Author Topic: Need help reading transistor datasheet  (Read 2666 times)

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Offline robvanderveer

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Need help reading transistor datasheet
« on: January 12, 2014, 11:18:53 am »
I was working on a project and found that an arduino signal through a 100k ohm resistor does not fully saturate my bc547 npn transistor, but a 1k resistor does.

Where in the datasheet can i find the info that tells me to use the correct resistor to drive the base of the transistor?

Edit: add link to datasheet

http://www.nxp.com/documents/data_sheet/BC847_BC547_SER.pdf
 

Offline mariush

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Re: Need help reading transistor datasheet
« Reply #1 on: January 12, 2014, 11:26:42 am »
Determine the hFe of the transistor, then you can do the math.

This video explains very well how to determine the resistor value:

ps. Considering the transistor is 100mA max and the minimal typical hFe is 180 and the minimum hFe being 110, I would do the math for 1 mA going to the base.
« Last Edit: January 12, 2014, 11:29:30 am by mariush »
 

Offline KJDS

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Re: Need help reading transistor datasheet
« Reply #2 on: January 12, 2014, 11:28:46 am »
The parameter you need is Hfe and then use ohms law.

So if the Arduino drives 5V and there is 0.7V on the transistor base, then that's 4.3V across the resistor. 100k resistor allows 43uA through, with a Hfe of 100 then that's 4.3mA of collector current. If you have a 10k in the base, you'll 43mA.

Offline lewis

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Re: Need help reading transistor datasheet
« Reply #3 on: January 12, 2014, 11:49:57 am »
And because hfe varies widely with temperature, age, voltage, between identical transistors from the same batch and by phase of the moon, you'll want to half the resistor value calculated by the maths for reliable operation.

Or, if the datasheet specifies a range of hfe, use the lower number to calculate the base resistor the reduce it further by 20%.
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Offline robvanderveer

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Re: Need help reading transistor datasheet
« Reply #4 on: January 12, 2014, 11:52:02 am »
Awesome! Thank you for explaining. Excellent video btw.
« Last Edit: January 12, 2014, 01:03:41 pm by robvanderveer »
 

Offline qno

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Re: Need help reading transistor datasheet
« Reply #5 on: January 12, 2014, 02:16:02 pm »
If you want to drive the transistor in saturation the situation changes.

In saturation Vce is about 0.1 volt.
With a grounded emitter this means the collector voltage is 0.1V.
Your base is 0.7 volt.
This means a part of your base current will leak to the collector and not contribute to the drive of the transistor.

A rule of thumb is to use 10 times the base current you need for the collector current when calculating with the lowest HFE from the datasheet.

One of the appendixes in The Art of Electronics discribes saturation in detail.
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