After the experience of building a fix to my old Philmore PS123 power supply, only to find that the I couldn't get the obsolete LM338K regulators that I designed in, and worse still, ordering LM338Ks from China only to find that they were counterfeit devices that blew out when current went above 1 Amp, I put away the project and worked on other things. See this link for my post describing the experience with the counterfeit regulators in May 2023.
I finally redesigned the project using a 2N3055 pass transistor. See the circuit below.
I was going to use a 2N2102 to drive the base of the 2N3055 but that device is apparently obsolete these days. Browsing between Amazon.com and Digikey, I found a 60V 1.5 amp NPN device that should work well. I bought an assortment of BD135 family devices from Amazon, 60 total for about $9. The transistors should arrive next week, so in the mean time I used a BC337 50V 0.8 amp device I got from a TO-92 transistor assortment I bought last year from Amazon. This transistor will only handle about 635mW dissipation. Worst case dissipation is 2W if the 2N3055 has only current gain of 20 at 3 amps collector current. The worst case assumes the 2N3055 is the datasheet minimum, but more likely the current gain will be 50 to 70, so it's hard to determine the exact dissipation for the base driver transistor.
At any rate I made the modifications and the unit appears to work as expected. I tried three different currents to test the regulation:
Load current Output voltage
0.036 A 14.3V
1.390 A 13.9V
1.710 A 13.7V
This corresponds to 4.2% load regulation over about 2 amps, which is probably pretty good for this type of pass transistor regulator.
Photo above shows front view of power supply with cover removed
Photo above shows side view of power supply without cover. Circuit board with filter and regulator appears vertically on the right side. 2N3055 heat sink is on the back of the unit.
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