Homebrew PCBs

correction, single-throw is enough, newly-added thermostat can be just
switched paralel to the existing one.


On Fri, May 10, 2013 at 2:04 PM, KPL <kpl.listes@...> wrote:

> 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@oneradio.net> 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
>



-- 
KPL


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