An "active" device

One definition is something which requires a power source distinct from the signal.

Trying to define it so that a relay is excluded[1] but semiconductors used in digital mode aren't is kinda tough. The definition offered by "Nobody"

?? doesn't do a good job at that. . . [1] or does someone consider a relay an active device? Konrad Zuse perhaps?

I think I'd have to consider a relay "active" in a broad enough sense of that term - and I'm also having problems narrowing it down such that it doesn't exclude some other things that ARE commonly thought of as "active."

Some other possible definitions:

A passive device would be any which either dissipates energy, (resistors) passes it along (ideally without change - I'm trying to think of the right terminology to cover transformers as well as simple conductors and such), or temporarily stores it (reactances), with no other function; active devices are pretty much everything that's left.

Active devices (a) require an external source of power in addition to the signal or current being controlled or acted upon, and/or (b) contain one or more non-linear elements (tricky, though - "nonlinear" could be a problem when we start looking at magnetic components with hysterisis and such.

We could, of course, just start by throwing things into bins - all the wires, resistors, inductors, capacitors, transformers, and simple switches go into the "passives" bin, all the transistors, tubes, diodes, and ICs go into the active, and we leave anything not yet covered to the reader as an exercise, with extra points to be awarded for sufficiently clever explanations as to why they chose to put a given sort of part into a given bin....:-)

Bob M.

I think this is a good question because it made me thing a little more about the idea.

Basically, I used to look at an active device or function block as one that requires attachment to a power source in order to properly operate.

In contrast, a passive device or function block is one where it can perform its service without access to a separate power source.

So a passive block looks like:

,---------, | | | | signal in -----> | | -----> signal out | | | | '---------'

and an active block looks like:

pwr | ,---------, | | | | signal in -----> | | -----> signal out | | | | '---------' | gnd

A transistor is active because it cannot perform most of its usual actions (control action, voltage or current amplification, or power gain) without access to a power source. A resistor is passive because it usually doesn't require the supply of power to perform its action.

However, none of that is perfect.

Is a diode active or passive under these definitions?

Jon

Under this definition, it's passive, as it requires no external power source. That was the same problem I had with that definition.

Or, just to throw more JP-4 onto the fire..:-) - what's a solar cell? A photodiode?

Bob M.

Bob Myers wrote:

REALLY broad, IMO.

Yup. Kirwan mentioned diodes.

...and when considering wound components,

*saturable reactors* are a fly in the ointment.

But really, all that's needed is a broad definition, and then add that there are expections.

The poster is trying to wrap his head around the concept, not fully define the term.

Start with why there is a defining line, either it serves a purpose or there was no good reason for drawing that line. Once one understands why there is a need to divide between active and passive, then the definition becomes more obvious.

Transistors, capacitors and relays (or no relays) are merely examples of active and passive devices. They don't define the terms.

So once someone figures out a basic definition, that should suffice, avoiding the examples completely, or more likel using the examples to reinforce the definition, rather defining with the examples.

SO long as you use examples to define, there will always be exceptions, and likely even some devices that can fall into either camp, depending on useage.

Michael

A diode is a non-linear resistor and it requires no 'external' power to function. So passive.

Graham

A relay can provide power amplification. That certainly rules it out as passive in my book.

Graham

Bob Myers wrote:

JeffM wrote:

Michael Black wrote:

Well, now you're just making it too easy. 8-)

An active device controls an output voltage or current by means of an input voltage or current or it has an intrinsic turn-on threshold or non-linear curve.

On Fri, 03 Aug 2007 04:35:44 +0100 in sci.electronics.basics, Eeyore wrote,

How about a tunnel diode?

"David Harmon"

** The short answer is - there isn't one.

The term " active " is a name used to classify strictly electronic devices as opposed to merely electrical devices and is defined by popular usage, not science.

It includes all semiconductors, valves ( or tubes) and maybe even the humble neon lamp.

Electromechanical devices, like relays, are not included because they are electrical rather than electronic.

...... Phil

That's the example I was thinking of that can be both passive and active, depending on useage.

Michael

Eeyore disagrees with you on this. I think you should insult him.

Only if he wants to start a religious war. ;-)

Cheers! Rich

Usage.

But, notwithstanding that, the one device that to me perfectly straddles the active/passive line is the diode.

Stick a diode in the feedback loop of a voltage follower, and you have a "precision rectifier". Is the diode "active" or "passive" in this role? >:->

Cheers! Rich

On Thu, 02 Aug 2007 21:22:40 -0700, JeffM wrote: ...

Boy, that would simplify things!

Active == Non-linear.

;-)

Cheers! Rich

Gaw, we could drag this on for months! Is a solenoid active? Is a switch active? Switches certainly do get "activated", or maybe "actuated" would be more accurate. ;-)

But, when you control a switch with a solenoid (i.e., "relay"), I believe it suddenly becomes an active device.

Then again, I've seen solenoids that certainly do some activity! Ever played pinball? ;-)

Cheers! Rich

^^^^^^ USAGE!

Misspellings like this really impugn your credibility.

Thanks, Rich