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Soldering station for Weller soldering iron.

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Evalon

New Member
Hello,

I'm about to make this solder station for a Weller LR-21 soldering iron. I have some confidence it'll work, however, just in case there's something I've missed I'd appreciate your comments ... Please note that I do need the circuitry to be fully DC so AC voltage are not possible.

It's basically a comparator which compares the voltage across the PTC resistor in the soldering iron with a voltage that is regulated by the "temp-regulator" variable resistor. When the value of the PTC resistor drops relative to the "temp-regulator", Q5's basis is raised which starts current flowing through Q5 and which fully opens Q1 which outputs ~24.5 VDC to the soldering iron.

If you spotted anything in this that doesn't work I'd appreciate your comments....

Best regards,

Jesper
 

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MrDEB

Well-Known Member
I can assume your wanting to construct a temp controled soldering iron?
over at pcb heaven he has a pic controled soldering iron.
 

Evalon

New Member
Hi MrDEB,

& thanks for answering. Yes I would like to make a temperature controlled soldering iron. However, I would prefer to keep it analog (which a pic controlled iron is not, isn't that correct?) and have come up with the design shown in the attachment in post #1. I have now built a basic version of the circuitry (please see attachment to this post) and now see that it does not work as intended.

No matter what the voltages are relative to eachother (V- and V+ on the opamp), the voltage on the output of the opamp is never above 0,43 volts. But when I actively lift the voltage on the opamp's output to 5 volts (V1's output level - a 7805 IC) the output of the circuitry is about 0.6 volts below V2's voltage.

On all transistor bases and on the inputs to the opamp I have placed ferrite beads just in case there might be oscillations....

Any of you can help figuring out what is happening? It seems to be the opamp but it should be ok ...

Best regards,

Jesper
 

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BrownOut

Banned
I see no purpose in R11, R12, c3 and Q4. Why not just build a wheatstone bridge for the PTC?
 

Evalon

New Member
Hi - I guess I could do that but it still would require an opamp, wouldn't it? Any idea why this circuitry doesn't work (I'd like to stick with it if I can make it work)?

Regards,

Jesper
 

BrownOut

Banned
If I remember correctly, the lm311 is a comparator, not an opamp. Yes, it would still be needed, and the wheatstone bridge would feed into it. I see no reason to try to make the other junk work. I don't know why the comparator won't work. Seems like the + input should be around 5/3V, and so your sensing circuit should be centered around that value. I don't see a "temp regulator" resistor, as all your resistors are fixed values. The voltge at the - leg will be approximately: 5V - 680*.3V/R(PTC) but I know nothing of he PTC you're using.
 
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MrDEB

Well-Known Member
I went the easy way out and bought a Komec 936 soldering station.
I paid $56 on ebay but same item is now $98
Found same for $40 in another post I have. See Komec 936
 

ronv

Well-Known Member
Most Helpful Member
Maybe you could give the specs or PN on the themistor? I don't think the 2907 will be big enough, but maybe you just put it in to simulate the circuit?
 

Evalon

New Member
Hi,

Thanks for replying :) I have, however, now found out what the issue was: When the LM311 is to go towards its V+ it does so through 3 diode "stretches" and a 600 ohm resistor (can be seen on the schematic in the datasheet). This means a relatively big voltage drop inside the LM311 and since the LM311's exterior possibility to supply current elsewhere from V+ was a 15k resistor there altogether was a significant voltage drop - the output was 0,43 volts when it should have been about 4-5 volts. Replacing the 15k resistor with a 330 ohm resistor solved the issue.

Thanks for looking & suggesting.

Greetings,

Jesper
 
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