered they are randomly mixed up blocking light transmission. When power is applied the crystals orient themselves according to the current and allow light to pass.
When the current is removed the crystals return to the randomized state and once again block light transmission. This means I should be able to partially “polarize” the crystals and let specific amounts of light in but adjusting the voltage.

Other data sheets even go so far as to place a limit on how long the panel should be energized (16 hours out of 24) to prevent damage which leads me to believe that clear=powered and isn’t as simple as bringing it to the tint level desired and then leaving it rather powering it at a specific amount.

It is basically a single pixel lcd screen with no backlight or color filters if you know how those work it’s the same concept.

Joined Jan 15, 2015
4,883
OK, things make more sense now. Thanks to your link and that of be80be.

Ron

#### Bordodynov

Joined May 20, 2015
2,469
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#### Alec_t

Joined Sep 17, 2013
10,708
I vaguely recall reading that square-wave drive for a LCD is fine, but duty cycle must be close to 50% to keep the DC average at 0V.

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#### Bordodynov

Joined May 20, 2015
2,469