Why is this burning up my 2n3906

[be80be]

I made this tone generator went threw the wiring it's hooked up just like it shows but it burns up the 2n3906 when I power it up. Is the book just wrong.

 

[be80be]

I did some reading could it be the capacitor I used a 1uf didn't have a .1uf

 

[mneary]

The circuit is just lousy. 56k ohms with 12V across it will draw about 214 microamps. If you have a particularly good 2N3906, it may have a gain (HFE) of 300, creating a collector current of 64 mA. This is a power dissipation of 77 milliwatts which is above the transistor's rating. As the transistor gets hot, the gain and leakage increase and destroy everything. 1K between the 2N3906 collector and 2N3904 base might allow the circuit to work without blowing up. No promises, though.

 

[be80be]

I didn't make it It's in a book that I read when I was a kid it didn't work then and it still don't. I was reading about tone generators and it had all most the same circuit. But it had a waring to check the 2N3906 to make sure it didn't get hot it said if it did to change the
resistor and capacitor

 

[mvs sarma]

I didn't make it It's in a book that I read when I was a kid it didn't work then and it still don't. I was reading about tone generators and it had all most the same circuit. But it had a waring to check the 2N3906 to make sure it didn't get hot it said if it did to change the
resistor and capacitor

you try to use 0.1uF cap (disc cap serves) and, reduce the supply voltage to 5V.

 

[UTMonkey]

This is a power dissipation of 77 milliwatts

Don't you mean 770 milliwatts, 12 * 0.064 = 0.768 Watts.

Either way you are right the transistor will NOT like it.

Mark

 

[flat5]

The circuit has been published without being proofread.
It asks for an 8000 ohm speaker.

 

[ericgibbs]

I made this tone generator went threw the wiring it's hooked up just like it shows but it burns up the 2n3906 when I power it up. Is the book just wrong.

hi,
The 8KΩ 'speaker' could be referring to a 8000R earphone.

 

[flat5]

Perhaps but Google does not find 8000r, 8000 ohm, or 8k ohm earphone. Not a standard item. However, you did include a smiley.

 

[ericgibbs]

Perhaps but Google does not find 8000r, 8000 ohm, or 8k ohm earphone. Not a standard item. However, you did include a smiley.

hi,
No joke, I have seen earpieces with impedances as high as 8K, the type that fits inside the ear,, bit like a deaf aid.

In fact the 'design' looks like one I saw some years ago for a 'tone generator' that used the earpiece from a deaf aid.

 

[arhi]

you can find 8K speakers in some ooooooold tv sets (I found some in some old lamp driven tv's from ages ago)... so they were probably common some time ago... you can still find old earphones (mono ones that plug into small old battery operated radio receiver) and those are mostly 8K too

 

[audioguru]

It is a stupid circuit that does not have a current-limiting resistor between the two transistors.
It is also stupid to put DC through the speaker.
It is also stupid to cause the poor little 2N3904 transistor also to burn out.

 

[transistor495]

Suppose he has used an 8Ω speaker in place of 8K. Which will burn out first guruji?
2n3904 or speaker or 10Ω resistor??

 

[audioguru]

I think the 2N3904 will burn out first since its peak current will be about 600mA but its max allowed current is only 200mA.
Then the 2N3906 will get too hot and burn out.
The speaker and 10 ohm resistor should have high enough power ratings or they will also burn out.

 

[be80be]

Like I said I didn't draw the circuit it's in a tab book that I used at school I no why it burned up I just wanted to no if I was right to much current going threw
the transistors and the capacitor i used made it happen that much faster. thanks for your input.
why are you not counting the pot it makes it 106kohms all the way on so you have 113.208 Milliamps at the base of 2n3906 . The only way to learn is to ask and see your sure not good to get it in school if thay use books like thay do lol thanks agin for input. I'm just learning all over or maybe i didn't get the first time

 

[audioguru]

When the pot is max (50k) then it plus the 56k resistor total 106k.
They have 11.4V across them so their current and the base current of the transistor are 108uA. The capacitor and the 10k resistor supply additional base current of 1mA. The 2N3906 PNP transistor amplifies it to about 200mA. Then its heat is over 2W for a moment. I don't think the 2N3904 NPN transistor will survive a base current of 200mA.

Look again. The polarity of the capacitor in this stupid circuit is backwards. It doesn't need to be polarized anyway.

The supply voltage is too high causing the 2N3906 PNP transistor to have avalanche breakdown of its reverse-biased emitter-base junction when the NPN output transistor turns off.