Potentiometer on 12V Soldering Iron

i also want to measure the temperature of the iron with this thermometer, but it only measures up to 158F which is cutting it a bit close, but i don't think its software limitations, just the point where the plastic wires will start melting, at least thats my theory. anyone know if I'm right, or if there is a way around this? i don't want to buy an expensive thermometer. could i just buy some high heat resistant wire, and use it to replace the wires on the thermometer?

1. Yes you can put a potentiometer in series and theoritically reduce the temperature (I²R ) but then these Irons are supposed to have the temperature controller and the input power is regulated by it to maintain the temperature. In that case this will not work.

2. 158^oF is only about 71^oC I don't think the plastic/Insulation will be limitation for this, nor the element. It is only the scale calibration. Could not see the exact spec hence unable to comment.

these irons aren't regulated. how big of a potentiometer would i need?

with my 120 volt (regular house outlet type) 25 watt iron i can melt through project box plastic, and project box plastic is quite thick and hard.

P=I*E, or I=P/E=30W/12V=2.5A This is the normal current drawn by your 12V pencil.

E=I*R, or R=E/I=12V/2.5A=4.8Ω This is the normal (hot) resistance of your pencil.

If the 10Ω, (20W or bigger) variable resistor suggested by PWSlack is too expensive (You should be able to find one for a few bucks or less at a surplus electronics place), then you could experiment with one or more fixed resistors, A 4.7Ω resistor (a common standard value) would cut both your voltage and your current in half, so would cut your power to one quarter. It would be carrying around 1.3 A, so it would need to have a power rating of around 10 Watts (including a safety factor). If that is still too hot, get a slightly higher resistance; if it is too cool, get a lower value. Any old electronics buff like myself has a box or drawer with a bunch of such resistors, and would be willing to loan you a few to try, and either give you the correct value or sell it for under a buck.

Note that you are going to need some form of cage or enclosure around the resistor to avoid burning your upholstery or yourself.

12VDC-120VAC Inverters are so cheap now, that I always have one in my car. That way I can use my computer without running down its battery, and also use my 120V soldering pencil when necessary.

I'm borrowing your circuit, it has a whole lot of applications besides the iron control.

Thanks

Yahlasit

Me too! GA!

Thanks!

Dick

In fact I liked it so much, I drew it up in CAD. Here is the result. Please check it for accuracy!

Thanks again!

Dick

what components do i need to "tweak"? sorry, i don't know much about how these different components work.

Surplus MOSFET suggestions below. First one will run a little warm (~0.85W) and may need small heat sink. Second one should be fine without a heat sink.

http://www.goldmine-elec-products.com/prodinfo.asp?number=A20393

http://www.goldmine-elec-products.com/prodinfo.asp?number=A10095

The values shown should work for your 12v soldering iron. Try it as-is first. Tweaking may only be needed if you want to change the frequency or trim the min/max duty cycle limits. I could describe how each value affects the circuit, but I'd be typing all night. There is a golden IPA calling my name and it has a higher priority. Sorry. ;-) Cheers.

Hello mjb,[p]

GA to you Sir........

Rakesh is right on the money. Throw a lamp dimmer in the mains line.

Uh, those light dimmers are great stuff for AC where they chop off part of the waveform but they won't do squat on a DC circuit... except burn up that is.

Does it matter operating an iron on AC or DC? I can not understand how it matters?its mealy a resistance, Its AC or DC its going to heat.

It doesn't matter as far as the soldering pencil is concerned (I refuse to call anything that small a soldering IRON), but it sure does matter when it comes to controlling the input power! A transformer (fixed or variable) works fine on AC, but it's useless on DC.

Transformer is out of context here, Its nowhere in talk. Question is will a resistive load work on both AC and DC and answer is yes, So its a AC or DC regulator,both are going to work fine with a soldering iron no matter how big is it.

so i can use a regular dimmer and it will control the voltage going into the iron?

Yes,of-course you will have to customize it to your requirements.and if you plug it, in between an AC source and soldering iron only.No cigarette lighter,

well it needs to be for my 12 volt car socket

You can try this duty cycle controller.

A standard light dimmer plugged into a 120Vac source will control a soldering iron designed for 120V operation. I use one like this all the time.

A standard light dimmer plugged into a 12Vdc source (car battery) will NOT work with any soldering iron.

Radio Shack (online) does appear to have the basic parts needed to build a version of the simple chopper circuit posted. I'm used to getting these parts at bulk/surplus pricing (whole circuit < $1.25), but I think it will cost you closer to $10 at RS. Is it still worth it? Your call.

yeah $10 is good. but i cant find either the lm393 or lm2903, and i don't know what would be equal to them, i also don't know what an n-fet is.

i have now gone through the radioshack website and have found everything aside from the c1 and c2, the n-fet, and the pot. i don't know which pot to get. there are three

1

2

3

Radio shack does appear to have an N-Channel MOSFET which should work, but will require a small heat sink (worst case ~2.8 W). Suggestions below.

data sheets
http://www.irf.com/product-info/datasheets/data/irf510.pdf
http://www.national.com/mpf/LM/LM339.html

N-Channel MOSFET
http://www.radioshack.com/product/index.jsp?productId=2062618

LM339, basically the same except 4 comparators inside chip
http://www.radioshack.com/product/index.jsp?productId=2062593

linear taper pot 100k
http://www.radioshack.com/product/index.jsp?productId=2062287

heat sink for TO-220 package MOSFET
http://www.radioshack.com/product/index.jsp?productId=2102856

thank you very much for the time you have given me!

i only need one N-Channel MOSFETand one lm339 correct?

Correct. Only one of each is needed.

Be careful when initially testing circuit. A bench-top power supply is preferred because they usually have a current limit setting. Incorrect wiring of parts and testing directly on a car battery can lead to large current surges which can EXPLODE parts and melt wires. Double (triple) check your circuit wiring before connecting to the battery.

ok, i have soldered together most of the circuit. But i don't know which leg from the nfet goes where. and I'm not completely sure how the pot should be wired. so i drew a bad picture of what i think is the right way to solder them

The pot connection looks OK. Full CCW should be min power and full CW should provide max power. Just like a normal volume control.

The FET wiring looks incorrect. See clip below for pin labels.

thank you!

it works great!
thank you so much mjb1962853!

I'm glad it worked for you. Best wishes.

As you should have seen by now the requirement is for a method of controlling energy from a 12VDC source (cigarette lighter type output) to power a 12VDC soldering pencil.

To my knowledge, there are two basic possibilities: 1. Throw away part of the energy somewhere else, as in a rheostat controller, or 2. Use some form of chopper (PWM) to turn off the power part of the time.

Exactly, and you'll se how much money you can save on your car's electricity bill.

Thank you Rakesh

Thankyou Tripcalspeed

1. A big ol' 10Ω rheostat would do the job, though it's a bit wasteful.
2. Another possibility would be a speed controller for a model railway application - preferably one intended for Gauge 1, as it needs to deliver some amps.
3. How about a bench-top power supply unit, appropriately rated?

Yes you are correct.

rakesh_semwal

Dimmers are made to be used on your houses power lines (120 volts at however many amps). my cars cigarette lighter plug only has an output of about 14 volts, i don't know the amperage here either, but i doubt its very high.

"Why to put a fuming hot goblet beside you?" what?

PWSlack

1 quick search. seems like this coasts more than my thermometer and the iron put together. I'm meaning very low here

2 don't have one of these, so again too much money, also i want the iron to run off my cars power.

3 i do have one of these, but like the last one, i need it to run off my cars power

"Why to put a fuming hot goblet beside you?" what?

Rheostat, isn't it?

Come on Rakesh, you're very good at other stuff, but this time you're not even close, just get over this OK?

How come? Don't you know controlling power with resistance means something heating somewhere? Is it first time someone saying?

come-on!!!!!

You can connect a wire wound variable resistor (Potentiometer) of 5 Ohm, 10 watts capacity with suitable arrangement for mounting, considering heat dissipation - there will be more heat generation (in potentiometr) at lower setting. The power output at Soldering iron will be in the range of 7.2 Watts to 30 Watts depending on setting of the variable resistor. This will be connected in series with the soldering iron.

Very good. Precisely what I was thinking of. The cheap way to go. GA

Very good, yes, but not precisely what your previous posts reflected.

Yahlasit

In my previous post, I threw out there an AC operating device without reading throughly and discovering that the application requires 12VDC. A simple PWM circuit with a load-control will do nicely. As a footnote, why should I explain myself to someone who is logged on as a Guest anyway?

Hmm, interesting, you posted the best answer in this thread along with a schematic but didn't even get a reply; but when you posted basically the same without the schematic you even got a GA !

I wander what it means...

Yahlasit

If you want to keep it simple there are low voltage multi tapped transformers available, one I am looking at has 0v-15v-17.5v20v-24v-27.5v-30v, you can switch to any of these taps to give you a range of voltages. Also you will know that the temperature output will be repeatable for any of these tap positions.

You will have no power loss and a quiet circuit with no switching noise.It will be cheaper than a 3amp WW potentiometer.

Hah sorry just noticed you want to run it off a car battery not stated in your origonal post.

Just strap a 10Amp power mosfet to a heat sink and regulate it gate bias with a cheap potentiometer feed your soldeing iron through this.

Power Mosfet? thats Ok for switching, not for proportional linear output.

Yahlasit

Rubbish its perfectly fine I have used them many times for power regulation. Go and study MOSFETs circuits some more.

Garth

The key words are "12 volt" and "car". Lets brute force this and just wire tailight bulbs in parallel to get the wattage to power it and then add them in series to change the temperature needed. Headlights could be used since they are higher amperage (resistors).

The added plus is that they can also be used as work ilumination!

Regards.

Is it an AC or DC operated?

If AC put a diode in series & a push button NO [Normally open] in parallel to diode; on handle.

Iron will have nearly half the voltage [one 1/2 cycle on 1 off]

Push to solder & release to standby.

In case of DC Pot will not do unless it is at least 20W & you have to select its value for your optimum requirement.

dc. I'm going to run the soldering pencil/iron off of the cigarette lighter socket (whats the proper name for these?) in my car.

i just want a real simple (cheap) solution, a solution were i can buy everything i need from radioshack (no other electronics store near by)

Hello vn,

I have found a complete circuit and switch for your iron. I have not read any other posts so please forgive me if this has been posts already.

Can you please let me know if it is suitable for your use? Tell me what country you live in and I will try and find another if this does not suit. Take care.

Item Number : C6724 Unit Price: $11.05 Quantity Refer this page to a friend

sorry, but not what i was looking for. the input voltage would have to be around 12 volts, not the output. I'm trying to power my soldering iron using a car 12 volt cigarette socket. my problem has been fixed already though. but thank you for trying to help :)

Wouldn't work to power the iron off AC. Although a nice little power supply, current is too low @ 0.125A. Project needed 12V @ 2.5A.

Hi mjb,

Thanks for the nice reply.

I realised about an hour after I sent the answer in that I had made a mistake. My usual mistake of not reading the request correctly!. Sorry to you and all!

I do not know what the iron is used for, but, there is some really handy gas powered 'pen' type irons around which may have been better?

Thanks again

bb

Many years ago I ran across a circuit for precisely controlling the temperature of a heater by measuring the resistance of the heater during the AC zero crossing, and controlling the phase shift triggering of a triac. I have been looking for this circuit ever since to no avail, anybody know of it?

Thought occurred to me that the chopper circuit/s provided as good answers could incorporate such a circuit for measuring the heater resistance during the "Off" time of the switch, modifying the PWM to accurately control the temp of the soldering iron (Excuse me "Pencil"). Also perhaps incorporating a display of the temp as a function of the controller circuit.

Regards CEKM

Hello CEKM,

Sounds interesting!

I have searched and found this page if you would like to view it?

http://www.google.com/search?client=safari&rls=en-gb&q=control+circuit+measuring+the+%22AC%22+crossing&ie=UTF-8&oe=UTF-8

Take care.

Thanks for the info, I'll check out those sites.

regards CEKM

Consider the following

1. Try an iron with temp controlled tip.

2. Yes make sure the pot is rated to carry the current of approx 3 amps.

3.How to check the temprature

4.heating effect proportional to square of current and or voltage.

5.pot should not be linear but wound to square law.

6 lifes too short see 1 !

Have you ever seen a temperature controlled soldering pencil that ran on 12VDC?

I actually have a home brewed 12VDC "pencil"...has outlast all my soldering irons that ran on 240VAC

Yes!!!

The good tips have a magnet in them that holds a contact together and supplies current. When the temperature is reached where the magnetism is gone, a spring separates the contacts and removes current....

A very well known company called Weller, used this system with a mains to 12 volt transformer for years!

The tips are stamped with the temperature that the magnet drops out at.....I would guess they are still available.....not having personally tried though.....

They will work fine on 12 volts DC too....

BUT, I like the PWM approach better as you could calibrate the pot to select the temperature you wish.....whereas the Weller approach, you need to swap bits for different temps....

Sounds like you need to upgrade to an iron with temp. control. Not that expensive.