#Mica insulators for 2n3055 transistor full#

The transistor is mounted on the heatsink using a mica insulator and thermal compound. Its junction to case thermal resistance is 1.52РІСљВ¬/W.

#Mica insulators for 2n3055 transistor full#

When working at full current, the 2N3055 would drop around 8V at 5A, dissipating 40W of heat. It has a thermal resistance of 1.16РІСљВ¬/W and can handle up to 50W (with a 58РІСљВ¬ raise). The pass transistor is mounted on a large heatsink. The three trimmer pots are as follows: the bottom left is the current limit (clockwise is higher current), the bottom right is voltage adjust (clockwise is higher voltage), the top one is the overvoltage trigger point (clockwise is higher trigger point). The voltage regulator circuit is mounted on a small perforated prototype board. Front view (cover removed) Top view (cover removed) The right one has the diode rectifier bridge and the BD239C. The left one has the SCR and the current shunt resistor. This were originally mounted on the PCB, but IРІР‚в„ўve mounted them to the sides of the case. IРІР‚в„ўve also reused the heatsinks from the computer power supply. The AC transformer that IРІР‚в„ўve used is a little bit taller than the case, but since the case is made of thin aluminium and can bend a little this is no problem. IРІР‚в„ўve used the case from an old computer switching power supply. In the photo below you can see how I have built the power supply. It would be better to use an IC, such as the MC3423, but sadly this and other similar ICРІР‚в„ўs are out of production. IРІР‚в„ўve found this to be a bit troublesome to adjust properly. The crowbar is triggered on overvoltage using a simple voltage divider. There is also a crowbar SCR using the TN2015H-6T. The LM723 provides voltage regulation and current limiting. Any small power BJT transistor which is able to pass 1A will do.

The choice of this transistor is not critical. Since the current gain of the 2N3055 can get quite low (perhaps down to 10 or 5) and the LM723 can only source 150mA, a BD239C is used to provide the base current to the 2N3055. Here I describe my design.Р”Р„s you can see in the schematic below, the voltage regulator used is the venerable LM723, which drives an external 2N3055 pass transistor. The main difference is that IРІР‚в„ўm using only one pass transistor instead of three. The design is based on the 12V 15A power supply by Ed WA1TWX that appears in the 2015 ARRL Handbook. I wanted to build something easy and useful this summer, so I have gone with a 12V (also adjustable to 13.8V) 5A linear power supply.