AC regulator?

[stev.dk]

I have a 230VAC / 24 VAC transformer, but i want to be able to adjust the voltage from (close to) 0 - 24 VAC. I cant find much info on this, so i guess im searching the wrong places. Is it possible to use a thyristor circuit, as used for ac motor control?

Can anyone link to some schematics or sites adressing this subject?

[Simon]

What exactly are you trying to achieve? A Thyristor will very the voltage by producing pulses from the input voltage which would change the average voltage but you would not have a pure sine wave. Do you just need a variable alternating voltage of some sort. Or do you need an actual sinusoidal voltage waveform.

[Richard Crowley]

As Simon says, it is difficult to respond because it isn't clear what you are trying to accomplish.
If you need true variable-voltage AC, then a "variac" is the typical solution.
It is a transformer made into a circular shape (like a toroid), and with a movable "tap"....

[PA0PBZ]

... then a "variac" is the typical solution.

but please use this in front of the 230/24 transformer you already have, and not directly because it will be connected to 230AC and kill you.

[Simon]

That is a good suggestion, I'm not sure how variacs work as I've never used one or looked into them. But I'm going to guess that they were not exactly designed to drop 240 V down to 24 V as I think usually they just knock a bit off the input voltage. If you're cariact can do a complete swing from 0 to 240 V then using it at low voltage will mean that you can only draw a small amount of power. I am going to guess that in theory the current output will always be the same in relation to the diameter of the secondary coil wire. So directly dropping to 24 V would be extremely wasteful unless the variac is designed to work for lower outputs.

[stev.dk]

Sorry for not explaining, what im going to use it for... its really really hot, and im kinda tired, so my brains not working very well.

Im building a "bending" unit for bending acrylic plastic/plexiglass. So im using some nichrome wire for heating the plastic. I want to be able to dail in, the needed voltage / heat, for different thickness sheets. So i wont be needing a perfect sinewave... at least i dont think so.

[Simon]

Know you will be fine with pulsing current, I'm not that familiar with regulating A/C, your life may be made simpler by rectifying it and then using PWM to regulate the effective power delivered to your heater. Either way you will be using some sort of PWM strategy to control the amount of power delivered to your load.

[mariush]

You could probably use a triac and a 555 to control the pwm frequency. Hakko used a triac and a microcontroller to to turn the  triac on or off.

Here's a schematic for a Hakko 936 clone.. you can easily see a triac being driver by a cheap micro : https://www.eevblog.com/forum/reviews/hakko-936-%28original-or-clone%29/

[PA0PBZ]

The problem with variacs is this:



So there will always be a direct connection between the input and output, much like an autotransformer:



So that could be a little uncomfortable, and in countries with outlets that do not have a specific phase and neutral connection lethal.

Always use them like this, and forget the word 'optional'

[Richard Crowley]

Assuming your load is a simple resistor, and is completely isolated from contact by users,
You could use a simple and inexpensive "lamp dimmer" to control the power through a resistive heating element.
Here in the USA, the popular "go-to" standalone controller is a "Router Speed Control" from Harbor Freight.
It is quite commonly used by photographers and videographers as a portable lamp dimmer, also...

[stev.dk]

The reason i want to use AC, is because it requires less AC voltage to heat nichrome wire. I dont know if pulsing positive voltage, give the same heat gain, as true AC current.

A variac seems like a costly solution, and might be able to get a used variable psu for the money. I was hoping for a cheaper fix :/

The lamp dimmer might be a solution. I will look into that.

[Richard Crowley]

It requires exactly the same amount of power to heat from AC or DC.  A simple resistor is the easiest kind of load to drive, and a very inexpensive phase-angle "dimmer" is the easiest and least expensive solution.  There are many problems with regulating power to various kinds of loads. But a simple resistor, as in this case, is dead-simple and you don't need to spend a lot of time and worry about it.  I have constructed many stand-alone controllers using ordinary "lamp dimmers" (the kind you install in your wall) but installed into a small box with an AC mains cord for input and output.

EDIT: Sorry, I kept thinking that you have a resistive heating element that is rated for the full power mains voltage.
But if you have one rated for only 24V, then you need a 24V transformer and THEN the phase-angle SCR/Triac power controller.

[Simon]

Yes a resistor is a resistor. If you want simplicity of control then rectify your 24 V A/C and used a 555 timer to generate a PWM signal to drive a suitable transistor. Alternatively I think a 24 V version of the mains light dimmer circuit could also be used.

[Richard Crowley]

These industrial-style temperature controllers are now so inexpensive, I am thinking of adding one to my toaster!   



Temperature Controller, 0-400Deg. C, Programmable  US$  14.95
http://www.mpja.com/Temperature-Controller-0-400Deg-C-Programmable/productinfo/31489%20MI

[stev.dk]

So a temperature controller, would do the job?

Connecting the controller to mains, and the nichrome wire to the controller, set it at the desired temp. and voila?

[Richard Crowley]

Yes, a temperature controller (properly configured for 24V) will work.
But that seems like vast overkill for such a simple application.

It is curious why you aren't using a heating element that is rated for your mains voltage..
Using one at 24V makes it more difficult to apply simple solutions.

[stev.dk]

I think finding a heater that fits my plans, and for my mains is more excessive, than a few feet of nichrome wire.

Im going to use a light dimmer in front of my transformer, that should do the trick right?

[mij59]

It requires exactly the same amount of power to heat from AC or DC.  A simple resistor is the easiest kind of load to drive, and a very inexpensive phase-angle "dimmer" is the easiest and least expensive solution.  There are many problems with regulating power to various kinds of loads. But a simple resistor, as in this case, is dead-simple and you don't need to spend a lot of time and worry about it.  I have constructed many stand-alone controllers using ordinary "lamp dimmers" (the kind you install in your wall) but installed into a small box with an AC mains cord for input and output.

EDIT: Sorry, I kept thinking that you have a resistive heating element that is rated for the full power mains voltage.
But if you have one rated for only 24V, then you need a 24V transformer and THEN the phase-angle SCR/Triac power controller.

Using a dimmer on the primary side of the transformer should work, you may need a special dimmer for inductive loads to get a good regulation.

[stev.dk]

Its not an actual heating element, its just a piece of nichrome wire, so its not rated for anything. It depends on the lenght and gauge. It can be heated using ac and dc :-)

[Richard Crowley]

A common way of dealing with this is to wind the nichrome wire into a tight coil and then stretching the coil out to the required length.  That is preferable to simply using straight lengths of nichrome.  For several reasons. Not the least of which is controlling the heat.  To my view, creating an oddball-voltage heating element is "excessive"

 

If you use a "lamp dimmer" BEFORE the transformer, then you will have an inductive load, and many (most?) common dimmers don't like inductive loads.  That is part of the problem using an oddball-voltage heating element.  A piece of nichrome wire is a HEATING ELEMENT by definition.

[SeanB]

If you want to do it quick and dirty, and you are using less than 500VA of transformer just go out and buy a light dimmer rated for inductive loads, and get one rated for more than the transformer rating. That then goes in series with the primary and you adjust for the desired heat in the wire load. Simple and works well. Most light dimmers that will work with low voltage downlighters using a magnetic transformer will do.

[Simon]

if your using a temperature controller you need a sensor that will pick up the temperature and feed it back to the controller so that the controller knows what power to output. If your going to heat a lot above ambient and ambient will never vary by more that +/-5C then an open ended pwm / dimmer type controller will be quite accurate.

[stev.dk]

Okay, i heard in a YouTube video that using ac was more easy, cause it returret lower voltage. But if that not the case, i'll just rectify the voltage and controle pwm With a 555. I should have All the parts for that

[jakeypoo]

Boy howdy. Could this possibly be the wrong way to heat your nichrome?

Check out the first two handy charts on this page:
http://www.wiretron.com/nicrdat.html

Now, if you know your target temp and wire size you can simply build a DC variable voltage (or even current) source  (assuming you're using a reasonable length and thickness of wire, otherwise ignore the rest of this post). You just need to spec it so it can supply the amperage you need (AMPS @ Target temp & Wire size) and the voltage you need (Amps * R/ft * feet of wire).

ie: 2ft of 24AWG wire up to 400C will take 3.4A at 12.7V (there's a 7% increase in R with Temp at 400C).

Personally, I'd say screw the variable part and just PWM a 16-18V source that can source the current .

AC / DC, either way - don't forget your fuses!

[Simon]

Whoever said that you need less voltage with A/C to get the same power is an idiot. A/C voltage is usually specified in RMS. The whole point of RMS voltage is to determine that A/C voltage which will produce the same amount of heat in a resistor if supplied that same voltage in DC.

[cs.dk]

If you need a "large" light-dimmer, i have some in my stove - The only cost for you is the shipping. It's output is some squarewave, or pulsed sine. I used it to run some fans, but they become unstable with the pulses. A VFD replaced them, so as said, they are here for free.

[stev.dk]

I have 16.4 feet / 5 meters 20 AWG nichrome wire. I dont think i'll be needing 400C to bend acrylic? At least not the thicknes i'll be working with. I think i'll be bending 5mm tops...

I cant remember the specs of my toroid transformer, but i'll post, when i get back home, tommorrow.

[stev.dk]

Ok, so my torrild transformer spec. is: primary 230v, secondary 24v 80VA. I have 16.4 feet of 20 awg nichrome wire. The chart says 5.1 amps is needed to heat 2 feet of straight wire to 315C, but devided by 2 if element is coiled. I should have enough wire to coil it, as my heating strip only needs to be about 9 inches long. Will my transformer be big enough for this job? Im planning on rectify the ac to dc, and use a 555 timer to pulse a transistor, that will power the heater element.

[Simon]

No it won't, 80VA is just over 3A at 24V and you want 5.1. I'm not sure how your current will go down if you coil it unless it is very tight. If you plan to PWM control it at much less than 3 A then you can use the transformer if it has a large enough input capacitor, I once did this to run a 80W fan off a 30W power supply.

[dentaku]

I wonder if this is useful for you?
http://jacobs-online.biz/power_supply_design.htm

[stev.dk]

Ohhh. the table http://www.wiretron.com/nicrdat.html is based on 115VAC. That in mind, it will take 586,5 Watts, to heat up 2 feet of straight nichrome wire to 315C... Im assuming, i'll need the same amount of watt, using 24VAC, which will result in 24.4 Amps... way above my transformers capability. Think i'll have to rethink this completely.

[SeanB]

Depending on duty cycle your transformer might work. Bar sealers use a 56VAC transformer running grossly in overload, but as they typically are on for 5 seconds every minute they last quite well. The transformer does get very hot if the user does 10 seals in a minute though, but they do at least come with a thermal trip to protect the transformer. Not always effective though.

[jlmoon]

To the original op.. Stev.

This might be a tad off topic but still pertains to a portion of your overall project:
Following this thread, I notice you're employing nichrome wire for your heater.  Might I suggest a much better solution for heating wire?  Take a look at Rene Wire.. or 'Rene 41'  This is considered a superalloy with outstanding tensile strength when hot.  Some of my business dealings involve foam, plastics cutting and forming which requires a much more robust wire than nichrome.  You will be pleasantly surprised with the performance of this stuff. 

my 2c worth! 
Jon

[jakeypoo]

Ohhh. the table http://www.wiretron.com/nicrdat.html is based on 115VAC. That in mind, it will take 586,5 Watts, to heat up 2 feet of straight nichrome wire to 315C... Im assuming, i'll need the same amount of watt, using 24VAC, which will result in 24.4 Amps... way above my transformers capability. Think i'll have to rethink this completely.

Just Look at the first two charts, that's all you need.
Ditch the transformer and PWM a DC source. It will make your life easier.
If you need denser energy output, use a coil as others have suggested.

[stev.dk]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

[jakeypoo]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

You can calculate all of this with the first two charts. Check my first post.

I have 16.4 feet / 5 meters 20 AWG nichrome wire

Look at the first chart, under 20AWG:  5.1A at 315C, 6.3A at 427C
Looking at the second chart: 20AWG has an R of 0.66 Ohms per foot.

Your voltage requirement is then 6.3A * 0.66Ohm/foot * feet   = Round up to 4.5V per foot.

Now lets go the other way around:
If you end up using a 12V supply, use about 30 inches of wire that will give you about 1.65ohms of resistance (cold) so peak current is 7.3A.   

[stev.dk]

Im thinking about modding a computer psu, that can deliver 24 amps at 12VDC, and pwm this. But will 12VDC be enough?

You can calculate all of this with the first two charts. Check my first post.

I have 16.4 feet / 5 meters 20 AWG nichrome wire

Look at the first chart, under 20AWG:  5.1A at 315C, 6.3A at 427C
Looking at the second chart: 20AWG has an R of 0.66 Ohms per foot.

Your voltage requirement is then 6.3A * 0.66Ohm/foot * feet   = Round up to 4.5V per foot.

Now lets go the other way around:
If you end up using a 12V supply, use about 30 inches of wire that will give you about 1.65ohms of resistance (cold) so peak current is 7.3A.   

Thx, i understand now. I think i'll be modding my old pc psu, and use a 555 timer to control pwm I should have the parts for that.