Switching Transistors with DPDT?

[KerryF]

Was there a way to switch between 2 transistors with a DPDT switch, or do I need to just do it with a 3PDT like I have been?

I know that theres a way to switch between the OpAmps in a TS like how Dano shows on his site: http://www.beavisaudio.com/Projects/ScreamerLab/ .  There you only need 2 seperate connections so a DPDT would work fine for that.

Thanks, I know its a little noob question.  We all have our days 
Kerry

[Ucho]

I made a similar question  a couple of weeks ago.

Here's the thread.

IMO it can be done.

[Crouton Forest]

I'm pretty sure you can use a DPDT switch for bipolar NPN transistors on most circuits. Just solder the emitter and collector to the two poles on the switch (or on the PCB depending on your design) and solder the bases of both transistors to each other. In fact I think I even seen something on the beavis audio website where they use a DPDT to switch transisors: http://www.beavisaudio.com/Projects/FuzzLab/images/FuzzCloneMods.gif

I used a DPDT to switch bipolar NPN transistors on a couple circuits and it worked, not positive if it will work on all circuits. I've mostly used and wasted money on 3PDT toggle swtiches for switching transistors in the past.

My questions would be: what if you have too many connections on the base pin or something like that? And can you do the same on PNPs or FETs and if you can used a DPDT on FETs what pin would be tied together on FETs, Drain, Gate, or Source?

I wish some engineer would post on this thread and answer these questions.

[R.G.]

For DC up to top of audio at impedances under about 100K-1M, in general, you can always switch a device in/out with a switch that has at least one fewer poles than the part has pins.

This is because electricity only flows in current LOOPS, and you must have a completed CIRCUIT around which it can flow. So monodes - things with only one pin connected - can never take part in a circuit's operation.

The only reason this is low frequency and low impedance instead of an absolute is because there is capacitance from every conductor to every other conductor in the universe, albeit small. This capacitance makes another "connection" at high frequencies and impedances. The stray capacitances keep monodes from being true at high frequencies.

So if I were switching between two transistors, I'd common up the emitters, those being in general the lowest impedance point on the transistors, and switch between two bases and two collectors with a DPDT.

My questions would be: what if you have too many connections on the base pin or something like that?

Too many parts connected makes the parasitic capacitance rise. Not bad at low frequencies and low impedances. Generally, bipolars at audio frequencies would be fine. Details always matter, though. In general, no generality is worth a d@mn, including this one. 

And can you do the same on PNPs or FETs

Yes. Same caveats. The gate of a FET is a high impedance point, so the added capacitance may cause problems or not. Details matter.

and if you can used a DPDT on FETs what pin would be tied together on FETs, Drain, Gate, or Source?

Like with the bipolars, I'd use the source, that being the lowest impedance point, in general, for most FET circuits.

I wish some engineer would post on this thread and answer these questions.

Free engineering advice is kind of a catch-as-catch-can thing. I sure wish I had a good engineer to answer some of my questions too.

[pappasmurfsharem]

Free engineering advice is kind of a catch-as-catch-can thing. I sure wish I had a good engineer to answer some of my questions too.

You could talk to yourself in mirror

[R.G.]

You could talk to yourself in mirror

I do that sometimes, but I catch myself repeating myself repeating myself repeating myself...

Give me a headache. 

[LucifersTrip]

just a quick note...you can in many instances switch transistors with a SPDT.  When I do that, if I remember correctly, I switch the base and leave  E & C connected.

just take voltages and do sound tests of each separately (before and after switch) to make sure it works.

I have even used a SPST...It was discussed in another thread, but many times one transistor "takes over" the other, so you can leave one connected all the time and just switch in the other.

[PRR]

> ... ... ... in general, you can always switch a device in/out with a switch that has at least one fewer poles than the part has pins.

I knew this rang a bell. Aside from your authority, and general reasonableness, it turns-out that the question was studied (in terms of metering) and solved in 1893.

Blondel's theorem

The minimum number of meters to ensure ALL power is paid-for is one less than the number of conductors.

Switching is equivalent to metering. If a meter reads dead-zero, we may replace it with an open switch yet still be sure the customer can't sneak ANY un-metered power.

Utility Power is a brute crude thing; audio systems sometimes use devious paths. We can imagine that stray capacitance could matter in a high-impedance audio circuit. However Blondel is still valid IF we understand "conductor" to include not just wires but also capacitances. (And in the case of old cardboard tag-strips, all leakage paths.)

But more often a 3-leg transistor has no appreciable audio path except its legs, and a 2-pole switch completely cuts-out a 3-leg device. Thanks Blondel!

Blondel's paper is in Google Books, find page 112 (pull-down menu marked "Front Cover" or "Contents").