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How to calculate base resistor value of a transistor Answered

Hello all. I'm trying to make autologht OFF and ON . Compnent: 1. Relay : 12 DC volt 2.tramsistor: BC547 3. LDR (I don't know specificatiom) 4. Source supply volt: 12 DC volt. 5. Resistor (need to know value) So how do I calculate of resistor? I have attache my circuit design n type of relay I'm using Thank you


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2 years ago

Prototype the circuit with a potentiometer first, then when it's working, measure the pot and replace with a resistor.

Jack A Lopez
Jack A Lopez

2 years ago

I think I would try for a base current that is about (1/10) = 0.1 the magnitude of the saturated collector current.

For example if the current needed to turn on the relay coil is 100 mA, then I would try to make the base current to the transistor be about 10 mA.

I am guessing that will correspond to a value of approximately 1000 ohm = 1 Kohm.

I mean, in the event of complete darkness, I am expecting the resistance of the LDR to be 100 Kohm or 1000 Kohm, basically, so huge it looks like an open circuit (infinite reistance) compared to R.

Then I assume the base-to-emitter path looks like a forward biased diode (Vforward = 0.6 V), so that the current flowing through R, and into the base of the transistor, is

Ibase = (12.0V - 0.6V)/(1 Kohm) = 11.4 mA

Which is approximately the size of Ibase that I wanted.

In the event of light reaching the LDR, then R and LDR together will resemble a voltage divider, and the amount of base current, Ibase, will be much smaller. Will it be small enough to strangle the collector current, and turn the relay off?

I dunno. If the relay seems to never turn off, even with bright light on the LDR, then I suggest you try making R larger, e.g. 10 K, or 100K?

I mean, I cannot tell you for certain what is the optimal size for R, but I am guessing it will be somewhere between 1K and 100 K.

If you want to want to know more about your BC547, sites like alldatasheet,
have the data sheet for this transistor.

If you want to know more about how BJT NPN transistors work, the Wikipedia page for BJT (Bipolar Junction Transistor)
can maybe give you some hints about that.

By the way, I am naively guessing the way this circuit is supposed to work (if it works at all) is the transistor is saturated when the relay is on, and in active mode (with a small current too weak to turn on the relay) during those conditions when the relay is off.