I've done it now....

[Duo]

Well, here it is, the lamest project of the end of the year 2005.

I found that a cheap soldering iron of mine simply got too hot to be really useful. I was always tinning it to keep from oxidation and the heat output was just too much.

My solution to the problem? PWM, yes, Pulse Width Modulation.

But the way I implemented it is what's so demented..

The idea came about easily and soon I began constructing the circuit from the top of my head. I started with an astable multivibrator using two transistors, then an RC integrator, then a single transistor comparator, then relay driver. Boy what a novel PWM device. It happens to operate at about 4Hz and with a pot varying the voltage to the emitter of the comparator, I can vary the duty cycle anywhere from 0% to 100%.

The verdict? As ugly as this is, it performs beautifully. The temperature of the iron is now easy to control at the turn of a knob and there's no longer any high heat output mess happening.

Just thought I'd add this archaicly styled circuit to the ideas of diy'ers...

[Christer]

Do you have any temperature sensing, or do you just try out what setting works?

[Duo]

There is no sensing. I didn't need it to have feedback.

I just base it on duty cycle. 50% duty is half the power, therefore half the heat.

It works well. I now can solder properly and not worry about the tip burning away.

[richie00boy]

50% duty is 1/4 the power

[Duo]

Yes, you are correct.

It's 4:00am, can you lay a blame on me?

[dnsey]

Mine's even more archaic - uses a simmerstat (cooker control) to do much the same thing

[Christer]

Originally posted by richie00boy
50% duty is 1/4 the power

No, only if you low-pass filter the voltage, which is hardly the case in this application. In this case, 50 % duty cycle would probably mean that there is 100 % power 50 % of the time.

[Duo]

You guys have me spinning now. I'm too tired to really think.

You are correct Christer in that it is 100% power half of the time.

Then again, in typical PWM systems, I believe richie00boy is correct.


EDIT: But let us not argue over the numbers too much. I just wanted to show my solution to the problem that faced me.

[richie00boy]

The thermal inertia of the iron will perform the integrating action akin to low-pass filtering so the net result will be the tip effectively sees 1/4 power

But yes electrically it's 100% power 50% time.

[Christer]

Yes, the thermal inertia will work as a low-pass filter for the temperature. That means the temperature will be fairly stable and the same as if we had had 50 % power 100 % of the time. What temperature we actually get is a different issue, though. The relationship between power and temperature is all but linear.