TIP3055 as switch

[conntaxman]

I tried to figure out the math for the base pin and can,I cant do the math to figure out the voltages. I know a lower voltage needs to go to the base and the 12vdc goes to the collector, and the emitter goes to ground with a resistor. Im making a PIR motion detector,well i bought that from ebay, 3 dollars, i also have the cir. that the pir goes to, but they use a DPDT relay and i want to change that to a transistor or a mosfet.
Could someone draw up a small cir just to replace a relay.I will be running 12vdc.and here is the power req, for the string of led's. Input is 12vdc--Power is 36 watts -- current is 5 amps
tks
John

 

[MikeMl]

Here is a 2N3055 connected as a common-emitter switch. The load R=E/I=12/5 = 2.4Ω is connected between the collector and the positive end of the 12V power supply.

I am showing that to turn on the 3055, your PIR will have to source more than 130mA at 10V to get the base current up to a level where the 3055 turns on. Look at the sim plots. The X-axis is the voltage of V2, the base source. The blue trace is the current into the base of the 3055. The green trace is the load current. Note that the input voltage V2 has to get to >9V (which corresponds to a base current of >130mA to turn on the 3055.

Note that by rights, when the 3055 is used as a saturated switch, the base current should be much higher, about Ic/10, or 0.5A to turn the 3055 on as well as it should be. It is doubtful that the PIR can source 0.5A, so you would do better to use an NFET, or possibly put a small NPN in front of the 3055 to make a Darlington pair.

 

[audioguru]

36W into LEDs at 12V is a current of only 36/12= 3A, not 5A. Maybe the LEDs use 36W and their current-limiting resistors use 24W.
The datasheet for the 2N3055 shows that its base current must be at least 400mA for 4A output and an output voltage loss of 1.1V max.

 

[alec_t]

As MikeMl suggested, use an NFET instead of the current-hungry '3055.