Tenma Digital Soldering Station - Page 1
Forum section: electronics / projects

author: SpiderElectronics
author_rank: Contributor
timestamp: May 08, 2013, 02:51:52 pm
content: In our workshop we recently bought a TENMA digital soldering station as it was going for a good price at a local retailer.In the true spirit of EEVBlog we didn't turn it on, we tore it apart.I was suprised to see that the is only 2 wires going to the iron, despite using a 5-Pin plug. Each of the two wires is soldered to 2 pins, presumably for current rating.So, how does thing measure the actual temperature of the tip? Does it just guess, or measure the current going through the element? Would that even work?I disassembled the iron itself and there is definitely no temperature sensor of any kind in there.In use you set a 'set' value and it then heats up to that value. Once there there display shows the 'real' value - but this never wavers, even if you dip the tip into something cool.

author: mariush
author_rank: Super Contributor
timestamp: May 08, 2013, 03:10:33 pm
content: Which model? There's lots of them.Tenma is a Farnell brand, they just buy OEM designs from Atten and other companies and sell them under their label.Your model may be one of those super cheap ones which just reuse the same handle but are a model without feedback.

author: SpiderElectronics
author_rank: Contributor
timestamp: May 08, 2013, 03:13:45 pm
content: Hi - it is this one:http://cpc.farnell.com/tenma/21-10115-uk/soldering-station-digital-esd-uk/dp/SD01738?in_merch=Featured%20Products&MER=e-bb45-00001001...even if it uses no feedback, the original question remains... hows does it know when it has reached the set temp, and how does it maintain that?

author: elgonzo
author_rank: Unknown
timestamp: May 08, 2013, 04:01:17 pm
content: Quote from: SpiderElectronics on May 08, 2013, 03:13:45 pmWithout pictures of the particular station it is hard to tell what is really going on inside this thing. (Perhaps other chains or no-name brands use the same plastic casing, but other cheaper/more expensive innards, who knows...)If it measures current to deduce temperature, you should be able to find a shunt resistor, likely with an op amp attached. But that would not make much sense, if the iron is controlled digitally. The control circuit would already know how much current it feeds into the iron and would not need current sensing (except for short/overcurrent protection if that would be necessary -- but even then a simple fuse would be much cheaper).My guess is, that the iron is controlled by a small, cheap micro which just uses a table of set current values vs. expected temperature stored somewhere in its memory. That would probably allow the cheapest manufacturing.

author: mariush
author_rank: Super Contributor
timestamp: May 08, 2013, 04:04:15 pm
content: That model is Atten 938D :http://www.atten.eu/solder-rework/atten-at938d-soldering-station-60w-150-450-c.htmCircuit Specialists also sell this model under their brand.If you can, post some pictures of the insides... as good quality as possible.

author: xygor
author_rank: Regular Contributor
timestamp: May 08, 2013, 04:32:11 pm
content: It apparently measures the resistance of the heater during the off time, or maybe by measuring the current. I'm not sure which.

author: elgonzo
author_rank: Unknown
timestamp: May 08, 2013, 04:52:59 pm
content: Quote from: xygor on May 08, 2013, 04:32:11 pmJust to clarify:It does seem to be measuring during off-time (like, the display signalling the iron being off), but not during on-time (like when you cooled it down). Is that correct?

author: xygor
author_rank: Regular Contributor
timestamp: May 08, 2013, 04:58:59 pm
content: It's pulse width modulated, so it would measure during the off time many times per second. I don't have actual experience with it; just what I inferred from looking at replacement heaters on ebay.

author: elgonzo
author_rank: Unknown
timestamp: May 08, 2013, 06:03:54 pm
content: Quote from: xygor on May 08, 2013, 04:58:59 pmIt's one of the possibilities how to do it, yes. However, if that is the working principle of your iron, then the temperature measurement seems to be broken (if i understand the term "not wavering" correctly, meaning "not changing").Well, look at the circuit, or post pictures of it. All about the iron's temperature measuring principle as of now is guessing...

author: xygor
author_rank: Regular Contributor
timestamp: May 09, 2013, 12:00:34 am
content: See this thread for a discussion on the similar atten 938d.https://www.eevblog.com/forum/reviews/atten-938d-soldering-stationDirect link to schematic:https://www.eevblog.com/forum/reviews/atten-938d-soldering-station/?action=dlattach;attach=33276