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Hello Kevin, Sorry i cudnt understand you, theres no input resistor that Im using. Only the 10 ohm 5W current sensing resistor at the input of 7805 and base. Im not using 3.3 ohms anymore, im using 10 ohm, which turns on my 2955 when ever load current increases 70mA.

Hello Kevin, Currently the my setup is working fine with 10 ohm 5 W resistor. I am just worried about the TIP2955 getting very hot! And since datasheet mentions it can easily support currents of 10A then why is it getting so hot on only 1.5A current (even with an adequate heatsink installed) ? Thanks.

im using 7805, its a 5 volts regulator so i think 12 v input to it is perfectly fine.

the input voltage is 12V DC from a maintenance free 20A/hr 12V battery.

Hi, i think you can remove 1 cable. Your supply ground and signal ground can be shorted so only 1 cable can be used for both of them.

Thanks for the replies guys. The current boosting technique I followed is exactly as it is given in the book "Electronic Principles" by Malovino 6th edition. The value for the current sensing resistor is calculated according to the desired current we want 7805 to pass. Like if I want 7805 to only pass 70mA and the rest of the current I want to pass through the pass transistors then I would have the current sensing resistor of 10 ohms. Actually a little over 0.7 volts is required at the base of power transistor 2955 to turn it on. So If i use the 10 ohm current sensing resistor then whenever current through the 7805 increases 70 mA then the transistor turns on and allows the rest of the current to flow through it (70mA * 10 ohm = 0.7 volts). This was its mentioned in Malvino and in some other books and websites. Here is the latest update. I attached a heat sink to the 2955. used the 10ohm current sensing resistor (5W; anything below 5W was burning like hell) and ran it. I could see that a little over 70mA was flowing through 7805 and the rest through 2955, i had a total current drain of 1.3 A (in the form of 6V bulbs). Thats fine! BUT the 2955 along with the heat sink was getting very hot, infact very very hot! the data sheet of TIP2955 says that it can handle peak current of 15A otherwise 10A continous current, but it was already very hot on 1.xx A!! Am i doing something wrong? I added another 2955 to the setup as another pass transistor now the current would divide between them, this time the pass transistors werent as much hot. Im still confused as when 2955 can support 10A continous current emitter to collector then why is it getting soooo much hot on only 1 A? Any comments on this whole setup would be more than welcomed. Thanks

Hello, I am building a power supply for ATX motherboard. I have two 12V batteries which I want to use for this purpose. My plan is to use regulator ICs and add current boosting capability to them. For this purpose consider the LM7805 IC. Im boosting its current capability like shown in the attached image Here 3.3 ohm is serving as a current-sensing resistor that will switch on the transistor as soon as the current draw increases 212 mili amperes, hence according to theory 212 mA current shud flow through 7805 and the rest through the power transistor, BUT this isnt whats happening (atleast with me). The voltage across the current sesing resistor shows 3.3 V! The resistor heats up rapidly! The transistor doesnt heat up, The 7805 heats up! I cant understand whats going on. I am testing with a load of 1A in form of bulb. Waiting for your help. Regards