# Soldering iron conversion?

Discussion in 'The Projects Forum' started by Max12345, Oct 4, 2013.

1. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Greetings
My old 25W Antex soldering iron has just packed up and I was given an old Weller. The problem is, is that it's a 60W iron, and I don't want to burn components or lift tracks.
Our AC supply is 220V.
Can I use a 390R, 50W resistor in series with the iron's supply to drop the voltage? It doesn't have a transformer in it.
The math I used is as follows:
Voltage: 220V AC (required voltage is 110V AC)
Power: 60W (Which should drop this to 30W?)
Required voltage drop over resistor: 110V AC
Current drawn by iron at 110V AC, and power of 30W: I=P/V = 30/110 = .28A
Therefore R=V/I = 110/.28 = 393R or 390R.
And power=VI = 110x.28 = 31W
Is my math correct. and will it work?
Thanks
Max

2. ### #12 Expert

Nov 30, 2010
16,681
7,322
P = E^2/R
60 = 48400/R
60R = 48400
R = 48400/60
R = 807
P = E^2/R
P = 110^2/R
R = 110^2/807
P = 15W

That's one way. You could also buy the right heating element from Weller. I did. They last for decades!

Or, you could put a diode in series with the heating element, but that doesn't cut the power in half. Some strange math about average voltage of the single polarity lumps of a sine wave. Somebody else will contribute the math for that one.

Two to one transformers aren't all that easy to find in some old appliance.

3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,150
769
For 25W you will need 142V ac, V^2 = WxR,

your iron is 60watts at 220volts, thats a resistance of 220x220/60= 806.66 ohms,

25Watts x 806.66Ohms= 20166 sqroot= 142Vac

best to use a light dimmer to alter the wattage.

Last edited: Oct 4, 2013
#12 likes this.
4. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Hi
Thanks #12 and Dodgydave for your replies and help with the math!
I really would like a temp controlled iron, but that's not on the cards right now. I like the diode idea 'cos it's quick and cheap. If that does not work well, I'll look at the other options.
Regards
Max

5. ### #12 Expert

Nov 30, 2010
16,681
7,322
I gotta say, the Dodger came up with a good one. Plain old light dimmer.

6. ### sheldons Well-Known Member

Oct 26, 2011
616
101
orrrrr you could just BUY a replacement soldering iron off ebay from Maplins etc....throw old iron away-buy new iron job done

7. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,971
1,135
Absolutely not. You'll need that for heavy-duty jobs

8. ### atferrari AAC Fanatic!

Jan 6, 2004
2,663
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Exactly what I did.

9. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Does the "old Weller" make a click sound? Most fo them do, and they ARE temperature controlled. These use a Curie magnetic switch behind the tip, which clicks on/off to maintain the right temperature. Temperture is set by the tip itself, which has a number stamped in it to show what temp it will operate at.

10. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
I've uploaded 3 pics of the iron with the inline switch which has the IN4007 diode inside. Seems to be working well, even if the wattage is more than half of the 60W.
I think I'll look at the dimmer soon. These irons and their heating elements are prohibitively expensive here.
As to the option of throwing this one away and buying one, especially from the UK at ZAR16 to the £...the shipping costs alone would force me to sell my house...lol.
The Weller does not make a click and there is no number stamped on the tip that I can see, although there is a spring behind the tip which I assumed was for better contact.
Anyway, the diode helped me thru the weekend, worked brilliantly and I have the option of going 60W or 30+W.
Thanks guys

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11. ### crutschow Expert

Mar 14, 2008
13,492
3,372
It does cut power in half for a constant resistive load. It's just simple math -- each half the waveform contributes 1/2 of the power so 1/2 the waveform equals half the power. It won't for a load whose resistance varies with power, such as an incandescent bulb.

Last edited: Oct 7, 2013
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12. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Cool, Crutschow. So I do have a 30W/60W iron now. Thanks for the response.

13. ### #12 Expert

Nov 30, 2010
16,681
7,322
Thanks crutschow. I must have picked up some bad information in another thread.

Jul 31, 2013
196
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I think the same. It looks like a temp controlled iron to me. Did you undo the ring nut and remove the tip, then look at the circular rear end of it to see a number ?
I have had a few problem over the years though of the contact failing and staying NC. So you might have a non temp controlled iron. But you can buy spare parts and fix it.

15. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
I must apologise to you, THE_RB. The After reading Cornishlad's post, I went back and had another look at the tip and there is indeed a number stamped on the "head", a 7. I googled this and it means that the iron is controlled at 700° F.
I have uploaded a pic which I think shows the Curie magnetic switch, or part of it anyhow.
I still have not heard any clicks, but I'll make a point of listening out for them now. It may also be that the Curie switch no longer works and it is permanently on.
Max

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16. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Yep that's a standard old temp controlled Weller.

All the parts should be readily available, those irons were popular and sold in large numbers.

17. ### crutschow Expert

Mar 14, 2008
13,492
3,372
If it's a temperature controlled iron then you don't need to reduce it's power. Actually the extra power is handy if you need to solder a large wire.

18. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
crutschow, I need to reduce the power as 60W is a bit hectic for PCB and components. However, I have the capability to switch between 30W and 60W (for the heavier applications).

The RB, I think that maybe I don't hear the clicks becaust the iron only operates at half power, so it never reaches full operating temp...I think.

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Jul 31, 2013
196
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There is nothing better than a temperature controlled iron with 50 or 60w rating for soldering small components. It will heat up quickly and stay at exactly the correct temperature. Lower powered irons can loose heat especially when soldering a number of joints in quick succession.

This can result in overheating a component while waiting for the heat to come back. There are a number of tip styles available for the Weller iron and the you should choose the right one for the job. The small point is fine for ic's.
Certainly if you can't fix the temp control I suppose you could stick with vagaries of the 30w...but if you can fix it you will never look back !

20. ### Max12345 Thread Starter Member

Aug 27, 2013
63
0
Any ideas about how I can check whether the temp control is working? I don't have a thermometer, but I sure would like to know.