Homebrew PCBs

I did a bit differently - did not replace existing thermostat, but added
another one, with a switch to choose between them. That way, laminator can
be used for original purposes too.
Even better way should be to add also the lower-temperature one, so the
half-power heating starts a bit later, otherwise it takes a very long time
to heat to 160C. But I did not have a small double-pole double-throw switch
handy at the moment...


On Mon, May 6, 2013 at 7:17 AM, w9ran <w9ran@...> wrote:

> **
>
>
>
>
> > Have you traced out the schematic and can post for us?
>
> I have uploaded a copy of the wiring diagram into the Files Section under
> "Laminator Mods".
>
> I'd appreciate comments from anyone who is interested in modifying this
> cheap (<$25) laminator that uses bimetal thermostat switches for temp
> control. Ultrakeet in OZ replaces the thermal protector (fuse) with a new
> 250 deg. one, and swaps a new 160 deg. thermostat for one of the existing
> ones. My question to the group was "which one" to replace, as the CS-923
> has three of them instead of two as in the Ultrakeet example.
>
> Here's how the laminator control system works: The heater elements
> (connected in series with the thermal fuse) are powered from either
> half-wave or full-wave rectified AC line voltage, depending on temperature.
> At power-up, both thermal switches are closed, which provides full-wave
> rectified voltage to appear across the heater element. Once the temp rises
> to either 115 or 125 deg and the low voltage thermostat opens, one
> rectifier will be disconnected and the heater will only see negative
> half-cycles, reducing the average voltage by about one-half. When the heat
> rises to the point where the 145 deg. thermostat finally opens, all power
> will be removed and the heater will begin to cool and the green LED will be
> off until the stat re-closes.
>
> Thus in normal operation the temperature is controlled in the range of the
> selected lower-temperature stat and 145 deg. The behavior of this control
> loop is going to depend a lot on the thermal response of the heater-roller
> mass. If the temp were to drop below the threshold of the low-temp switch,
> it will close and again supply the higher average voltage to the heater to
> bring it quickly up to temp, but otherwise it seems that the 145 deg.
> thermostat will do most of the controlling. It's a pretty clever design,
> based on a few inexpensive parts.
>
> So the answer seems to be to substitute the new 160 deg. stat for the 145
> degree one, which will allow the laminator to reach a higher temperature
> before it shuts off. However, one could also replace one of the low-temp
> stats and thus reverse the function, so that the full voltage would be
> applied until the 145 stat opens, which would presumably maintain the
> temperature between 145 and 160 deg.
>
> Thoughts? I guess its time to take some temperature measurements.
>
> 73, Bob W9RAN
>
>  
>



-- 
KPL


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