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Author Topic: How to calculate how I can fully open (saturate) a BJT transistor  (Read 4789 times)
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Ukraine
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Good day!

I'm trying to figure out how can I know from datasheet what current  and voltage I should supply to, say BC337 base to fully open it, so it will act as a switch. What value I should look for

I've the following datasheets:
http://www.translocal.jp/radio/micro/bc337.pdf (very short)
http://www.nxp.com/documents/data_sheet/BC817_BC817W_BC337.pdf (from NXP)

Thanks in advance!
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Make sure the base current is more than enough to overdrive (say by a factor of two).  For this you need to know the minimum current gain (hFE parameter) for the sort of current levels involved.

Say the transistor has an hFE from 70 to 200 at the current levels involved (it tends to vary especially for high-current conditions). Then assume the current gain is actually 70 (the minimum).  If you want to pass 300mA in the collector circuit, then the base current (non saturated) is 300/70 or about 4mA.  Double this to 8mA and drive the base with this - should guarantee saturation.  You don't want to try and set the base voltage, it depends on temperature in a very sensitive way, control the base current.

You are using 'open' in a confusing way - a current switch is open when it carries no current (the opposite from a water tap.  Closed circuit - current can flow, closed tap, no water can flow...

Your particular transistor is available in various gain bands, so without knowing which one choose the lowest hFE value (100) for the BC337.
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Thank for the response!

Futher questions smiley

- What parameter shows max current for base?
- If I know target collector current, should I use Hfe curve to find correct Hfe value?
- Are darlington transistors really have only on and off states due very high gain or partly on state is possible?

You are using 'open' in a confusing way - a current switch is open when it carries no current (the opposite from a water tap.  Closed circuit - current can flow, closed tap, no water can flow...
Yes, I was unclear, but you understand me right. smiley Fully saturated transistor = closed switch (current flows), closed transistor -- opened switch (no current).

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