# Variable Voltage Control



## speedre9 (Oct 5, 2017)

I know this may be the wrong forum for this but, I didn't know where else to get an answer. Is this a safe
schematic?  Want to use it to control power ( how hot it gets ) of s soldering iron. I will be useing a wall socket to provide 120 VAC and then pluging in the soldering iron at the other end. The dimmer has an off switch.


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## RJSakowski (Oct 5, 2017)

For a soldering iron, it is perfectly safe to use a light dimmer.  They are both resistive loads.  If your soldering iron has a grounded plug, make sure that the ground is properly connected and that the neutral side of the iron goes to the neutral side of your wall plug.


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## brino (Oct 5, 2017)

All of the incandescent dimmers I've seen use a TRIAC internally to adjust how much of the AC cycle is passed thru to the load.
So they don't really vary the voltage, but do control the average power.

As long as you respect these three rules, it should work fine:
1) It is a basic soldering iron with only a simple resistive heating element (ie. NO fancy digital temperature display, NOT CPU controlled, etc.)
2) Be sure the wattage rating of the dimmer is higher than the wattage of the soldering iron.
3) Be sure to insulate all connections, or better put it all in a box so everything is enclosed.

-brino

EDIT: fixed my mistake; changed SCR to TRIAC above.


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## jim18655 (Oct 5, 2017)

Did that for years for stained glass work and it works well.


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## speedre9 (Oct 5, 2017)

O.K. Thanks guys I knew I could count on you for the right answers. I will build it up soon. Thanks again, I feel safe about useing it now.


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## markba633csi (Oct 6, 2017)

what Brino said- it would not work well for a Weller-type solder station with the temp switch in the handle, but for a plain resistive iron it's fine.
Mark S.


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## speedre9 (Oct 6, 2017)

It is for a standard 30w iron, nothing fancy just the iron.


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## Silverbullet (Oct 8, 2017)

I did it years ago using an old jig saw that ran to fast for cutting something or another. I was surprised it worked, I expected it to just pop and spit.


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## speedre9 (Oct 11, 2017)

O.K., So how can I measure the voltage at the end that gets the iron plugged in?. I have only an analog multimeter.


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## gr8legs (Oct 11, 2017)

speedre9 said:


> O.K., So how can I measure the voltage at the end that gets the iron plugged in?. I have only an analog multimeter.



Your analog meter will work just fine for that.


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## whitmore (Oct 12, 2017)

speedre9 said:


> O.K., So how can I measure the voltage at the end that gets the iron plugged in?. I have only an analog multimeter.



Voltage measure will be nonsense, you want to use the AC ammeter function.   Beware, of course, the
wires you're connecting are live.   
I've wired an outlet strip with (insulated banana plug) wires soldered to each side of the
power switch: with the switch ON, that outlet strip just powers sockets.  With the switch OFF,
all current goes around the switch, through an AC ammeter plugged into those two wires...

That kind of rewired outlet strip is a convenient (safe) way to monitor AC current, but
there's also a nifty commercial gizmo, Kill-a-Watt, with suitable metering functions.


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## speedre9 (Oct 12, 2017)

O.K. This is where I get lost. My meter has a section that is labled, ACV, with three settings, 10, 250, 500.
At the end of the wire where I would plug in the soldering iron I inserted the probes turned on the dimmer
and tried to see if it would show the volage increasing as I turned it more, but I got nothing, no readings at all the
needle never moved. Can someone explain how to do this and what I'm doing wrong.


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## RJSakowski (Oct 12, 2017)

Analog AC voltmeters are expecting a sinusoidal waveform.  They actually measure peak voltage by converting the ac to dc with a rectifier and  applying a scale factor to read out rms voltage. They are only accurate for the sinusoidal waveform.

AC dimmers use a triac which conducts partway through the cycle.  As a result the output voltage is zero until the triac conducts and then it tracks the input voltage until the half cycle returns to zero when it turns off again.  Triacs function on both positive and negative half cycles whereas scr's only conduct on the positive half of the cycle.

I haven't looked at digital voltmeter circuit but I would guess that they function in a similar fashion unless the meter is specifically rated as a true rms voltmeter.

For the uninitiated, rms means root mean s1quare and stems from the fact that power is the square of the voltage divided by the resistance.  Since delivered power is usually of importance in electrical work, we would like to know what the power squared is.  rms voltage is the square root of the voltage squared averaged over a period of time, usually one complete cycle.  This may seem like some double talk but the rms voltage is actually different than the average voltage.

Bottom line, a voltmeter will not give an accurate picture of the power delivered to any device  when using a dimmer unless the meter is a true 4rms reading meter.  However, how important is it to know the actual voltage.  All that is really necessary is to know that a particular reading givers good results when using the soldering iron.


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## RJSakowski (Oct 12, 2017)

whitmore said:


> Voltage measure will be nonsense, you want to use the AC ammeter function.


For a resistive load. amperage is directly proportional to voltage at any given point in time.  What you really want to measure is power.


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## RJSakowski (Oct 12, 2017)

speedre9 said:


> O.K. This is where I get lost. My meter has a section that is labled, ACV, with three settings, 10, 250, 500.
> At the end of the wire where I would plug in the soldering iron I inserted the probes turned on the dimmer
> and tried to see if it would show the volage increasing as I turned it more, but I got nothing, no readings at all the
> needle never moved. Can someone explain how to do this and what I'm doing wrong.


I expect that you need some loading to get a reading.  The meter will not be a sufficient load. Connect some device like an incandescent light bulb and then measure.


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## whitmore (Oct 12, 2017)

RJSakowski said:


> For a resistive load. amperage is directly proportional to voltage at any given point in time.  What you really want to measure is power.



True (you want RMS voltage, or RMS current, and might get something else from any given meter).

The Kill-a-watt does true RMS, or (my favorite) can meter true AC power.
<https://www.newegg.com/Product/Product.aspx?Item=9SIA2M043P9703>


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## speedre9 (Oct 12, 2017)

All that is way above my head,but I think I just realized something.
Isn't the dimmer switch actually adding power to the outlet.
I have one in my dining room over head lights.
Sliding it upwards increases the brightness of the light . Therefore
it is not decreasing power but increasing it, ergo, this curcuit does not work for what I want.
Any ideas as how to do it, or is all this totaly incorrect?


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## Doodle (Oct 12, 2017)

Why not make it a fun project? Get a temperature controller with solid state relay and thermocouple. Mount it in a box and control temperature to with a couple of degrees. This project will get you to use CAD, Milling machine, lathe, and some hand tools. You will learn how the controller senses heat, self adjusts for accuracy, and controls using little blasts of power with the solid state relay.

I did the exact project for 3D printed parts needing precise control of the temperature when inserting threaded inserts into printed plastic parts using a soldering iron.


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## speedre9 (Oct 12, 2017)

Thats way too much work. I'm looking for simple easy DIY project.


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## Doodle (Oct 13, 2017)

Get a battery powered soldering iron and just keep taking the individual cells out until you get the right temperature.


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## brino (Oct 13, 2017)

speedre9 said:


> Sliding it upwards increases the brightness of the light . Therefore
> it is not decreasing power but increasing it, ergo, this curcuit does not work for what I want.



Ahhhh, but sliding it down does dim the lights! Right?

The light dimmer cannot truly "add power to the outlet" what with those pesky thermodynamic laws and all.

Think of it like this:
If your dining room lights had just a simple on/off toggle switch, they would either be OFF (no brightness/no power) or ON (full brightness/full power).

All the dimmer is doing is giving you a range of brightness/power somewhere between OFF and ON as selected by the slider/knob.
It does this by allowing the load (the lights in this case) to only get power for a certain amount of time.....some fraction of the 60Hz cycle.
It does not change the maximum amplitude of the voltage supplied to the load, just the amount of time the load gets power.
It's like a little man throwing the toggle switch on and off really fast.....so fast you can't see it!

Once turned on, those lights look like a simple resistive load. They pull some amount of current based on their resistance.
Your "heating element only" soldering iron will also look like a simple resistive load.

You should be able to control the heat of your soldering iron very well with this approach.
(........as long as you follow the advice in post #3 above)

-brino


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## speedre9 (Oct 13, 2017)

Doodle said:


> Why not make it a fun project? Get a temperature controller with solid state relay and thermocouple. Mount it in a box and control temperature to with a couple of degrees. This project will get you to use CAD, Milling machine, lathe, and some hand tools. You will learn how the controller senses heat, self adjusts for accuracy, and controls using little blasts of power with the solid state relay.
> 
> I did the exact project for 3D printed parts needing precise control of the temperature when inserting threaded inserts into printed plastic parts using a soldering iron.
> 
> ...


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## speedre9 (Oct 13, 2017)

Doodle said:


> Get a battery powered soldering iron and just keep taking the individual cells out until you get the right temperature.


I've got too much money in this allready to buy another iron. I've bought three so far and a new digital mulimeter.


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## Doodle (Oct 13, 2017)

I have a bunch of these






It's a variac, a variable transformer. I have  perhaps 10 like new. I will draw a schematic and throw it in the box. You pay shipping and I will give it to you for free.

I would stick it in a Post Office box that sends flat rate.

If that sounds good to you send me a private message.


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## speedre9 (Nov 4, 2017)

O.K. Lets get back to my first post regarding the dimmer switch circuit.
 I searched "how to measure voltage" found too many to be clear but, most were like this image, is this correct?? If not put me on the right road.
	

		
			
		

		
	

View attachment 245911


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## RJSakowski (Nov 4, 2017)

No attachment!

Personbally, I use a 50 or 60 watt iron.  My 30 watt irons were way too slow.  But I think you are overthinking this.  If it were me, I would adjust the dimmer until I got the results I wanted.  I have two variable temperature irons,one with a temperature meter and the other without.  Either works for me as I judge the setting by how the iron performs.  

When I solder pc boards, I will turn the iron all the way up to 500 C because I m making about 1 solder joint a second and pulling heat away from the tip fast enough to keep the temperature down.  On the other hand, if I am doing some delicate work, I will set the temperature at around 300 C so I don't overheat my work and the iron doesn't oxidize.


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## markba633csi (Nov 4, 2017)

Have you tried your dimmer circuit with a lamp plugged in instead of an Iron to see that it actually dims? 
If it doesn't work then you must have miswired it or the dimmer is bad.
Mark S.


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## speedre9 (Nov 7, 2017)

I have decided to give this project a rest for now. Thank you all for the help you have given me.


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