11/8/2023 0 Comments High voltage bjt transistorUsing molex female sockets for regulator and transistor connection, just something I though made dismantling any semiconductor really easy. I've used 16 Amp terminal blocks for quick disconnect from the circuit board. I should have took some readings on paper, the circuit is out of the PSU case at the moment. I have taken readings with a multimeter around the circuit etc, but haven't noted them or written them down. No to make poor excuses, if my math was better, I would better understand what's going on with the TIP147 transistors. I only want the 7812 regulator to stay with in a comfortable margin, or safe operating area, and the power transistors take up the heavier work load. If i can get my head around the formula to select a better value resistor for the transistors base. I think for now i will stick with single BJT transistors, and look in to lowering the base bios resistor. For me the poor math is areal handicap, especially when you need these figures to make adjustments that get the job done. My weak point in electronics is my maths, i can work out a fare bit of the figures, but once it gets really complicated i get lost in the equation. I thought darlington transistors would do this, as i thoght the huge gain would amplify the current and power better. What i was trying to achieve was stiff output voltage that had very little voltage drop under heavy loads. i can see the gain of the darlington transistors is not helping much, i was just amazed that just switching transistors with little to no load doubled my output voltage. the only difference is my filter capacitance is quite high, as is my final output capacitor. Thank you for the replys, the schematic above is the circuit i am using. If you lower that resistor, it will lower the minimum load requirement, but also lower the maximum current. TIP147 has Hfe of 1000 (minimum) MJ11015 has Hfe of 1000 (minimum) MJ2955 has Hfe of 20-70 TIP36C has Hfe of 10- leaks 8.5 mA That means the minimum load for that circuit using the TIP147 or MJ11015 will be 8.5 Amperes Actually, it's even worse than that, since the maximum Hfe is over 5000! If you don't load it enough it will be running higher than 12 V and the 7812 will be outputing zero current, not regulating, but sucking down that 8.5 mA Using the MJ2955 the minimum load will be somewhere between 170-600 mA Okay, this was not a Spice analysis, it was back of envelope ignoring the 100 ohm resistor. If there isn't, that's your problem right there. With the 7812 removed a 1k resistor should be able to take the output to zero. If you remove the 7812 there should be very minimal current flowing through the transistors. Without a load you're going to get the full input voltage due to leakage. If anyone knows what is going on, that would be great. But it's not, I know transistors amplified current and power, but not the voltage by twice it's expected output. I had started an earlier thread on a zener diode going short, but I though that was my only problem. I've no idea what is going on here, I am at a complete loss to be able to work it out. When I swap back to the TIP36C transistors, all is fine at 12.80 volts. About 23.00 Volts, I did try a 12 Volt halogen lamp expecting it to blow, it didn't, but dropped the voltage to about 17.00 volts. I have other TIP147 transistors from other reputable suppliers, and it's the same. But something odd is going on, I'm getting virtually double my output voltage with these TIP147 transistors. Yesterday I thought I would swap out the TIP36C transistors for two TIP147 darlington transistors. Although I did note a little volts drop at 100 watts, not tragic though. The circuit can power 50 watts and 100 watts, all is as expected. I have one silicone diode in the negative rail to get the voltage up just above 12 volts. What I've got is a 7812 linear positive regulator, and a pair of TIP36C transistors, it's putting out 12.80 Volts as expected. I've tried looking carefully for a problem I have.
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