Current Sources

Or maybe the exact opposite.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I thought your first explanation nailed it...

The second analogy is a good one to describe power.

I always found current sources harder to imagine than voltage sources. Batteries for the voltage source. But you can't buy stand alone current sources. Some beta decay source inside a metal sphere is not a bad example.

(I mostly think of current sources, as how I make the electronics.. which seem to all go back to a voltage and a resistor... hmm I guess we could argue about jfet current sources.)

George H.

Beta batteries are cool, but it's hard to use a 400KV nanoampere power source. It has been done.

I know of no simple-physics sources (like, for example, an illuminated photodiode, makes photons into electrons) that are as good as an electronic cc circuit.

Another interesting gadget is the pumped single-electron transistor. Every clock pulse, it lets through exactly one electron.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Huh, I didn't know there was such a device.

Sounds like it didn't have 400 kV of compliance, but blasted the beta's into a PN junction and collected the ionized electrons.

George h.

Is this because you can buy voltage sources in the shops but not current sources?

You know that a voltage source does its best to change the current so as to keep the voltage constant.

Any real voltage source has a very low but finite internal resistance. Within limits it can produce whatever current you want and the voltage doesn't change very much, but the current can only go up to some limit and then it will fail. If you put 8 AA cells in series you can have a curent of

1 mA or 100mA and still measure 12V. You can make the load resisrance go lower and lower and still measure 12 V but you'll measure 0V if you try to start the car.

Do you know that a current source does its best to change the voltage so as to keep the current constant?

Any real current source has a very high but finite internal resistance. Within limits it can produce whatever voltage you want and the current doesn't change very much, but the voltage can only go up to some limit and then it will fail. So a 1mA current source, unless it's designed to do so, won't likely be able to produce the 100,000 volts needed to sustain 1mA through 100M ohms and will produce 0mA instead, but if it's connected to 1K ohms then it probably won't find it hard to produce the necessary 1 volt. And if it's connected to 2K ohms then it will likely be happy to produce the necessary 2V.

Probably not: the collector of a transistor is such a current source, and they're CHEAP, sell by the millions. The reason you can't buy "a current source" is that they're so easy to build. Well, until you get to the megavolts-of-compliance current sources, they're easy.

You can buy current sources too, if you don't mind adding one resistor to an LM7805.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

There are 2-terminal current-limiting things available, typically used as LED regulators. I'm guessing that they are depletion fets inside.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

If you don't mind about a 7V drop. I think you meant those regulators that use an LM317-style chip architecture? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
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             I'm looking for work... see my website. 

Thinking outside the box...producing elegant & economic solutions.

You can do that too. The OP was apparently asking a rhetorical question.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Sure but you have to wrap a voltage source around it.

It would be cool to have a stand alone current source*. I guess more so when I was younger and didn't have an intuitive feel for a current source.

George H.

*OK a battery operated lm7805/ lm317 current source, but that is somehow not the same.

Tape it to a battery. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

I once (for a school project) built a 2-terminal negative 1K resistor, battery powered. It executed all of the standard circuits, with a negative R value plugged into the equations: voltage dividers, RCs, LC's with negative Qs, all that. It was fun. My prof seemed unimpressed.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

You mean like an arc welder, a generator, a current transformer, even a capacitive dropper makes a reasonable approximation of a current soruce.

OTOH:

A CR2032 cell is sufficiently current-source like to make it useful for testing LEDs

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This email has not been checked by half-arsed antivirus software

Simplest thing is a current regulator diode (jFET with G-S short); compliance circa 200V, power supply not included. <

A small van de Graaff generator, battery powered, is just a little too... complicated to be an 'ideal component'. Imagination doesn't grasp the intrinsic simplicity when confronted with whirs and zaps.

... compliance circa 100V, power supply not included.

Forgot the link:

Sure or just a high voltage and a big resistor. But I meant a current source that was like a battery. A two terminal thing, that spits out ~1mA (or anything) at 1 kV of compliance. You'd keep it short circuited with a kilo ohm or so, so it wouldn't kill you.

It's really just a pipe dream.

George H.

1 mA wouldn't kill you, or even shock you. But a just-right bad skin contact could dissipate 1 watt, which might make a small burn.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics