GM3XIJ 13.8 volt 20 amp PSU.

The following are Construction Notes of particular importance.

1: All high current connections, both positive and negative, between the transformer and the bridge, the bridge and the 68000hf smoothing capacitor, the bridge and the 2N6328 collector, the 2N6328 emitter and the thyristor, and to the output, must be in heavy duty wire. If solid wire is used it must be a minimum of 1.5mm.

The heatsink is a deep finned 98.4 mm wide x 53.2 mm deep x 75 mm high, fitted to the rear of the case. Before fitting, a oval hole 60 mm x 28 mm is cut in the back of the case and the heatsink fitted over it. Air blown into the case from the fan exits via this hole over the heatsink. The PCB is mounted on two nylon standoffs across this hole. (see photographs)

Soft start is provided by a 100 ohm 10 watt WW resistor in series with the push to start button. This button starts the supply and energises the 12 volt relay. This relay remains latched until the 13.8 volt is removed. The on/off switch is to isolate the PSU from the mains.

Please note: The DC negative rail of the PSU is above the case AC mains earth. This ensures that the plus and minus DC supply rails feed directly to the load (transceiver), and the transceiver negative rail, usually at AC mains earth, provides the connection to the PSU case earth. This arrangement stops earth loops, which in some cases cause false triggering of the over voltage protection.

2: The MOXIE, 57 Centigrade, thermal sensor, is a TO18 package with a tab. It MUST be fixed to the back of the heatsink immediately behind the 2N6328. To do this, take one of the TO3 fixing bolt insulators and pass it through the tap on the MOXIE. Then pass the insulator through the heatsink and TO3 and bolt into position. Make sure the MOXIE lies along the back of the heatsink, between the two pins of the 2N6328.

The connections between the PCB and the MOXIE should be by 2 x 7/0.2 hook up wires, twisted to about 6 TPI.. The reason for this is that the MOXI is capacity sensitive and the use of twisted wire will stabilise the connection. Finally, set the 5K pot on the PCB to approximately 2K. This controls the current through the MOXIE, ensuring that it is insufficient to heat the MOXIE and cause false triggering of the 2N3053 transistor switch.

The FAN I use is a 12 volt DC, 60mm x 60mm x 25mm Pabst, which draws a running current of 120ma, easily handled by the 2N3053. If you wish you can fit a TO39 heatsink to the 2N3053.

3: The resistor values around the 3423 IC are important. DO NOT alter them. The 12K,47K and 10K pot resistive network connected to pin 2, has been calculated to ensure there is no drift in the circuit. Drift in the circuit will either cause, false triggering by lowering the trigger point or, more importantly raise the trigger point above that set by the 10K pot. This can cause the set trigger voltage to rise above a safe level.

The 12 ohm resistor connected to pin 8 of the 3423 and the Thyristor gate, has been calculated to ensure that sufficient current is available at the gate to switch the Thyristor instantly the set trigger voltage is reached. DO NOT alter it.

The capacitor between pins 3 & 4 of the 3423 and the 13.8 volt negative rail is a small 0.1 ceramic. As the trigger voltage is so close, 0.2 of a volt, to the PSU output voltage, this capacitor ensures there is no false triggering of the circuit caused by spikes. If you wish you can use a value of 0.01. 0.1 gives a trigger time of 1 ms and 0.01 gives a trigger time of .01 ms.

The Thyristor used is a THY 500-40 amp device, mounted on an alloy L bracket, , and insulated from it. The Thyristor has a turn on time of 1.5 us when triggered by the 3423 with a 12 ohm resistor in series with its gate. When the Thyristor is triggered, by an over voltage condition, it instantly shunts the 13.8 volt output rail to negative earth and removes the supply output. The 25 amp fuse blows, the 12 volt relay opens and removes the mains supply to the PSU, and smoothing capacitor then discharges through the 2.4K resistor across it.

4: The resistive values of the set output voltage divider network connected to pin 4 of the L723 have been calculated to give a voltage swing of between approximately 11.5 volts and 15 volts, adjusted by the 1K pot. This restrictive range of voltage adjustment is required because, the close over voltage trigger point of 14.0 volts is only .2 of a volt from the PSU output voltage of 13.8 volts.

The TIP31 driver transistor is mounted on the PCB and does not require a heatsink.

The over current connections CL & CS, pins 2 & 3 of the L723, are not used in this design. In the event of a short circuit or over current condition the transformer core will 'saturate' , input volts to the stabiliser will drop and output will cease.

The LED shown connected to the PCB is mounted to the front panel of the case, using 2x7/0.2 twisted together, and indicates 13.8 volts at the output.

All connections to and from the PCB are made using 7/0.2 hook up wire. NO heavy wire connections are required. However, please note, the plus 13.8 volt and minus 13.8 volt connections to the PCB are sense connections, and MUST be taken from the OUTPUT terminals of the PSU and no other point in the layout. If you do not do this, the efficiency of the voltage stabilisation circuit will be effected.

5: To test the supply after construction perform the following. You will need two Voltmeters, one digital and one analog. Set the digital to auto or the 30 volt range and connect it to the PSU output. Set the analog voltmeter to its 10 volt range. Initially, do not connect the Thyristor gate to the PCB, but connect the analog voltmeter plus lead, to the wire from the PCB that will go to the Thyristor gate, and the voltmeter negative lead to PSU output negative. Connect the digital voltmeter to the PSU output terminals. Set the 1K and the 10K pot on the PCB to half range.

Then switch on the PSU with the on/off switch and then press the push switch. Then adjust the 1K pot on the PCB for exactly 14.00 volts at the PSU output.. Then carefully turn the 10K pot to the end of its travel where the analog voltmeter shows no indication. Then very carefully and slowly turn the 10K pot until the analog voltmeter needle just flicks up scale. Repeat this adjustment until you are sure that you have the 10K pot exactly set at the point where the needle just begins to move off the stop. This is the 14.0 volt trigger point. Then readjust the 1K pot on the PCB to show 13.8 volts on the digital meter. Switch off the supply and then switch it on again and check that you have 13.8 volts and that the analog meter is at zero.

If you notice the analog voltmeter is not at zero, ie it flicked upscale at switch on and stays there, repeat the above adjustments, but set the PSU output volts to 14.1 volts. If after this second check you still have the analog voltmeter up scale at switch on increase output volts to 14.2 volts and repeat adjustments.

If you still have trouble with the thyristor gate drive triggering, it is more than likely caused by 'spikes' at switch on. If you have fitted a 0.01 capacitor to pins 3 & 4 of the 3423 change it to 0.1 and try again with the PSU output volts set to 14.0. When the test is satisfactory, reset the PSU output volts to 13.8, remove the voltmeters, and connect the Thyristor gate wire from the PCB to the gate of the Thyristor.

Please note: During testing, you cannot substitute a digital voltmeter for the analog voltmeter connected to the Thyristor gate lead. A digital voltmeter has to slow a count time for accurate setting of the exact Thyristor trigger point.

6: The supply is now ready to use. If the 2N6328 reaches a temperature exceeding 57C the fan will come on and switch off when the temperature falls to about 35C.

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