Hakko936 clone - won't stop heating  - Page 1
Forum section: electronics / projects

author: martik777
author_rank: Newbie
timestamp: April 29, 2017, 02:55:42 am
content: The iron heats up enough to easily melt solder in about 30secs but the red light never goes out. The transformer I used generates 26v rather than the 24v specified. Would this make a difference?Watt meter measures a constant 50.1w

author: jeroen79
author_rank: Frequent Contributor
timestamp: April 29, 2017, 05:44:19 am
content: What do you mean by "the transformer I used"?Did you modify it?You'll need to test the various parts.Is the thermocouple in the iron working?Is the TRIAC in good order? (ie open until triggered)What can you measure around the inputs of the opamps?What test equipment do you have?

author: martik777
author_rank: Newbie
timestamp: April 29, 2017, 07:44:44 am
content: I purchased the board on ebay and used my own transformer, made an enclosure and added a switch.All I have are several multimeters and the energy meter and a Infrared digital thermometer.Not sure how to test the tracs or opamps.I measured approx 20mv on the end of the cable after 2 mins of heating so the TC is working. The reading decreases as it cools.Probably best to buy another, they are under $5.

author: DBecker
author_rank: Frequent Contributor
timestamp: April 29, 2017, 08:05:05 am
content: What control board are you using?Does the soldering iron heater match the control board type? A thermistor control board won't work with a thermocouple heater, and vice-versa.But if it's a digital display board, you may have simply fried the output MOSFET.I've been trying to find an inexpensive digital display control board. I've been surprised at how utterly crappy the designs are. They typically use a 30V P-channel MOSFET as the output switch, while claiming to support an input voltage above that. (A 24 VAC transformer might peak about 40VDC with a switching load.) Many are switching the MOSFET gate with the full input voltage. (Most MOSFETs spec +-20V as the maximum gate voltage, rarely 25V.) And there is no protection against inductive kickback. (Typically ceramic encapsulated tungsten heaters use a zig-zag pattern to minimize inductance to about a single turn, while nichrome wire heaters are wound just like an inductor. This doesn't matter with triac control, but can generate quite a kick with a hard-switching MOSFET.)The typical thermocouple amplifier is a LM358. Anyone here would reject that opamp for the purpose as it has a high DC offset, but it seems to work. A T12 tip puts the thermocouple in series with the heater. It's a clever design that simplifies the construction, puts the thermocouple close to the tip, and uses only two wires (plus a ground). The challenge is handling the full heater voltage (35V when using a 24V transformer) immediately before sensing a millivolt-scale signal. Some use a series resistor and a diode clamp (usually 10K and a 1N4148). Other boards simply run the signal directly into the opamp input, with no limiting.

author: martik777
author_rank: Newbie
timestamp: April 29, 2017, 09:08:00 am
content: That may be the problem. The controller is for an A1321 heater/ (thermistor) and the handle probably contains a 1322 heater/(thermocouple).Here is a similar item:http://www.ebay.com/itm/HAKKO-936-Soldering-Iron-Station-Controller-DIY-for-907-Iron-A1321-Heating-Core-/181794950587?hash=item2a53d2cdbb:g:DRQAAOSwHnFVm4hXFrom what I've read the 1321 heater has a resistance of ~ 3.6 and the 1322 is ~12 ) Mine reads 13ohmshttps://www.banggood.com/For-HAKKO-936-Soldering-Iron-Station-Control-Board-Controller-Thermostat-A1321-p-1014469.html?rmmds=searchOn that board, you can supposedly change 4 components to enable use with a 1322.I may just replace the 1322 with a 1321 heater.

author: DBecker
author_rank: Frequent Contributor
timestamp: April 29, 2017, 02:57:48 pm
content: The ebay link will be obsolete when people read this later.The pictured product is a circuit board plus front panel for a Hakko 936 "replacement". (It's printed with the Hakko trademark, so it's a counterfeit rather than a clone or work-alike.) The circuit board is a full-width single sided brown board marked "936 v3.5" and "A1321". It has a single 8-DIP opamp, a triac and a bunch of through-hole discrete components.At the low end of the market you get a lot for your money, but the sellers generally have no idea if the $3 soldering iron has a A1321 or A1322 heater. They probably just copied the listing from another seller that had a similar looking item.The heater resistance isn't useful for identifying the sensor type. There is some correlation with the sensor, but it's not reliable. What is your sensor resistance? Are the wires color coded to indicate sensor polarity, or only to differentiate them from the heater pair? A thermocouple has polarity and a 1-3 ohm resistance. A thermosensor resistor has a 40-60 ohm cold resistance and no polarization.

author: martik777
author_rank: Newbie
timestamp: April 29, 2017, 08:13:38 pm
content: You are right, sellers probably have no idea of the heater type, interestingly, the ones that specify a 1321 charge 3x the price.Sensor resistance is 2.3ohms increases with temp rise.It has polarity evidenced by readings while heating up and switching leads.So, it appears to be a thermocouple.See colour coding in the photo