Trasistor Replacement Question

[idylldon]

Hello everyone,

I'm trying to determine if the follow substitutions will work.  It seems they match up well for specs. but I'm not sure they're as suited to the purpose.  They're used in the meter brightness circuitry of my Neotek mixing board.  I have some new ones coming on Monday, but not in time to save a session I have scheduled so I thought I'd see if something I have in stock will work and this is the closest match I could find in my stash:

Original            Replacement
PN2222            MPSA42
MPSH54          MPSA92

The "unregulated" voltage is 35VDC.

Thanks,
--
Don

 

[idylldon]

For future reference, I went ahead and installed them and they work fine.  Probably overkill for the application.

Cheers,
==
Don

 

[JohnRoberts]

the pn2222 is a plastic version of the old metal can 2n2222 a common general purpose npn. I don't recognize the pnp #, so not a  common part I ever used. 

JR

 

[idylldon]

I've been staring at this schematic for quite a bit and can't really figure it out.  Could someone kindly explain what's going on here or point me to an explanation of this circuit?  Seems like a lot of parts for the application.

Thanks,
--
Don

 

[Andy Peters]

I've been staring at this schematic for quite a bit and can't really figure it out.  Could someone kindly explain what's going on here or point me to an explanation of this circuit?  Seems like a lot of parts for the application.

Q102 is a "high side switch" although here I think it's basically a linear pass element. Q101 acts as a level shifter to bias Q102 as desired for the brightness. Q101 collector voltage establishes Q102's base current.

What feeds the "+" node at the top of R136?

-a

 

[idylldon]

Q102 is a "high side switch" although here I think it's basically a linear pass element. Q101 acts as a level shifter to bias Q102 as desired for the brightness. Q101 collector voltage establishes Q102's base current.

What feeds the "+" node at the top of R136?

Thanks!

What are Q103 and Q104 doing?

If this schematic has the same nomenclature as the rest of them for this board, then the "+" is +18VDC.

Cheers,
--
Don

 

[PRR]

> What are Q103 and Q104 doing?

Current Limiter, or Constant Current Source.

It's a building-block. If you're gonna stare at transistors, you may want to know this one.

Overall, the only critical spec is that Q102 be able to dissipate the power/heat _not_ taken by the "brightness" unit (lamps?). I don't know why the whole thing is so elaborate. A big wire-wound pot would do the job? Or barrel of salt-water with two nails?

 

[JohnRoberts]

I've been staring at this schematic for quite a bit and can't really figure it out.  Could someone kindly explain what's going on here or point me to an explanation of this circuit?  Seems like a lot of parts for the application.

Thanks,
--
Don

It is arguably over-engineered, but the parts do have a purpose.

Q101/102 form a variant darlington connection where the current gain of the two transistors multiply but this particular topology enjoys a few extra benefits. Instead of two Vbe drops in series you just have one forward Vbe drop, from Q101. Additionally that drop is relatively load independent since Q101 is running roughly constant current at the Vbe of Q102/1K so around 1/2 mA. Current draw more than that 1/2 mA pulled from Q101 is actually supplied by Q102.  This topology flipped around so a low power PNP fed the bass of a higher power NPN was often seen in in quasI-complementary (as in NOT-complementary) power amp output stages, back before well behaved PNP power devices were widely available. We also see the topology used inside some mic preamps to linearize the input stage devices by running them constant current.

Q103/104 is a simple current source to create the stable voltage drop across R131 and R132. This voltage will track as a constant voltage "less than" the unregulated voltage. Which may be useful or not. Without seeing what this connects to, I can't tell if this is a benefit or inconsequential to the design (my guess). It seems sourcing current would make more sense being stabilized wrt ground not below the + unregulated, but who knows?.

JR

 

[idylldon]

Thanks to all for your explanations!

This is a brightness control for the LED meters on my Neotek mixing console.

Cheers,
--
Don