What method you use to calc the Base resistor of a transistor? - Page 1
Forum section: electronics / beginners

author: Chriss
author_rank: Frequent Contributor
timestamp: February 25, 2018, 01:33:06 am
content: Hi!Here I would ask members to share their method to calculate the base resistor for a switching transistor.I mean, this should be helpful for beginners who are not pro's but they make everyday DIY small project andare faced with this question "Uh! how can I on the simplest way calc my base resistor now?".So, here is my option how I would prefer to make the calculation:This method is the 1/10 method.This means the base current Ibshould be 1/10 of the collector current Ic.Example:We search for a base resistor Rbfor the BC337 transistor where we know the needed collector current Icwhat we need for our aplication is I=50mA and the signal voltage of base Ub=5v.So the base current Ibcalculation is:Ib= I/10I=50mAIb= 50mA/10Ib=5mAThis means we search for a resistor on the base of the transistor which can deliver 5mA into the base.And the formula how to calculate the base resistor Rbfor 5mA on Ub=5v is:From the datasheet of the BC337 check the Base-Emitter-On Voltage which is the VBE(on).And it's stand for a max value of VBE(on)= 1.2v.So I would get a bit more then VBE(on)=VBE(on)max/2 what is actually lets say 0.6vbut I would say I will use VBE(on)=0.7v.This 0.7v is a common used VBE(on)voltage in the field of average transistors would I say.And here is my formula how I would calculate the base resistor Rbfor the given parameter above:Ub=5VVBE(on)=0.7VIb=5mARb=(Ub-VBE(on))/IbRb=(5-0.7)/0.005Rb= 860 OhmA resistor with 860 Ohm is not a standard value, so you can use the nearest value lets say the 910 Ohm or 820 Ohmbut you can use a 1k resistor for sure.There is more specific calculation for the base resistor value than mine but let as stay simple as it can be, so beginnerscan also use the math and calculate their values.--------------------------------------------------------------------------------------------------------------UPDATE #1Please readreply #1fromdanadakThere we come to a more tuned base driver for a transistor and with a nice schematic alsofor a complete relay driving stage.I didn't got far enough but thanks to danadak we have an additional resistor betven the base and emiter.This transistor is connected by one end directly to the base and the other end to emiter.The calculation for this transistor for a simple home DIY-er I vould say it is Rb* 10.So we come out from my example before if you used a 1K resistor, does this so called pull-down-resistor valuewill be 1K * 10 = 10K.This resistor will for sure hold down the transistor base and secure the closed stat of the transistor untila signal voltage isn't feed to the base.Thank you danadak for this valuable share.---------------------------------------------------------------------Please share any other options too if you have.Simplicity must not be a wrong way...

author: danadak
author_rank: Super Contributor
timestamp: February 25, 2018, 01:46:30 am
content: If you are driving from a UP the output high voltage, for a 5V part,is significantly less than 5V.Also if the output of the UP is tri stated, you need an additional R from base to ground,on the order of 10K or higher to absorb any transistor leakage to keep it from turningthe transistor on. Leakage specs in transistor datasheet. This complicates the R compu-tation somewhat it robs some current from going into base.Regards, Dana.