philpem’s blog

Once again, I’ve started an expensive project.

A few days ago, I was building up an LCD controller board, which involved soldering down a lot of small parts. Worst of all were the flatcable connectors for the LCD — these are made out of a plastic that seems to melt at about 100 Celsius. Not exactly good when your solder needs 175C just to melt, and its recommended working temperature is closer to 300C…

I figured there had to be a better way. I’ve been aware of [Kenneth Maxon's article in "Encoder"](http://www.seattlerobotics.org/encoder/200006/oven_art.htm) for a while, in which he explains how to turn a “toaster oven” (also known as a “mini oven” on this side of the pond) into a fully working soldering oven. An article on the same thing in the November 2007 issue of “Elektor” sealed the deal — I was going to build one of these things.

The first problem was finding a suitable oven. After a ton of Google searches, I came to the conclusion that nobody documented their projects to the degree of mentioning how powerful their SMD oven was, or even how big. There was a note in the Elektor article that an SMD oven should be “around 1500 Watts”, but nothing about the size of the oven cavity (perhaps furnace is a better word) itself.

I eventually settled on a Cookworks oven which set me back nearly £60, has a volume of 25 litres, and four solid-rod steel-cased heating elements providing a total of 1640 Watts of power. So that’s 410 Watts per element, or 820 Watts on the top and bottom of the oven. The built in thermostat tops out at 250C, and the whole thing is put together with screws, not rivets — meaning it should be fairly easy to dismantle, then put back together.

I’m currently waiting for some parts from Farnell (some K-type thermocouples, thermocouple connectors, a syringe of 63/37 solder paste, a box of 22-gauge dispensing needles, and a couple of [Maxim MAX6675](http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3149) thermocouple controller chips), so I’ve gone back to the design side of things for a bit.

I’m a bit stuck at the moment, however. I’ve more-or-less sorted out the power supply side of things (rest assured I’m going to need a pretty meaty IEC cable), but the power switching is proving a little troublesome. All the 8-to-16-amp AC relays I’ve seen seem to have a break current of less than an amp at 240V AC. If I understand correctly, that means that they’re useless for switching the heating elements on and off, simply because an attempt to break the circuit would probably weld the contacts. It’s nice to see that the manufacturers are still wording their datasheets very carefully — “Oh yes, it’s a 240V 16A relay, but you can have either 16A *or* 240V, not both at the same time.” Just like transistor manufacturers, really.

I think I’m probably going to end up using a solid-state relay or a triac/opto-triac combination instead of the mechanical relay. I’m just not keen on their well-known tendency to fail shorted. At the very least, I’ll probably need to add some form of failsafe to kill the power if something **really** bad happens… What that failsafe will be, I don’t know. I’d rather not use a crowbar circuit; deliberately dead-shorting the AC line to blow the 10A fuse in the IEC inlet seems really silly to me.

My current plan is to test the oven’s ramp-up speed — that is, how long it takes to go from ambient to the full 250 Celsius — and if it’s too long, find another oven. If the oven does happen to heat up quickly enough (that is, from 25C to 250C in less than 5 minutes) then I’m going to buy the rest of the parts and build myself a temperature controller for the oven. If I ever find a solution to the power switching problem…