Transistors are always very confusing to me. I read and try to understand the specsheet always but never confident about my understanding :( I have PCB (readymade), which gives me a 2.8V output when I switch on the PCB. The 2.8V is coming out from the MCU directly. Now if I drive a BC109 with that output and put a 75ohms relay in a common emitter fashion then will the transistor be able to supply enough current to turn on the relay? According to my calculation, the BC109 max collector current is 200mA and a 12V/75ohms relay will require around 160mA current. If I put an additional resistance in serial with the relay's coil, then I can bring down the current more. Can someone please tell me if I am theoretically all correct or not. I am not attaching any drawing as it is really a tiny circuit and can be visualized easily.


After reading the comments, here is the question in a more mathematical way. Please let me know if I understand it correct.


simulate this circuit – Schematic created using CircuitLab

Transistor : BC337 Relay: 12V / 75 Ohms

Connection type: Common Emitter

Input Voltage (applied to base) = 2.8

Required Current to turn on the relay = 12V / 75 Ohms = 160 mA

Max Ic = 800mA (Safe to drive the relay)

Current Gain to hard on BC337 = 10

Base current required to switch on the transistor (thus the relay) = (Current Gain / Load Current) = 160/10 = 16mA

Vbe = 0.7

Effecting Base Voltage = 2.8 - 0.7 = 2.1

Required base resistor = Vb / Ib = 2.1 / .016 = 131 Ohms (approx)

Transistor's power dissipation: (Vce * Ic) + (Vbe * Ib) = 196mW Approx

  • 2
    \$\begingroup\$ You cannot learn how to use a transistor by just staring at the datasheet. The datasheet is for engineers who already know what they want. What you need to do is look at examples how to use a transistor to drive a relay. Search for them on the internet or even this site. There's a list left right on this page on the right. Driving a 160mA relay with a 200 mA max transistor is pushing it though. Better use a 500 mA transistor. BC109 is also "stone age". Try a BC337 \$\endgroup\$ – Bimpelrekkie Oct 8 '17 at 9:58
  • 1
    \$\begingroup\$ Well, you're still unsure about the correct name. In addition to what @Bimpelrekkie said I recommend you get a book and pick 100 questions or more (about BJTs, MOSFETS, etc) and solve them one by one. This is the only way to become confident in transistors or else. \$\endgroup\$ – dirac16 Oct 8 '17 at 10:14
  • 1
    \$\begingroup\$ BC109 emitter to ground. Collector to relay. Relay between collector and V+ (12V). Reverse diode across relay coil (essential) . Drive base much harder / more current than minimum suggested by current gain. Transistor will turn on harder and have less Vce drop. Limit is what drivce source happily supplies. Probably eg 330 R drive if load is acceptable to drive source \$\endgroup\$ – Russell McMahon Oct 8 '17 at 11:07
  • 3
    \$\begingroup\$ @RakeshMehta: There is a schematic tool on the editor toolbar. If you draw a schematic we will all know exactly what you are describing and you will get good answers. \$\endgroup\$ – Transistor Oct 8 '17 at 14:39
  • 1
    \$\begingroup\$ Calculations appear rightish. Beta of 10 is low but acceptable. 16 mA is too high for many mcus and odds are is nit needed here. Using a high beta BC337 eg BC337-40 ensures good Beta. \$\endgroup\$ – Russell McMahon Oct 9 '17 at 2:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.