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Saturday, 3 February 2018

Modifying the Fellows A75 A4 laminator for making PCBs. Toner transfer the posh way!

Now, if I'm making something up, it's often easy to make a simple single-sided PCB up quickly, and professionally by using the toner-transfer method of PCB manufacturing. I've been doing it for years, but the results are always a bit hit-and-miss. Use google if you haven't heard of it before, there are many guides on the web (although no two the same!) , and I don't intend to duplicate those here.

I have tried many different types of transfer paper over the years, from the proper stuff, to plain old A4 paper, the best I found for years was a copy of the New Scientist magazine! Sadly they changed to thicker, shiney paper a few years ago, which doesn't work well at all. The best at the time of writing seems to be "The Economist", which is a shame, as it's nowhere near as good a read as "New Scientist" before it get's recycled into PCB making materials!!

... anyway ...

The last board I did, I tried an experiment, by borrowing Mrs Doz' laminator from her office. I idea was to see if it would transfer the toner from the paper to the board, whilst keeping a constant and steady pressure on it. Well, results were mixed... it really pushed the board through too fast for proper transfer to take place. I ended up transferring the toner with the conventional clothes iron, and then passing the board through the laminator, which resulted in an improved board after a couple of goes. It did, however give me an idea ... make the motor run slower....

So, rather than incur the wife's wrath and modify her laminator, I picked up a Fellows A75 A4 laminator from Argos for £16....




I initially tested it would take the thickness of the board by passing a blank piece of PCB stock through it... yep, works a treat.

So, to disassembly... There are 4 pozi screws underneath (great, not even some torx or bizarre security screws to bother me!)

So (from left to right) we have an essential mug of tea, the motor, drive rollers and heaters, and finally a PCB on the right....







A couple of things to notice about the PCB... 

There's some sort of timer in there to shut the unit down after it's been on for 30 mins (just in case you leave it running). It's unlikely to bother us much....

The low voltage electronics (the timer, and LED's etc) are driven from a power supply formed by a wattless capacitor dropper... so a word of warning... all of these voltages on this board will appear to be referenced to the mains. This would present us with a shock hazard and a problem if we want to interface to them...

On to the motor....


It's something I used to refer to as a "Le Cruzet" motor. A synchronous motor with built in gear box. As we can see from it's label, it rotates at 4 RPM (4.8 RPM if you live on that funny bit of the planet that uses 60Hz mains). That's a bit of a pain. I was hoping for a small DC motor I could drive with some PWM to get the speed nice and regulated (although if I was building a cheap laminator, I'd be using exactly this motor too!) 

So, how can we control it's speed?

Well, it's synchronous, so we'll need to lower the AC frequency driving it .... Ugh ... don't fancy this idea much.. generating a few tens of hertz at 240 V RMS is not going to be a cheap fix....

What about phase modulation? Daft idea. It may vary the speed, but will destroy the motor's torque, and won't vary the frequency. May cause overheating and failure in the motor's windings because of the resultant unpleasant waveform.

What about PWM? Nope, DC only ..... or is it????
How about really, really slow PWM? Really really slow.... 
I'm talking about switching the AC to the motor on for ~.5 second, off for a second, and so on ......

It'll inch the board forward, cook it for a bit, and inch it forward again ....

It's just stupid enough to work....

So what do I need? 

A 555 timer, a slack handful of passives, a transistor and a relay...
or an arduino (or similar, running a modifed blink sketch!),a transistor and a relay...?

I'll go with the latter, as it'll be easy to adjust by fudging the software, and most of the hardware is built for us.

Enter the relay module, and the weird and wonderful Digistump Tiny85 module... just the ticket, and small enough I should be able to squeeze them into the case.

I've used the normally closed relay contacts, so if I do want to use the laminator without the speed control, it will work fine with the speed control switched off. The relay contacts are connected in series with the motor, and the relay control pin to P0 of the digistump.



A design is laser printed onto a piece of paper from "The Economist", and a test piece of board stock is cleaned up (I know they're a different size, but it's only a test)
















The paper is tightly folded round the board so it doesn't move, and stuffed into the mouth of the hungry laminator! The "printed" side needs to be facing the bottom roller, as that appears to be the roller that's heated.

I left the speed controller off (so the unit was running at normal speed) whilst it reached temperature.

I ran a couple of tests, trying different speeds.




and into the sink!













I finally settled on 4 seconds off time, and 0.5s on time.... which produced nice, repeatable results ....




and left us with the digistump code looking like this....

void setup() {
  pinMode(0, OUTPUT);
}

void loop() {
  digitalWrite(0, HIGH); //Turn the motor off
  delay(4000);
  digitalWrite(0, LOW); //Turn the motor on
  delay(500);
}

So the electronics is all boxed up, and driven from a redundant mobile phone charger...



A few words of warning....

We're putting paper in here, which is being exposed to high temperatures for more time than it would be in normal use.... If it should jam, there's a risk it could smoulder and catch fire, and ruin your day... DON'T USE IT UNATTENDED!

It clearly states "Up to 80 Micron" on the box. We're really taking the P*$$ here, as my boards are 1.6mm thick, plus the paper (folded) so don't expect it to last ... but while it does .... get making!









... oh, and you can use the same idea to print a "slik screen"...


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