why did transistor blow up??

[patroclus]

I builded this circuit into a proptotipe board to see how it worked before final design. when I set R=10ko, Q1 was active, and Q2 was saturated. When I set R=330ko, Q2 blows up (I mean really, in a few seconds transistor blows and stops working, of course)...

why did this happend?? Shouldn't if have to turn Q1 off, and so Q2 ??
How do I calculate Ib and Ic in Q1 using this circuit??

thnaks

 

[John Sorensen]

There is nothing in the circuit to limit the current through BC547 (except hfe-- not a good design). The emitter-base junction of BC557 looks like a forward biased diode, so current can flow freely through BC557's E-B and through BC547's C-E straight to ground. You are probably burning up the E-B junction of the PNP. I'd give you a recommendation for a circuit, but I'm not sure what you're trying to accomplish. For the moment, though,a resistor on the base of the BC557 will save the transistors.

j.

 

[ljcox]

I builded this circuit into a proptotipe board to see how it worked before final design. when I set R=10ko, Q1 was active, and Q2 was saturated. When I set R=330ko, Q2 blows up (I mean really, in a few seconds transistor blows and stops working, of course)...

why did this happend?? Shouldn't if have to turn Q1 off, and so Q2 ??
How do I calculate Ib and Ic in Q1 using this circuit??

thnaks

You can calculate Ib for the NPN by assuming there is 0.7 V across the base/emitter. The current through R is then (5 - 0.7)/R then calculate the current through the 10k, ie. 0.7/10 and subtract this from the current through R to obtain the base current. The collector current is this multiplied by hFE. But since hFE varies widely from one transistor to another you can only calculate the range of current, ie. minimum to maximum by using the min and max hFE.

I would have thought that Q2 would have blown when R was 10k not 330k.

As John said, it is not a good circuit, and if we knew what you are trying to do, we could help more.

Len

 

[patroclus]

**broken link removed**

this is the circuit I try to simulate. Q3 is OFF when Q1 is ON, and the other way round. But I think substituting the Opt1 (infrared reflection detector sensor) with a variable resistor won't lead to same results (just for testing purposes)

 

[ljcox]

http://www.x-robotics.com/images/sch_sniffer.gif

this is the circuit I try to simulate. Q3 is OFF when Q1 is ON, and the other way round. But I think substituting the Opt1 (infrared reflection detector sensor) with a variable resistor won't lead to same results (just for testing purposes)

The circuit at the web site is not a good one fior the reasons we gave.

I suggest you put an 8k2 resistor in series with the base of Q2, ie. between the collector of Q1 and the base of Q2. This will limit the base current to about 0.5 mA.

A variable resistor can be used in place of the Opt1 for testing.

Len