Electronics > Beginners
key steps to calculate Transistor values
phaseform:
My understanding when calculating values for a NPN BJT as a switch are:
* BJT needs to be able to handle Ic
* then need a value Rb such that the BJT will be saturated, but not Vb must be not too highI am so confused at to the actual steps to choose and correctly wire a transistor...!!
I have a TIP122 with datasheet here from Jaycar.
In my case I will be using this transistor to switch on a 12v load using a 5v arduino. Worst case may be 2A load. (possibly unsuitable transistor?)
I am a little confused by the datasheet, it says "base-Emitter On voltage" max 2.5 VDC, this is the max voltage through base-emitter? If I understand correctly Vbe = 0.7v for saturation as a usual number? SO the only thing I need to do for this transistor is calculate Rb?
Where 5v - IbRb - 0.7 = 0
What will determine Ib? Again looking at the datasheet I get hFE Min = 1000. dont really understand this, current gain is at least 1000x? hFE is irrelevant for switching, where I should just use 20-40 as a current gain number? max for Atmel 328p is 40mA, *40 = Ic = 1.6A. I will be powering a 12v-5v car USB adapter between 12v and collector, which I think has max current ~2.2A (at 5v) so perhaps Ib = 40mA will be fine...??
:-// :-// :-//
Ian.M:
Its a Darlington pair so is two transistors integrated in a single package - see fig 2 on datasheet page 2. The 2.5V Vbe is worst case when passing 3A. See the left hand fig. 10 graph, line Vbe(sat) @ Ic/Ib = 250, which indicates that for Ic=2A and Ib=8mA (by definition from the ratio), you can expect Vbe to be about 1.7V. At 8mA output current the Arduino output will be a bit above 4.7V (see fig. 33-24 in the full ATmega328P datasheet), so the base resistor will be dropping about 3V while passing 8mA, which gives 375 ohms. 330 ohms is the next lower E12 preferred value, which will give a bit under 10mA Ib, plenty for saturation with a 2A load, and not too much for the Arduino I/O pin (unless you have too many pins driving the same type of circuit).
However Fig. 10 also shows you that the Vce(sat) @ 2A will be about 1V, (a penalty of using a Darlington) so with a 12V supply your load will only get 11V. Also the transistor will be dissipating 2W, so will need heatsinking. If you need to drop less voltage or want to avoid the need for a heatsink, you'll need to use a N-MOSFET, and to make sure it turns on properly, select one with a Vgs threshold <2.5V.
phaseform:
Awesome thank you, saved a bunch of frustration. Could you recommend a common N-MOSFET for this role?
Zero999:
The IRL540 springs to mind, but no doubt there are far superior modern parts.
http://www.vishay.com/docs/91300/sihl540.pdf
phaseform:
I found a great video
explaining MOSFETs.
I stumbled upon the exact recommended part IRF540N on Jaycar here. seems to have similar specs to the one in the video, so shouldn't require a heatsink or waste too much energy. This part has Vgs Min 2, Max 4. So this is the voltage required to be fully saturated, and will tolerate 5V directly from the arduino if I understand correctly..?
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