Current Sources

That's not true. You simply need a power source with enough voltage to power the circuit and enough current to supply the load. It can be neither a voltage nor a current source.

Instead of short circuiting with resistance, it would be better to short it with a... short. The short is the current source's equivalent of an open circuit for a voltage source. No resistance equals no power.

I bet it just requires more thought from the right perspective. A chemical battery works because of the work function of various elements and compounds. This work function equates to a voltage independent of whether current flows. What would produce a current that depletes if the current doesn't flow? I will say I doubt something as simple and compact as a battery will be found. I think someone pointed out that radioactive decay is like that but hard to turn into a battery. What else?

Aren't all power sources best approximated as voltage sources? I don't know that much about power generation.

George H.

Well as someone else said a photodiode with a bunch of light shinning on it. But the compliance stinks. I guess you could stack up a hundred or so.

George H.

I would not say all, as most of the old nicad batteries were charged with a constan current source.

Many of the most often power sources are mainly a voltage source. The power coming into your house is mainly a voltage source. It will most often have 120 volts plus or minus about 5 volts depending on many things. The Batteries are mostly voltage sources. The put out a relative constant voltage as long as they can provide enough current to the load.

Outside of charging some batteries and maybe a welder I can not think of any common use of a constant courrent source most ordnary people would come in contact with.

Not sure what "best approximated" means. Batteries are pretty much voltage sources as the output impedance is pretty low. But a transformer with a bridge diode can look a lot like a current source if the winding resistance is high. This is often done deliberately in car battery chargers (or were anyway) so they didn't run too much current into a dead battery and would survive a short without any fancy electronics. Now days they mostly use some electronics so they don't enable the full power until they see the battery voltage on the output. If your battery is totally down one of these chargers might not turn on. I've got a small 15 volt wall wart that I use to trickle charge a car battery. It looks exactly like a current source to the battery because it is always current limited to something less than half an amp.

Ok...

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I am sure the constant current sources are used in many places, but unless you are really into electronics and design, not too many people will see the current sources. Most will see constant voltage or almost constant voltage in many things.

Ralph Mowery wrote on 9/8/2017 6:09 PM:

I'm not sure who else would even know what that means. I don't get your point.

Another example of a current source would be some DACs. They aren't true current sources, but can only be used if you treat them as such with a current to voltage buffer.

My carpeting seems to manage it every winter, though. :(

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Laser psu.

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I use a lot of current sources, running magnetic field coils, controlling heater power (resistor or TEC) Biasing all sorts of diodes accurately.

George H.

I have a zillion current source scribbles. This one is fun, and lights up too!

The LED forward drop can just about cancel the transistor's Vbe temperature coefficient.

This is really goofy:

The current sink is the opamp's supply rail.

I do enjoy your creative use of the opamp power rail(s). We mostly forget it's there, 'cause it gets stuck down at the bottom of the schematic with a cluster of bypass caps.

George H.

In either union electrician's school or HVAC school, I think I remember hearing this equivalent: Air in a balloon is VOLTAGE. Poke a hole in the balloon - that airstream is CURRENT.

I don't quite have it, but if you think about a regulated current source it may get you there.

How about, a regulated current source is a water source with limit on how much water (current) can flow. No matter how hard you try only X amount of water (current) will flow. However, if your (voltage) water source (height of water) is not high enough the (current) water flow could be less if you squeeze the hose (more resistance). Best I got for now. Mikek

amdx wrote on 9/23/2017 7:01 PM:

There are pumps that create pressure but do not force any given volume. Centrifugal comes to mind. The fluid is spun by an impeller and centrifugal force creates pressure at the periphery. But it does not require the fluid to flow at any given rate or at all.

Other pumps force a flow rate regardless of back pressure such as the oil pump in your car or a piston pump. In each case there is a volume of fluid that is being squeezed like toothpaste in a tube so that the fluid will create a great force to get out.

The former is like a voltage source, the latter is like a current source.

I'm not sure these analogies help to understand the workings of a current source.

This seems perfectly clear to me now. I'm hoping no one will find fault with it. Are we all agreed?

If you understand it why does anyone else need to like it?

In the real world a current source is approximated by a high voltage and a high value series resistor. No need to adjust the voltage. As long as the resistance of the load is significantly smaller than the series resistance it will be very close to a constant current supply.