My problem is that the class course uses a 2N222 Transistor and he talks about \$V_\$th of 1A.) So this is starting to look like either a MOSFET solution or else a two-BJT solution. ![]() I got into the stage where we're suppose to build some sort of protection system so we can plug-in outsourced power supply and prevent the arduino uno from frying. I know the other part of that is a pwm driver, of course - but I want to be certain I knew exactly how much current is being sent and spent.I just started studying Arduino and I'm watching a online course. I was also interested in knowing this to better my skills in building a circuit to drive high intensity LEDs, control the power to give the most light without sending too much energy. But I got the circuit to work, I am not sure I'm doing the right thing, but it's not smoking or getting hot so far. I got confused because I just wanted to send +5v on or off to the power line, and not power the hc-05, and toggle a ground wire by a transistor. I started off to control the power-on to the hc-05, so I could reset the board into command or spp mode. The reason I am trying to do this, is for many reasons, mainly education so I can design better circuits. I guess if that energy isn't consumed by a device or resistor it's probably turned into heat and poof - goes the circuit. Bi-Polar high current NPN Transistor DC Current Gain (hFE) is 100 Continuous Collector current (IC) is 800mA Emitter Base Voltage (VBE) is 6V Collector. (IC 150 mAdc, IB 15 mAdc) PN2222 PN2222A (IC 500 mAdc, IB 50 mAdc) PN2222 PN2222A VCE(sat) 0.4 0.3 1.6 1. And I must have hit some type of limit because the transistor burnt out. I typically go for a forced beta of 1/2 the minimum hfe so assuming a minimum hfe of 100 for a 2N2222: 4mA/5080uA. I was playing around with that until the pot started smoking (lol not me).īut it seemed like the sweet spot was above 100k started limiting to the range I needed, but I had to keep giving it more 100k resistors for every 10 more ma of current to limit. If a CMOS output is used to drive the transistor, then R2 is not needed because the CMOS output can pull the base to ground. Like it didn't seem to start limiting current until I put 100k ohm resistor in. I think I noticed a sweet-spot in the circuit. And in fact, most real-world circuits (not just op-amp circuits) rely on the same principals so in production you don't have to tweak pots or select resistors. They have very-very high gain and with negative feedback the gain can be controlled with the resistors, and you don't have to worry about the exact open-loop gain of the "raw" op-amp. But, since there is so much tolerance in the transistor's gain, that's just a starting point. So, if the transistor has a gain of 100, we need 0.45mA through the resistor (and base) so that's about 11K. But, since the transistor's gain has such a wide range we don't need to be that exact and we can just assume 5V across the resistor. The base-emitter junction acts like a diode, so I'll estimate a voltage drop of 0.6V and that means with 5V applied you've got 4.4V across the base resistor. What first has to be known is the voltage and. This example shows operating a solenoid, but it could be any 12v device, motor, lamp, buzzer, etc. ![]() That's a wide range (typical for a transistor) so you either need a pot, or you can experiment and select a resistor, or design a more advanced circuit that depends on resistor values rather than the transistor itself.* This shows how a NPN transistor is commonly used to allow an Arduino output to switch devices on and off that require higher voltage and/or current then can be supplied directly by an output pin. According to the datasheet the PN2222 has a current gain (h fe) of between 50 and 375 depending on the conditions and the particular part.
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