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Friday, 18 May 2018

More modifications to the Fellows A75 Laminator for toner transfer.

If you haven't seen the first bit of this ... click here.

Made quite a few boards with the laminator and toner transfer method recently, and results have generally ranged from acceptable to good.

There's the occasional fail, but I want to improve repeatability.

I've made some measurements of the temperature of the rollers, and it's temperature controlled at about 100 deg C, and sometimes, the board just doesn't get hot enough to melt the toner and cause it to transfer. We need to warm things up a bit....

I removed the PCB and reverse engineered enough of it to work out how the temperature control is done... IT'S IMPORTANT TO REMEMBER THIS CIRCUIT IS "LIVE" AT ALL TIMES, AS THERE'S NO TRANSFORMER. I used a mains isolation transformer whilst making measurements and working on this unit.

Mains comes in at JP1. Mains is dropped via a capacitive dropper C1 (R2 prevents the cap remaining charged up once the unit is unplugged, and removes a shock risk) R1 provides a bit of current limiting, D1 and D2 rectify the output, which is stabilised by ZD1, and C2 is used to smooth the supply. N3 is a 5 volt regulator, and C3 provides a reservoir for the 5v rail. The temp sensor looks like an ordinary 1N4148 diode.... it may well be. It's pressed up against the lower rubber roller. When it's at room temp, it's got about 4.2V across it. When it get up to temperature, there's about 2.4V across it. This is loaded by R32, and connected to the inverting input of N2 via R7, and back to ground (neural) via R8. A voltage reference is provided by the chain R9,R11,R12 and R13 (approx 2.6V) and applied to the non-inverting input, forming a comparator. Stability is provided by some feedback provided by R6. The output of the comparator is fed to the gate of a triac, T1... Now a note on D7... it connects to N1, which the manufacturer has very kindly filed the number off.... I imagine it's some sort of micro controller. It most likely provides some timing (the unit shuts the motor off after 30 mins) and some zero-crossing pulses to D7 so the circuit can phase modulate the triac. I've not illustrated this part of the circuit, as it's not relevant, and difficult as I don't know exactly what's going on inside the mystery N1. 

OK, so we need to alter the voltage reference at pin 5 of N2. I remove R9, and place a 10K pot in it's place.

I adjust the pot until the temperature stabilises at 130 deg C.

The pot's then disconnected and measured, and it's 3.6K, which is rather convenient, as that's a preferred value! R9 is replaced with a fixed 3.6K part.


Tests prove transfer is definitely more "robust". The toner seems thicker, and more difficult to remove.

We're now operating this unit way outside of it's design spec (as if it wasn't bad enough before!), so don't leave it operating unattended!!

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