Home built hot air preheater - Page 1
Forum section: electronics / projects

author: CoffinDodger
author_rank: Regular Contributor
timestamp: September 09, 2016, 03:27:17 am
content: A few of the hot air pre-heat systems (Hakko FR-830, Zephyrtronics AirBath) are a tad out of my price range. So i want to make one my self by deconstructing a hot air gun and using a rheo to control the current to the heating element. I would like the output heat to be predictable and stable so what kind of extra control should i be educating myself on to add into the circuit? I'm assuming some kind of thermocouple and micro controller pairing. is an on-off switch and a rheo enough, and i'm just over thinking all this? as far as even heating i'm sure i'm going to need to get some sheet metal to form an even dispersing nozzle. Not too big of a problem there. Some feed back would be awesome. Once i get free time to build and test i'll share the results.

author: Berni
author_rank: Super Contributor
timestamp: September 09, 2016, 06:08:42 am
content: I bought a cheap no name brand one from china that works pretty well.The thing is basically a glorified hot plate. It has a ceramic dipped heating element inside of it and aluminum rails for holding the PCB about a cm or two above it. It only regulates the temperature of the heater element but when the board sits this close its a good enugh. The convective currents between them and the infrared radiation bring the board up to temperature nicely. It just wont heat up as quickly as it might if it had forced air blown up against it, but then again you don't get blazing hot air blown in to your face as you work on it.The danger of using a temperature sensor on your PCB is that if it falls off the heater goes full blast and burns your board to a chrisp. Also when the PCB is cold still the heater will go full blast while a plastic connector on the bottom side of the board has a bad thermal connection to the PCB since its plastic and if its tall its closer to the heater than the PCB. In that case the PCB warms up to set temperature nice and quickly but that poor connector ends up chared to a chrisp.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 09, 2016, 06:34:52 am
content: It's my understanding that preheating probably has the most tolerance of fluctuations or inaccuracy in temperature, because it's only the time spent at the higher temperatures that really can do the damage. With that said, I'd think even heating is the most important thing, so just making sure whatever heating element or air being directed is actually getting to the entire space you want to heat and trying to reduce the strength of any hotspots so they stay within acceptable levels.I don't know how it's setup to interface with your soldering/desoldering work - but you could probably do things with a standard hot air gun and a mount for it. Rely on the temp control from the gun to keep it stable and then use an IR thermometer to get an idea of how hot it is where the board is and adjust the temp setting accordingly. The IR therm does have potential emissivity problems, but if you use a known material as a target the first time, you can get pretty spot on, and again, I don't think accuracy is key with a preheating stage.You could get really fancy with ducting or something, but you could also go the cheap route and probably get good results with a hair drier attached to a stand of some sort - low setting for the fan and one of the three heat settings can probably get fairly stable temperature for a decent sized space with enough distance to work with, and cheap hair driers are typically less than cheap hot air guns :pAs a side note, I've done some PCB assembly with solder paste in an electric griddle and it does work pretty well, again using the IR therm to check and moving it occasionally to reduce the impact of the hot spots. A lot of electric griddles will also go down to pretty low temperatures - so while you'd have to check for evenness and temp first - if you're only dealing with a single sided load, you could just put the board in a griddle on low and then do what you will with the top side. Just don't use it for food afterwards!