Easy schematic reverse-engineering project - Page 1
Forum section: electronics / beginners

author: edy
author_rank: Super Contributor
timestamp: August 19, 2013, 08:40:18 pm
content: Hi,I'm trying to understand what is going on with this wood-burner instrument, namely to figure out the exact schematics and parts (and how safe it is). I've worked out most of it but need help. Being AC design I am not as clear on some things.I can't take the entire thing apart due to switches soldered in after placed in housing.Basically, AC mains goes into one side and gets regulated and variable adjusted through a pot and sent out to a 120V to 20V 10A transformer. The white wire pair side (everything After the transformer) I have figured out... It just throws a switch to drive either one plug or the other plug (they share a common "ground", with switch selecting which center hole is active).Looks like LEDs are just bridging across the + and - with no resistors. If this is AC then they are getting only lit up half the time?Ok so back to the part of circuit between transformer and mains, which I don't completely understand.The switch closes the circuit and also allows the LED and 22k resistor to bridge across the white and black, lighting up when unit is on. However, there are 2 pots... One adjustable knob and one screw-only adjustment (Hi-Lo) which may set the Max limit.I will post more pictures.This basically runs like soldering irons. The handpiece has a 2-prong tip which you can change to a number of shapes, like knife, spoon, chisel, etc.Thanks. This should be easy for you more experienced folks to figure out.

author: edy
author_rank: Super Contributor
timestamp: August 19, 2013, 09:28:55 pm
content: These photos show the "after transformer" side... Just a switch selecting A and B but you can see they share common "ground". Each has an LED bridging across the active to ground which I assume only works half the time (no resistor either).So the reason I am saying active and "ground" is because I'm not sure what to call these on the AC circuit. Don't the alternate back and forth? So +/- becomes -/+ and then back to +/- at 60Hz.Next I will post photos of the circuit between mains input and the transformer. It is basically just a fuse and LED ON indicator (with its own resistor), a fuse, 3-pin Voltage regulator?, 2 caps, 2 resistors and 2 pots.

author: edy
author_rank: Super Contributor
timestamp: August 19, 2013, 09:57:40 pm
content: Here is a rudimentary schematic (sorry about the symbols) showing what I could figure out looking at the traces and what components I saw. Hope that leads to the right direction.

author: edy
author_rank: Super Contributor
timestamp: August 20, 2013, 05:44:25 pm
content: I had another look... that 3-pin "voltage regulator" is actually a TRIAC. It looks like what is going on here is a TRIAC-based "light dimming" circuit. There are only a few parts and based on examples (like the attachment) it seems to be roughly the number of components being used (2-3 resistors, 2 caps, 1 triac, 1 main adjustable pot, 1 "trim pot" to set max level). I am hoping that by debugging this relatively simple circuit, I can acquire some basic understanding of how it works.