Continued Construction

As you already have noticed no transformer is used for isolation in this circuit. It is strongly recommended that a plastic enclosure be used for safety in avoiding accidental shorts and shock from the AC line voltage. If a metal enclosure is used make sure that a ground line is used and the case is connected (grounded) to it as well as the AC outlet.

On the face of your enclosure, install the switch , neon and make your hole for the potentiometer. On the back of the case drill a hole for your line cord allowing enought wire length to reach the switch and secure the cord. As well cut the square hole to accept the AC input plug and then snap it in.

Before installing the PC board connections must made for the AC wiring. Use 4 to five inch lengths of AC wire soldered to the bridge and SCR connection on the board. Make sure no bare, high-voltage wire is accessible to the touch on top of the board.

Insert the PC board in the enclosure. Keep in mind where you'll be making external connections. As noted above, if you're using a metal cabinet see that it is well isolated. Solder the AC (black & white) input wires to the switch and the bridge AC wires from the board to the centre of the switch. Twist connect the positive wire from the bridge to one side of the AC outlet plug wire and the SCR output wire to the other side. Then twist connect the neon wires on each side and use the plastic screw caps to secure the two connections. Finally twist or solder the green wire from the AC cord to the ground connection of the AC outlet.

Final test

Remove the ICs and check your wiring connections. Then connect the AC and switch on the power. The neon should be off as the SCR is not firing. If the neon is on turn off the power and recheck everything.

If the first step checks out reinsert the IC's and turn on the power. The neon should be low but blinking, an indication all is well. Turn off the power. Connect a soldering iron to the AC plug on the back and reapply power. The neon should now be blinking brightly. Rotate the potentiometer and observe the change in the neon's cycling.

Troubleshooting

• With the soldering iron connected the neon light is weak or unstable.
  • The gate trigger signal may be too weak. Remove R9 and reconnect with a jumper wire to the gate.

• The neon stays on all the time and is not cycling.
  • With a scope or a voltmeter check TP2 for a pulse signal. If no signal is present go to the junction of R8, R6 and check for a steady stream of pulse from the negative line of the bridge. If all is well there the CD4011 IC may be defective. Improper insertion may have damaged the IC. Replace it.
  • Go to TP1 and check that CD4001 is oscillating if not check D1, D2 for open or short circuit.
  • With the power on check for proper voltage supply at pin#14 of the IC's and the pos. end of C4. The voltage should be fluctuating within .5V if the SCR is working. If supply voltage is not present check D4 as it may be open. If more or less than 5Vdc check the junction of ZD1 and R7. If the voltage is too high or zero change the Zener diode.
    
  • One or both of the ICs may be defective. Check all the diodes and rectifiers for an open-circuit state. One more final point, it is sad to say, is that the SCR you have installed may not be compatible or sensitive enough to the gate signal level.

Notes

After you have everything working and you may find that the iron's heat range is too high or too low. I'm sorry to declare that all parts are not made equal. Consequently the C3 value may be too high or too low. You would do well to have several values on hand. I'd suggest from 2.2uf to 6.uf to test for the best range acceptable to you. Alternately install a DPST switch to add another heat range between the Neg. bus and pin 6 of CD4001. Another option is to increase the value of R11 to 2M ohms and lower the value of C3 .

One more point, stick with high-quality parts -- you'll be rewarded with a first class tool that you'll be proud to own and work with for many years. Have fun.

Parts List RESISTANCES (all 1/4 W / 10% ) Except for R7 * See text ¤ R1,R4 100k ¤ R2, R3 1Meg ¤ R5,R8 10K ¤ R6 100K ¤ R7 16.8K/2W or 2 @ 33k/1W ( R7a,R7b ) ¤ R9 100 ohms ¤ R10 470K ¤ R11 1Meg potentiometer CAPACITORS ¤ C1,C2 0.01uf disc. ¤ C3 3.3uf/15V electrolitic ( see text ) ¤ C4 200uf/25V electrolitic ICs ¤ IC1 CD4001 Quad 2- input NOR Gate 14 pins Dip ¤ IC2 CD4011 Quad 2- input NOR gate 14 pins Dip Diodes/Rectifiers ¤ D1 - D4 1N4148 ¤ D5 - D8 1N4003 ( 200V/1A ) or PC mount rectifier bridge with same rating or higher ¤ ZD1 8.2V/1W ( 1N4738, 1N3018 ) or 9.1V/1A ( 1N4739 , 1N3019 ) ¤ SCR MCR106 ( 400V/4A) or equivalent Other Parts ¤ neon One colour capsule with resistor ( Panel mounting type ) ¤ Socket 14 pins DIP 2-ea ¤ Switch One DPDT toggle , small panel mount ( 120V/6A rated ) ¤ AC Socket One square input plug for panel mounting "press to fit" ¤ Cabinet 3"x3"x4" sturdy plastic preferred for good isolation ( see text ) ¤ One control knob ¤ AC line cord 3ft with ground plug (see text ) ¤ AC wire connectors 2-ea plastic screw type medium size. ¤ Stick on rubber or plastic foot buttons 4-ea.