walid Posted January 18, 2007 Report Share Posted January 18, 2007 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 = 200mAI read from this page:http://www.kpsec.freeuk.com/trancirc.htmthat: 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 18, 2007 Report Share Posted January 18, 2007 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. Quote Link to comment Share on other sites More sharing options...
walid Posted January 22, 2007 Author Report Share Posted January 22, 2007 hi guruYou did not understand me, so I'll ask the question once again : The 2N3904 transistor have the following char. from datasheet:Po = 625 mWIc max = 200 mAVEC max = 40 Vfrom that, if we have VCE = 5V, the max IC = 625m/5= 125 mANow if at saturation, VCE=0.4 V, then Ic max = about 1.6 Amy question now, can i use a load draw 1.5A in a swtching circuit using 2N3904thanks Quote Link to comment Share on other sites More sharing options...
audioguru Posted January 22, 2007 Report Share Posted January 22, 2007 if we have VCE = 5V, the max IC = 625m/5= 125 mANow if at saturation, VCE=0.4 V, then Ic max = about 1.6 Amy question now, can i use a load draw 1.5A in a swtching circuit using 2N3904No, 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. Quote Link to comment Share on other sites More sharing options...
walid Posted January 22, 2007 Author Report Share Posted January 22, 2007 ok thank u guru Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.