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transistor saturation


walid
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Suppose that You used a transistor 2N3904, in its datasheet I found that the Po = 625 mW; the power which this transistor can dissipate (in the form of heat) equal to 625 m Watt. Now, if we assume that VCE = 0.4 V at saturation, this transistor can pass to 1.5 ampere as Ic.
But in the datasheet Ic max = 200mA
I read from this page:
http://www.kpsec.freeuk.com/trancirc.htm
that:
The power developed in a switching transistor is very small:

In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.
In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the power is very small.

can you explain to me to what value of Ic I can operate this transistor.
thank you.

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Hi Walid,
The power dissipation is max when the transistor is a linear amplifier. If it has 20V across it and only 31.25mA, or 30V and only 20.8mA then it dissipates its max heat of 625mW.
When the transistor is saturated then the voltage across it is very low so it doesn't get hot.

The absolute max current for a 2N3904 is 200mA. Its current gain drops sharply above about 50mA.

A 2N4401 has a max current rating of 600mA. a BC337 has a max current rating of 800mA. They all have the same small case so their 625mW heating rating is the same.

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hi guru
You did not understand me, so I'll ask the question once again :
The 2N3904 transistor have the following char. from datasheet:
Po = 625 mW
Ic max = 200 mA
VEC max = 40 V

from that, if we have VCE = 5V, the max IC = 625m/5= 125 mA

Now if at saturation, VCE=0.4 V, then Ic max = about 1.6 A

my question now, can i use a load draw 1.5A in a swtching circuit using 2N3904
thanks

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if we have VCE = 5V, the max IC = 625m/5= 125 mA

Now if at saturation, VCE=0.4 V, then Ic max = about 1.6 A

my question now, can i use a load draw 1.5A in a swtching circuit using 2N3904

No, it will break. You must not exceed any "max" rating. The max collector current is 200mA. It performs poorly above 100mA.
Use a power transistor like a TIP31 if you need a collector current of 1.5A. Its max collector current is 3A.
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