Hello,
I want to switch a 12v lamp through a transistor NPN with a switching voltage of 5v by the Arduino output.
I have a BC637 transistor: the emitter to ground; the collector on one side of the lamp, and the other side of the lamp on +12v; the base is driven by the Arduino output.
The negative of the 12v power supply is connected to the negative of the Arduino power supply.
The maximum absorption of the lamp is 160mA.
I have to put a resistor between the base of BC637 and the Arduino to limit the current.
I wonder what is the better value of the resistor and its wattage.
Thanks
Hi,
welcome.
A brief summary:
The BC637 has a gain of 25,
https://br.mouser.com/datasheet/2/308/BC637_D-2037207.pdf
so for a collector current of 160 mA, you need 160/25 = 6.4 mA at the base.
Let's use 8 mA for safety.
Considering a Vbe of 1.0V, then we will have:
5V - 1V = 4V; 4/0.008 = 500, (500 ohms),
and W = VI,
W = 4 * 0.008 = 0.032 W (a 1/8W resistor).
500 ohm base resistor and 1/8 W dissipation.
2 Likes
Maybe 470 or 560 as they are in the standard series...
And to really saturate the transistor, you need more current...
So maybe 330 is better.
Which is irrelevant for switching.
For switching (where collector voltage drops below base voltage),
a 5% to 10% base current ratio is commonly used (see the saturation graph in the datasheet).
Which is 8 to 16 mA in this case.
Leo..
Japanese Proverb : You're right.
For my personal knowledge:
Why would you not use a 270 ohm resistor?
From figure 4 of the datasheet, considering Ic = 160mA, Vbe (sat) is ~0.8V and Ic/Ib is 10.
4.2V / .016 = 262.5. Roll that up to 270 ohms. Am I missing something?
Yup, 270ohm is right on the 10% mark. 470 or 560 ohm (~5%) would still be ok.
More than that could bring the transistor out of saturation, which results in a hotter transistor.
Leo..
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