Reduce soldering iron element switching stress?

My home made temperature controlled iron's element failed again. I know these are cheap elements but they should last longer than this! (Hakko style but mains voltage)

I'm guessing since the iron is constantly being switched on and off it is putting abnormal stress on the element, so I was wondering is there a simple way to create a more gentle power cycle, perhaps giving one or two seconds to power up/down?

Almost unbelievably, I managed to repair the original element, so usable for now...

Two ways come to mind:

- Reduce the temperature limit when the element switches on, and increase the limit when it switches off. This leads to greater temperature fluctuation, but decreases the switching frequency.
- Increase the thermal mass (bigger tip). This stores more heat, therefore it takes longer to heat up and longer to cool down. This does not increase the temp fluctuation.. just decreases the switching frequency.

..or just buy a proper iron. They are not that expensive.

Actually it sounds strange that the element would fail because it is switched on/off too often.. I think the problem is something else.

Hello there,

Are you sure the element is made for AC and DC or is it just for AC? Some elements are made for AC only.

If you can use DC and you still think it is the switching, you can always incorporate an LC filter. In fact, build it as a buck converter.

How about a dual-current heating method. Can you estimate the power input to the iron while it is idling in its stand? Say you determine that it takes 20W just to keep it hot at slightly below the ideal soldering temperature, but the heater is capable of a peak power level 200W when the thermostat is full-on.

I would wire a power resistor across the thermostat contacts so that the resistor limits the power input to the tip tothe idling power. When you first turn on the iron, you will get 200W to heat it quickly, the first time it reaches its set point, the thermostat opens, dropping the power from 200W to ~20W, at which point the tip begins cooling very slowly. The thermostat will kick in, but much less frequently that if you shut the tip totally off.

To drop the dissipation in the resistor, put a rectifier in series with it. This will reduce the duty-cycle of the AC waveform by half.

MikeMl said:

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I would wire a power resistor across the thermostat contacts so that the resistor limits the power input to the tip tothe idling power. When you first turn on the iron, you will get 200W to heat it quickly, the first time it reaches its set point, the thermostat opens, dropping the power from 200W to ~20W, at which point the tip begins cooling very slowly. The thermostat will kick in, but much less frequently that if you shut the tip totally off.
...

Click to expand...

Great idea!

Another good idea is to just put a resistor permanently in series with the element, that drops the element power 5% to 10%. That power drop does not matter much although it does increase element life a bit, but the main thing that matters is that the turn-on surge current to the element is greatly reduced as turn on surge is now depending on cold resistance of the element PLUS the resistor in series.

The same trick works fantastic on incandescent light bulbs that are turned on/off a lot (like in an incubator).

MikeMl said:

...
To drop the dissipation in the resistor, put a rectifier in series with it. This will reduce the duty-cycle of the AC waveform by half.

Click to expand...

Isn't a series diode with an AC resistor pretty much the same as using a resistor of twice the ohms?

My Weller temperature regulated soldering iron is about 50 years old and still works perfectly. It is still manufactured. Its heating element is 24VAC and is fully switched on until the temperature sensor switches it off. It continuously clicks on and off to maintain a good temperature that never burns the tip. Then the tip lasts for many years.

It might be 60W.

My 25 year old xytronic uses zero crossing switching I thought that was to reduce switching noise for sensitive circuits, but maybe it does go easier on the heater too.

Its not that difficult to make a soft turn on circuit, but I've never heard of doing that on an iron element, maybe an adaptation of this circuit that gives you a controlled turn on plus a idle current as mentioned above, sort of a dimmer circuit with 2 settings full on and idle.

Might be better with a higher quality iron, your setup deserves it.

Mr RB said:

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Isn't a series diode with an AC resistor pretty much the same as using a resistor of twice the ohms?

Click to expand...

Not quite. Here is a test case where the voltage is 240Vac 50Hz. The heater resistance would have to be 279Ω to produce ~200W. Shown are two different ways of reducing the heater dissipation to ~20W. Note the required resistor values of R1 and R2 to do that. Then notice the power dissipation in the resistors (as computed by LTSpice).

**broken link removed**

What useful bunch of suggestions. I'll have to have a good think about the options. Possibly a bit of everything.

Mike I had to read your suggestion about 4 times before I understood it, what a good idea! You didn't attach the attachment for your test case though.

Oh I wish I could afford a better quality iron, dr pepper! I would have a genuine Hakko or Xytronic station or similar if I could afford it, I love the way this one handles and the style is the same.

Mr Al, the iron was sold as a 220v one, but when I looked inside the handle there was a diode in series with the element, implying it was really a 110v element. The replacement elements (pack of 2) were sold as 220v elements, implying they are intended for AC use, but when I turned off the tc (I have added an override switch to the station) I watched the temp climb up past 600 degrees before I turned it back to being controlled. The iron was originally sold as 40W (though don't think it really was - no power given in English on the the pack) and the replacement elements are sold as 35W

I did wonder about a diode in parallel with a resistor and a small capacitor, in series with the element (which on examination is just fine wire wound round a ceramic core, and inserted in a ceramic tube.)

Meantime, I have got a very cheap 30w Eagle iron. Horrible grip but they are supposedly reliable at least.

So next on this experiment, if mains "softening" doesn't work, I'm going to try to make a 24v 2.5A hfac supply, so I can put a 24v element in this handle.

throbscottle said:

...Mike I had to read your suggestion about 4 times before I understood it, what a good idea! You didn't attach the attachment for your test case though....

Click to expand...

Are you not seeing an image in post #8 of this thread???

MikeMl said:

Are you not seeing an image in post #8 of this thread???

Click to expand...

I'm only seeing the "broken image" icon.

I'm not seeing any images

throbscottle said:

I'm not seeing any images

Click to expand...

Ok, I'll try it again.

In reply to Romans comment:

Not quite. Here is a test case where the voltage is 240Vac 50Hz. The heater resistance would have to be 279Ω to produce ~200W. Shown are two different ways of reducing the heater dissipation to ~20W. Note the required resistor values of R1 and R2 to do that. Then notice the power dissipation in the resistors (as computed by LTSpice).

MikeMl said:

Are you not seeing an image in post #8 of this thread???

Click to expand...

No, there is no image.
Its properties says "Not Available". Please try posting it again.

MikeMl said:

Ok, I'll try it again.
In reply to Romans comment:

Not quite. Here is a test case where the voltage is 240Vac 50Hz. The heater resistance would have to be 279Ω to produce ~200W. Shown are two different ways of reducing the heater dissipation to ~20W. Note the required resistor values of R1 and R2 to do that. Then notice the power dissipation in the resistors (as computed by LTSpice).

Click to expand...

Thank you! I understand what you were saying now.