# Temperature control of a soldering iron

#### RichardO

Joined May 4, 2013
2,270
A recent thread on soldering irons reminded me of an idea a had for some time...

Do you think it is possible to measure the temperature of the soldering iron by measuring the resistance of the heating wire (probably NiChrome)? I checked -- Nichrome wire has a tempco of about 0.04% / degree C.

I visualize measuring the current or resistance during one half of the ac power line cycle to get the temperature. I would control the power during the other half of the cycle using an SCR to set the temperature.

#### cmartinez

Joined Jan 17, 2007
8,257
A recent thread on soldering irons reminded me of an idea a had for some time...

Do you think it is possible to measure the temperature of the soldering iron by measuring the resistance of the heating wire (probably NiChrome)? I checked -- Nichrome wire has a tempco of about 0.04% / degree C.

I visualize measuring the current or resistance during one half of the ac power line cycle to get the temperature. I would control the power during the other half of the cycle using an SCR to set the temperature.
Yes it would be possible... but I'd use a pwm MOSFET instead of an SCR if I were you, for accuracy's sake... Also, the wire would have to be part of a Wheatstone Bridge that you'd have to read periodically through an ADC

#### RichardO

Joined May 4, 2013
2,270
Yes it would be possible... but I'd use a pwm MOSFET instead of an SCR if I were you, for accuracy's sake... Also, the wire would have to be part of a Wheatstone Bridge that you'd have to read periodically through an ADC
I think the thermal time constant is long enough that I don't have to do PWM during the half cycle I am controlling the temperature. In other words, the PWM would be on full cycle increments using zero cross switching. This has the advantage of not generating a lot of RFI. Of course, a high voltage MOSFET could be used in place of the SCR if it is only switched on and off at the zero crossings.

I would hope that a bridge is not needed. I am hoping that a current sensing resistor or transformer followed by some gain will work.

#### paulktreg

Joined Jun 2, 2008
835
"Probably NiChrome" isn't good enough?

If you don't know the exact specification of the heater element wire then it's going to be hard to control its temperature without installing a temperature sensor.

Joined Jan 15, 2015
7,522
If this just a standard soldering pencil with just a simple heating element and you want to experiment a little for about $13 you can go to your local Lowes and buy one of these. Lamp dimmers (SCR) work pretty well for controling a soldering pencil temperature. As to the actual tip temperature, the temperature where the work is done, I have been seeing these Hakko FG-100 Soldering IRON Tip Thermometers for$12 to $15 by the truckload. Note a few temperatures and index the slide on the dimmer. If nothing else it should give you some data to play around with. Maybe to compare to what you get measuring current. Granted this is not true temperature control but for a few bucks it may give you something. Ron Thread Starter #### RichardO Joined May 4, 2013 2,270 If this just a standard soldering pencil with just a simple heating element and you want to experiment a little for about$13 you can go to your local Lowes and buy one of these. Lamp dimmers (SCR) work pretty well for controling a soldering pencil temperature. As to the actual tip temperature, the temperature where the work is done, I have been seeing these Hakko FG-100 Soldering IRON Tip Thermometers for $12 to$15 by the truckload. Note a few temperatures and index the slide on the dimmer. If nothing else it should give you some data to play around with. Maybe to compare to what you get measuring current.
Granted this is not true temperature control but for a few bucks it may give you something.

Ron
Thanks for the thoughts.

I already have some low voltage irons to experiment with -- no 120 volts needed.

My goal is to use a "high" wattage iron for soldering small SMD parts and yet have enough excess capacity to solder a ground plane when needed. To do this, I believe that temperature control is required.

#### GopherT

Joined Nov 23, 2012
8,009
I think the thermal time constant is long enough that I don't have to do PWM during the half cycle I am controlling the temperature. In other words, the PWM would be on full cycle increments using zero cross switching. This has the advantage of not generating a lot of RFI. Of course, a high voltage MOSFET could be used in place of the SCR if it is only switched on and off at the zero crossings.

I would hope that a bridge is not needed. I am hoping that a current sensing resistor or transformer followed by some gain will work.

Here is another option.
C1 must be non-polarized and good for 200 vac. The associated text says 0.1uF.
Standard AC lamps are used. You can see the power change with the brightness of the bulb. Use only incandescent bulbs if you can find them.

This is from an old G. Randy Slone's Guide to Understanding Electricity and Electronics.

#### RichardO

Joined May 4, 2013
2,270
"Probably NiChrome" isn't good enough?

If you don't know the exact specification of the heater element wire then it's going to be hard to control its temperature without installing a temperature sensor.
Agreed. The difference between tungsten and NiChrome is 10 to 1.
An equally hard to deal with problem is that the absolute resistance is different for different wattage irons. My (lofty) goal is to have a controller that works independent of soldering iron wattage rating.

#### RichardO

Joined May 4, 2013
2,270
Here is another option.
C1 must be non-polarized and good for 200 vac. The associated text says 0.1uF.
Standard AC lamps are used. You can see the power change with the brightness of the bulb. Use only incandescent bulbs if you can find them.

This is from an old G. Randy Slone's Guide to Understanding Electricity and Electronics.

View attachment 79067

#### cmartinez

Joined Jan 17, 2007
8,257
A few years ago I did a study on how to control a nichrome wire's temperature. @thingmaker3 helped me a big deal. Unfortunately he has not been around for a long time, and I haven't had the chance to thank him.... Anyway, nichrome wire does have a temperature dependent resistance, but it's not linear... also, on certain alloys, the resistance peaks at certain temp, and goes down after that, which is a big no-no for what you want to accomplish.
In the end, I came to the conclusion that nichrome 60 was the best alloy for my application.

Here's a graph that you might find valuable for you application:

Here's a link with nikrothal's 60 detailed properties.

#### Alec_t

Joined Sep 17, 2013
14,327
Back in the 70's, with the same intention of using the element's resistance as a temperature sensor, I found the hot/cold resistance difference disappointingly small. With modern ADCs etc perhaps that would be less of a problem. Time to revisit the concept perhaps. Where's my meter .......

#### cmartinez

Joined Jan 17, 2007
8,257
I found the hot/cold resistance difference disappointingly small.
That's why I suggested the wire should be part of a wheatstone bridge

#### ian field

Joined Oct 27, 2012
6,536
A recent thread on soldering irons reminded me of an idea a had for some time...

Do you think it is possible to measure the temperature of the soldering iron by measuring the resistance of the heating wire (probably NiChrome)? I checked -- Nichrome wire has a tempco of about 0.04% / degree C.

I visualize measuring the current or resistance during one half of the ac power line cycle to get the temperature. I would control the power during the other half of the cycle using an SCR to set the temperature.
A few of the big brands already do that.

#### RichardO

Joined May 4, 2013
2,270
A few of the big brands already do that.
Darn. It sure is hard to think of something new.... especially in the Internet era.

Do you know what brands do this? Some industrial espionage may be in order.

#### RichardO

Joined May 4, 2013
2,270
I guess its time for a first cut at some specifications...

Soldering iron controller features:
1. Works with nearly any inexpensive iron.

2. Accepts irons rated for wattages from 10 watts to 100 watts.

3. Controls the temperature (not the power) of the heating element.

4. A calibrated knob sets the temperature.

5. The temperature can be set from 200 to 450 degrees C.

6. A second knob may be needed to "tell" the controller the wattage of the iron.

7. Does not generate RFI/EMI.

8. Uses microcontroller for simplicity???

The iron's temperature versus resistance could be calibrated at three points:
Cold (room) temperature, half power and full power.
The disadvantage of this is that the iron would have to cool off before being turned back on.
This would be a nuisance when the iron is turned off to change the tip, for instance.

Research needed:
What is the temperature of an iron when run at half power?
What is resistance of NiChrome wire of different gauges?

#### cmartinez

Joined Jan 17, 2007
8,257

#### RichardO

Joined May 4, 2013
2,270
I guess you haven's seen my post #10
Oops. Those notes were written before I read your post. Thanks for the real-world data.

#### RichardO

Joined May 4, 2013
2,270
I finally got around to making some measurements of one of the low voltage soldering irons I have.

It is an Oryx model 7240 rated at 6 volts, 6 watts and 690 degrees F.

I measured 7.2 ohms at 12 degC (54 degF), 7.14 ohms at 112 degC (233 degF) and 7.14 ohms at 303 degC (576 degF).

So much for my theory of using the heating element to measure the temperature of the soldering iron temperature.
This is a really small iron. I am now wondering if larger irons act differently.

No doubt more to follow.
Too many projects, too little time. See my post on fast pulsers.

#### cmartinez

Joined Jan 17, 2007
8,257
I finally got around to making some measurements of one of the low voltage soldering irons I have.

It is an Oryx model 7240 rated at 6 volts, 6 watts and 690 degrees F.

I measured 7.2 ohms at 12 degC (54 degF), 7.14 ohms at 112 degC (233 degF) and 7.14 ohms at 303 degC (576 degF).

So much for my theory of using the heating element to measure the temperature of the soldering iron temperature.
This is a really small iron. I am now wondering if larger irons act differently.

No doubt more to follow.
Too many projects, too little time. See my post on fast pulsers.
I'm interested... why don't you link us to that fast pulsers post of yours, so wee can take a look?