You also have to worry about power dissipation in the switch. You would need an extra switch stage before to get the overall current gain high enough. Even darlington (2 stage) transistors are typically only 1000 and can drop down to 200 under high current loads. You are not going to find one with that current gain in a single stage. So that would indicate you would need a transistor with a beta of 6000. You don't really want to drive more than 6mA out of a Raspberry GPIO. If your motor current is maximum 6A then the base current should be 5 * 6A / beta. Typically that means providing 5 x more base current than required over that calculated from the collector current divided by beta (current gain). For a BJT that means providing enough base current so that the transistor is 'saturated'. You do not want the transistor to be operating in its linear region which would lead to all sorts of problems. You need to focus on keeping the switch voltage drop low for the current drawn. Don't worry about motor resistance in this case. That means you want the switch (BJT or MOSFET) to be able to pass that the maximum possible without significant voltage drop across it.Ģ. Yes The motor should be the determining factor for the current drawn. The rest of the supply voltage will be across the bjt - and because of that, its HFE will have risen just enough for the transistor to pass the current the motor is asking for.ġ. To allow 6A through its collector (with the motor close to stalling), even with HFE at 20, you'd need to feed 300mA into its base - the Pi's GPIO can't provide that current, so you'll probably be looking at a Darlington transistor.īut even then, since HFE will have dropped well below 20, less than 6A will flow through its collector, and the voltage across the motor will therefore be less than 12v. In your case, you want the motor voltage to be as close as possible to 12v, allowing it to take what current it wants, so you want as little voltage to be dropped across the bjt - not only to keep the motor spinning, but also to keep the power loss in the bjt to a minimum. Its HFE is between 20 and 100, as long as Vce is about 4vīut when Vce drops towards 0v, HFE also drops, so more base current is needed for the same collector current. However, the BJT's BASE seems controlling the current flow at COLLECTOR as well.Īs a result, motor draws current or BJT's BASE controlling the current at Ic? Or should I treat the motor as a variable resistor in the calculation?Ī BJT being a current amplifier, the base current does indeed control the collector current, but only while the collector-emitter voltage is reasonably large. In my understanding, motor "draws" as much current as it needs. input current, input voltage, HFE, Ic, etc.)?ġ. Once I get all my parameters, how should I find out which BJT should I buy? (Any site for me to filter those transistor e.g. There are so many BJT transistor in the online market. The other unknown variables are depend on what BJT will I getģ. Motor current range (or resistance): 1.5A to 6.0A (or 2ohms to 8ohms) I just need to choose a suitable BJT, Am I correct? since I have: If I treat the motor as a variable resistor: from 12 / 1.5 = 8ohms to 12 / 6 = 2ohms, seems that I got my required variable already. As a result, motor draws current or BJT's BASE controlling the current at Ic? Or should I treat the motor as a variable resistor in the calculation?Ģ. However, the BJT's BASE seems controlling the current flow at COLLECTOR as well. Provided is my basic scematic and I have a few questions.ġ. Working Voltage: 12V - 18V (I am going to run it at ~12V) Let me provide some basic information first. I am trying to use my RPI to control a 775 DC motor through a BJT (I know MOSFET would be better in my ocassion, but I want to get myself clear to BJT first). I know my questions maybe so dumb and hope you guys can give me some advice on the whole flow from a schematic to getting a correct BJT. I learnt some basic EE knowledge in Uni and going to pick it up again. Hi everyone, I am a super newbie to RPI and EE world.
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