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Topic: conversion help (Read 641 times) previous topic - next topic


Feb 09, 2012, 07:56 am Last Edit: Feb 09, 2012, 08:36 am by annonymous Reason: 1
How to I calculate the size of the resistor needed at the base of my transistor?


You need to know the current in the collector, that is the current you want to switch. Then you need to know the gain or Hfe of the transistor.
Then the base current is = gain X collector current.
Then double it to give yourself a margin.
The resistor is found by ohms law, with the voltage being 0.75 V less than the voltage on the other end of the resistor. So in the case of the arduino that is 5 - 0.75 = 4.25V


See what I thought is that if I'm applying 5v to the base, wouldn't the basecurrent be 5v? And make the collector current 500?


you will only aply 5volts directly to the base once
after that all the smoke gets out
you always need a resistor between the driving pin and the base
secondly, the emitter base "diode" has a voltage of about 0.7 - 0.75 volts when conducting
so as Grumpy Mike says the current is (5-0.75)/resistance
there are only 10 types of people
them that understands binary
and them that doesn't


wouldn't the basecurrent be 5v?

I hate to break it to you but 5V is not a current it is a voltage the clue is in the name.

If you connected the base of a transistor directly to 5V you probably would blow it up.


Sorry grumpy Mike. I'm new to all of this. I have never heard of any of this until a few days ago. I just program. Its a big change of pace! I guess this first project I picked wasn't for a beginner. Non-the-less, I have all the parts so I am going to see it thru! Its just the formulas, math, names of measurements such as current and resistance...are confusing! :smiley-eek-blue: Still having trouble understanding!

I thought 5v was a current but now I know its not. Let me Google some more on transistors. And see if I can't answer my questions. Whatever I don't/can't understand I will ask here. 


You might like to look at this on transistors:-

You need to understand Ohms law:-
Just remember
Voltage - pushes
Current flows
Resistance - resists the flow of current.

Good luck.


Suggestion - Get yourself a multimeter so you can measure voltage, resistance, and current*.   You can get one for around $10 USD.

Just FYI - Be careful about trying to apply Ohm's Law to transitors, because the "resistance" is not constant.   Ohm's Law always applies, but you need to know what's going on with the components and thier limits, etc.  There is also Inductive and capacitive reactance, and impedance, which are also measured in Ohms, and also "resist" the flow of current.

I don't want to discourage you.  It's fine if you want to play around with transistors.  But there's a LOT to learn and if you were taking electronics in school you probably wouldn't study transistors 'till the 2nd year...  maybe the 2nd semester.    Typically, you'd take "DC Circuits" and "AC Circuits" first and learn all about resistors, capacitors, and inductors in various series & paralel combinations, and batteries (and power suplies) in series & parallel,  and about something called "superposition".

* Read the instructions, or do some research before measuring current.   You need to break the circuit and insert the meter in series.    If you switch the meter to Amps and put the probes across your circuit (in parallel) you can blow a fuse in the meter, or blow something in your circuit.

And, you need to turn-off power before measuring resistance.   Nothing bad will happen, but you'll get invalid readings if there is current or voltage in the circuit you are trying to measure.

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