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Transistor dead again

I am trying to convert a pushbutton to a toggle switch. The circuit is
working fine for a few tests, then I turn off the battery, come back
the next day and it's not working any more. Replacing the transistor
Q1 fixed it once but now it's dead again which makes me think that
there is some underlying problem.

The light bulb is draining 60mA at 12.5V.

.---------o-----------------------------------.
| .-. | |
| | |10k .-. --- +13V
Q2 | | | ( X ) -
\| '-' '-' |
2N2222 |-------o---------------------o | |
<| | ___ | | ===
| | 2N2222 .--|___|----)------o GND
| \| | | |
| |-------o 330k | |
/+\ Volt <| Q1 | | ||-+
( )meter | | | ||<-
\-/ | | '---||-+ NTE67
| | | |
| .-. .-. |
=== | | | | ===
GND 4M| | | |10k GND
'-' T '-'
| --- |
o--o o----'
|
---
--- 220nF
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
 
J

john jardine

Jan 1, 1970
0
I am trying to convert a pushbutton to a toggle switch. The circuit is
working fine for a few tests, then I turn off the battery, come back
the next day and it's not working any more. Replacing the transistor
Q1 fixed it once but now it's dead again which makes me think that
there is some underlying problem.

The light bulb is draining 60mA at 12.5V.

.---------o-----------------------------------.
| .-. | |
| | |10k .-. --- +13V
Q2 | | | ( X ) -
\| '-' '-' |
2N2222 |-------o---------------------o | |
<| | ___ | | ===
| | 2N2222 .--|___|----)------o GND
| \| | | |
| |-------o 330k | |
/+\ Volt <| Q1 | | ||-+
( )meter | | | ||<-
\-/ | | '---||-+ NTE67
| | | |
| .-. .-. |
=== | | | | ===
GND 4M| | | |10k GND
'-' T '-'
| --- |
o--o o----'
|
---
--- 220nF
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Is the sketch right?. Seems it powers up and the bulb must come ON.
The 4meg and "T" in the right place?.
 
R

Ross Herbert

Jan 1, 1970
0
I am trying to convert a pushbutton to a toggle switch. The circuit is
working fine for a few tests, then I turn off the battery, come back
the next day and it's not working any more. Replacing the transistor
Q1 fixed it once but now it's dead again which makes me think that
there is some underlying problem.

The light bulb is draining 60mA at 12.5V.

.---------o-----------------------------------.
| .-. | |
| | |10k .-. --- +13V
Q2 | | | ( X ) -
\| '-' '-' |
2N2222 |-------o---------------------o | |
<| | ___ | | ===
| | 2N2222 .--|___|----)------o GND
| \| | | |
| |-------o 330k | |
/+\ Volt <| Q1 | | ||-+
( )meter | | | ||<-
\-/ | | '---||-+ NTE67
| | | |
| .-. .-. |
=== | | | | ===
GND 4M| | | |10k GND
'-' T '-'
| --- |
o--o o----'
|
---
--- 220nF
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I am not sure if the circuit is correct, but assuming it is, then Q1
is not likely to take a hit, but Q2 is - if that voltmeter connection
were to experience a momentary short circuit.
 
Is the sketch right?. Seems it powers up and the bulb must come ON.
The 4meg and "T" in the right place?.

Oops. There are mistake here. Q1's emitter goes to ground and the 4.7M
is connected to the collector instead. It's based on the third circuit
here: http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm

The voltmeter is actually a voltage comparator, but I ran into
problems just connecting a 10k in its place. Initially the FET is off
and thus minimal current goes through the lamp. Briefly toggling Q1
then raises the gate voltage and turns the light on. At least that's
what it did yesterday but not any more..
 
J

john jardine

Jan 1, 1970
0
Oops. There are mistake here. Q1's emitter goes to ground and the 4.7M
is connected to the collector instead. It's based on the third circuit
here: http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm

The voltmeter is actually a voltage comparator, but I ran into
problems just connecting a 10k in its place. Initially the FET is off
and thus minimal current goes through the lamp. Briefly toggling Q1
then raises the gate voltage and turns the light on. At least that's
what it did yesterday but not any more..

It's a nice circuit arrangement and will work fine.
There's no way that Q1 can be blown by overvoltage or overcurrent. Sounds
like there's a wire shorting somewhere.
 
J

Jon Slaughter

Jan 1, 1970
0
Oops. There are mistake here. Q1's emitter goes to ground and the 4.7M
is connected to the collector instead. It's based on the third circuit
here: http://ourworld.compuserve.com/homepages/Bill_Bowden/page9.htm

The voltmeter is actually a voltage comparator, but I ran into
problems just connecting a 10k in its place. Initially the FET is off
and thus minimal current goes through the lamp. Briefly toggling Q1
then raises the gate voltage and turns the light on. At least that's
what it did yesterday but not any more..

Could it be happing when you remove the power? It seems that if its working
while the power is on and if theres no power then it can't fail(except maybe
caps but doubtful) then it must happen when the power is changed.

You might try running it with the power on for a while and put it through
some stress test and see what happens. If it lasts for a long time then try
turning the power off for a split second and then put it through another
test for a few mins. Repeat this a few times. Then turn it off for a longer
period of time and try. You'll be able to determine which stage the problem
is comming from.
 
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