User:Jstapko/EngLab/ScrollSaw/PowerSupply

Scroll Saw Foam Cutter Power Supply
Attention - this component of the Scroll Saw Foam Cutter project has been abandoned in favor of the new power supply. This page is now for reference purposes only.

Overview
The heating element of the scroll saw foam cutter will use a low voltage, high current power source to generate the heat needed to cut the foam. The first attempt was based on a power supply with 6.3 volt filament transformer, based on experiments which established the current/voltage requirements with the first heating element. In an effort to reduce the saw kerf, a prototype of a new heating element has been tested, with promising results. However, the smaller wire diameter of the new heating element may lead to increased resistance, and therefore higher voltage than the first power supply can deliver may be needed. Before putting more time and materials into finishing the first power supply, tests will be made to determine compatibility with the new heating element.

The first plan for the new power supply consisted of

(the series impedance is discussed in detail on the Ballast Resistor Sub Project page)
 * the original power supply components
 * electrical circuit modifications to boost voltage
 * the attachment of an IEC cord socket for AC power input
 * the addition of a fuse to improve safety
 * the development of a top cover to prevent accidental contact with live wires
 * the development of a series impedance, such as a resistor, choke, or variable resistor to allow for adjustment of the heating element temperature, and to reduce the current slightly

Original Power Supply
[schematic]

The current power supply platform consists of a former low voltage isolated power supply which used a 6.3 volt, 6 ampere center tapped vacuum tube filament transformer (primary: 117 volt, 50/60 cy) feeding a pair of 1N250B (possible  Link to Datasheet)power diodes in full wave, center tapped configuration. (possible Link to Rectifier Circuits)The transformer secondary leads originally fed the anodes of the rectifier diodes, and the center tap fed the negative (black) output wire. The cathode ends of the diodes are bolted directly to the chassis, with a ring lug under one to which are soldered (poorly) the ground return electrical connections. The other diode’s cathode is bolted to the chassis, and the electrical return is through about 2 linear inches of the aluminum material of the chassis. The red output wire and one wire leading to a bleeder resistor are soldered to the grounding ring terminal screwed to the diode terminal. The remaining electrical connections are made on a piece of phenolic terminal board with 4 terminals and many holes. The bleeder resistor consists of 4 18 ohm, 5 %, what appear to be 1 or perhaps 2 watt carbon composition resistors, of 1950’s or 60’s style, wired in parallel, for a theoretical equivalent of 4.5 ohms, although these have not been checked for accuracy. One end of the resistor bank is soldered to the aforementioned grounding lug, and the other end is connected to the black output lead, effectively shunting directly across the output. Initial tests showed that the power supply’s output in the original configuration was too low to effectively heat the hot wire element, so the center tap was disconnected from the black output wire, and the end of the secondary winding was removed from the diode (with ring lug) and moved to the black output wire. The increased voltage across the bleeder resistor would increase the power dissipated in it, and no checking or calculations have been made to assure that they are still operating within their ratings.



Possible Next Steps

 * draw schematic of original supply in [CircuitLab]
 * bolt transformer securely to chassis
 * redo solder connections to ring lug and output wiring, with a 250 watt Weller soldering gun
 * add link to datasheet on 1N250B diode (above link is short data only)
 * check dissipation of bleeder
 * re wire bleeder as series resistor
 * mount other resistors on terminal board, maybe one for the primary side to cut voltage
 * check the power supply with the new heating element to make sure it will pass at least 3.5 amps into the increased resistance of the new element

Things This Page Needs

 * 1) links to all pages indicated in square brackets
 * 2) need to create page for ballast resistor project
 * 3) need to add photos and captions to supplement text
 * 4) make a photo of a fuse block with glass fuse, showing mounting screw and internal fuse construction

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