# Transistor as a switch for 3A collector current

I am using:

• ATtiny10 w/ 10Ohm Resistor in series to drive Base of transistor
• Transistor is ZTX688B
• Vcc is 5V and 3A source
• Load is a Raspberry Pi connected to Collector

simulate this circuit – Schematic created using CircuitLab

The ATtiny is programmed to turn on the Base for 10 seconds and then turn it off for 5 seconds. When I run the circuit it turns on briefly but only manages to 100-200mA (not enough to fully turn on the Pi. How can I make this work? The transistor is rated to handle 3A and up to 12V.

NOTE: I am also open to suggestions of replacing parts. I just need to control the Pis power supply with a MCU.

• Where did you get the ZTX688B from? – Spehro Pefhany May 3 at 18:57
• It is from Mouser. I added a hyperlink to the datasheet – SChand May 3 at 19:03
• Are you sure PB0 is set to be a push-pull output? That's a plausible current from the pullup resistor given the very high hFE of that part. – Spehro Pefhany May 3 at 19:16
• @SChand I'm just worried that you plan to use a part with the expectation that it can handle $3\:\text{A}$, continuously, when the specifications specifically say that the testing is done with a duty cycle of 2% (you want 100%, I think) and for a very short pulse width. Not to mention that they brag about a high $\beta$ but you can't use that value for considering the base current you will need. That TO-92 cannot handle much continuous dissipation. I am not sure you realize the implications. But maybe I don't know your use well. So it could go either way. I am just worried. – jonk May 3 at 22:09
• @SChand NFETs are recommended largely because the current you want is a kind of sweet spot for NFETs. At those currents, a BJT needs a lot of recombination current to act as a switch. You can do that with a couple of BJTs and some resistors. But usually people prefer to just go with the NFET. Me? NFETs are >10X the cost of the BJTs I buy (often 100X.) And I have a lot more of them floating around here. So I usually make the BJT work, anyway. But you may want to consider an NFET for this (trivial to use, so it's not even worth writing about it.) If you want a BJT design, I'll offer one. – jonk May 3 at 22:30

From your schematic, the current flowing from PB0 to T1's base-emitter junction is $$\I_B = (5V - 0.6V) / 10\Omega = 440mA\$$, which is extremely high for a MCU-pin's drive capability. Thus PB0 may limit the output (I'm not sure, but I hope).
To saturate the transistor, a base current of $$\I_{Bs} = \frac{I_C}{ (\beta_{min}/10)}\$$ is sufficient (I cannot prove this, but this comes from my experiences). From your circuit, this base current is 3000mA / (400/10) = 75mA, which is still extremely high for MCU's port/pin.