Shout out for help. Solid state jack switching...

[italianguy63]

I am losing my mind.  This is something comfoundedly simple; and I can't seem to solve it.

I have been using this solid-state switching scheme on my circuits for YEARS.  No problems.  All of a sudden-- major problems.

I think the original idea was from Jack Orman, but honestly it was so long ago, I don't remember, and I don't have any notes about it.


The problem started when I got a new batch of transistors.  They stopped working.  I used to use 2N3906 as shown, but found sometimes they did not pass but about 1/2 of the 9V supply.  I switched to 2N4403 many years ago, and have had happy sailing for a long time.

Now, NOTHING.  Circuit will not pass voltage.  I know I'm not crazy!  I have tested (even built a test platform) 2N3906, 2N4403, 2N2907, and BC557.  My OLD STOCK transistors worked.  Now, all the new stock transistors I have are not passing voltage! 

I can jumper the Collector to the Emitter, and everything is hunky dory.  But, with the transistor in place, no worky.  I have checked everything else manually.  (resistor values, continuity, etc.).. and it has to be something with the transistors.

Can anyone offer assistance? 

Losing my mind.
MC

[tonyharker]

Are you sure the leads are the right way round?  Some manufacturers swap E & C and some will have the collector in the middle.

[italianguy63]

Are you sure the leads are the right way round?  Some manufacturers swap E & C and some will have the collector in the middle.

Yeah.  For sure, the pinouts are correct.

MC

[italianguy63]

Found it.

Problem with the tester board I built.  Bad solder connection on the Base socket.

Weird because there was solder on both sides of the PCB via.  So, it flowed through.. however, no continuity from the top of the PCB to the bottom of the PCB!  Refloated it twice, and all of a sudden I'm getting voltage (F-ing imagine that!).  OK, I can come out of my rabbit hole now.

Sorry guys!

However, now I am able to replicate the original problem I was having.  Recent batches of trannies are not working properly!

In another post-- Tayda 2N4403's (on paper tape) are faulty.  As soon as I put a load on this circuit (an LED w/2K2 dropdown), the voltage drops to 5.2V.

I got some non-Tayda 2N4403's... and they are putting out 7.4V.
Some old 2N3906's I have are yielding 8.5V

and finally, the non-Tayda BC557's are yielding 9+ volts.  At least these are working short term.

So, the question remains-- WTF is going on here, and why won't these transistors latch solidly?

MC

[diffeq]

I remember the 2N4403 woes thread. If the same circuit platform works with one type of transistors, but not the others, something must be wrong with the parameters of the latter. What I would do, is measure the base voltage of "good" and "bad" ones under LED+2k2 load, and play with R1/R2  ratios (with trimmers), to see under what base voltage bad trannies work as expected in this load.  I suspect their gain moves bias point away from 1k/6.8k point.

[anotherjim]

I think R1 is the possible hitch.
Transistor E-B volt drop is whatever the particular junction wants. There is no absolute need for the voltage divider formed by R1 & R2 to go anywhere close to setting the junction voltage. R2 is essential in setting the base current at a sensible and non-destructive value, but R1 is really only a "nice to have" part.
All R1 needs to do is to "pull off" the E-B junction when the jack plug is out and the ring "floats", since we don't want Q1 to conduct out of its collector then. In reality, unless it's a bad & leaky transistor or pcb, the transistor won't conduct anything meaningful without the E-B forward bias.

If you know the minimum hfe of Q1, then ohms law with supply volts minus E-B drop with R2 gets you the base current and that times the hfe gives the collector current maximum. However, R1 ruins all that by adding a parallel path to the E-B which means some current bypasses the E-B junction and won't contribute to turning the transistor on.

That said, I think the idea may have been to provide some current limit protection by not having the transistor saturated, as you found when the output voltage drops with the collector loaded. In that regard, the idea isn't perfect since differences in the transistor parameters change everything, sometimes in the wrong direction.
R1 can be much higher, x10 of R2 or more. R2 can be the highest value needed to keep base current safe for the particular transistor while having it fully on.

[italianguy63]

I think R1 is the possible hitch.
Transistor E-B volt drop is whatever the particular junction wants. There is no absolute need for the voltage divider formed by R1 & R2 to go anywhere close to setting the junction voltage. R2 is essential in setting the base current at a sensible and non-destructive value, but R1 is really only a "nice to have" part.
All R1 needs to do is to "pull off" the E-B junction when the jack plug is out and the ring "floats", since we don't want Q1 to conduct out of its collector then. In reality, unless it's a bad & leaky transistor or pcb, the transistor won't conduct anything meaningful without the E-B forward bias.

If you know the minimum hfe of Q1, then ohms law with supply volts minus E-B drop with R2 gets you the base current and that times the hfe gives the collector current maximum. However, R1 ruins all that by adding a parallel path to the E-B which means some current bypasses the E-B junction and won't contribute to turning the transistor on.

That said, I think the idea may have been to provide some current limit protection by not having the transistor saturated, as you found when the output voltage drops with the collector loaded. In that regard, the idea isn't perfect since differences in the transistor parameters change everything, sometimes in the wrong direction.
R1 can be much higher, x10 of R2 or more. R2 can be the highest value needed to keep base current safe for the particular transistor while having it fully on.

If it helps.. my tester has shown all these varieties of transistors have shown an Hfe of between 180ish to 240ish.

MC

[italianguy63]

Jim--  I think you may be "The Man".

Swapped out R1 for 10K--

All outputs jumped up to 9+V  (within a 10th of a volt of each other).

Since I am not an EE and have not "done the math"-- do you see any inherent risks in this change?

MC

[anotherjim]

That will be fine. 47k is probably the first value I'd pull out of the air - but 10k will work too.
The 6.8k base resistor means, in the extreme of someone trying to power it with 18v, about 2.5mA will flow in the base. I think most of the small silicon BJT's we have can handle that. But if you have 180 hfe, then that means the Q could deliver nearly 0.5A from the collector if was capable and if the load demands it.

[italianguy63]

That will be fine. 47k is probably the first value I'd pull out of the air - but 10k will work too.
The 6.8k base resistor means, in the extreme of someone trying to power it with 18v, about 2.5mA will flow in the base. I think most of the small silicon BJT's we have can handle that. But if you have 180 hfe, then that means the Q could deliver nearly 0.5A from the collector if was capable and if the load demands it.

Thank you so much.. from what I remember seeing on the data sheets, these are all rated at about .5A

MC

[italianguy63]

I knew the threshold had to be low because some worked and some didn't-- that is why I started small.   

[italianguy63]

Final note... I dug the "self-proclaimed" bad Tayda 2N4403's out of the trash bin-- and, even those are working now!

MC