Current Sources

In general:

A curent source is a device that will supply what ever voltage needed up to a maximum into the load to achieve the desired current load, the device connected to it.

For example:

Lets say a device wants to see 20mA of current and its input load is 180 ohms. The supplied voltage from the current source would need to be

3.6 volts.

Since I = V/R = 3.6 / 180 = 0.20mA

If the current reeceiver device has an input R of 300 ohms the current source would need to generate 6 volts to get 0.020A/20mA.

Many circuits will supply a variable amount of (I) current to control devices, that is, exernal devices that do things via the input current. This means current will be variable and such as voltage from the current source will vary to maintain this current, which is why current transmission lines are used for long runs, where voltage control devices will suffer.

A current source cirucit that saturates at a specific set point has a feed back in the circuit that will stop the increase of output voltage when desired current is reached. Many sources use a simple low value R as the sense junction, note that I said low value. This is because using high values will also limit your current range when output voltage reaches the max.

This sense circuit is connected to a high gain (sensitive) voltage comparator which will regualate output.

Lets say for the moment the sense R is 0.10 ohms.. This would produce

2mV across the sense R. you need to have a circuit that will respond to this 2mV and shift it's output to so maintain it at 0.020A/20mA

A more cheaper and less accurate way is to use a transistor with the Sense R. Many examples around for that and are generally a fixed max current solution for safey limits.

Jamie

Correction:

Since I = V/R = 3.6 / 180 = 20mA

[....]

I'm sure you're only trying to help, but this leaves me no more better informed. I just want a simple way of visualising a current source. For example, using analogs like a lake, waterfall, a river, a dam and such like.

Chris wrote on 9/3/2017 11:32 AM:

This is just a matter of what you are used to. A voltage source will supply current as required by a load to maintain a given voltage. A current source will supply a voltage sufficient to drive a given current. There is no perfect voltage source and there is no perfect current source.

A voltage source can be used as a current source by making the voltage very high relative to the voltage on the load and adding a series resistor to set the current. Likewise a current source can be used as a voltage source by making the current very high relative to what is required by the load and setting the voltage with a parallel resistance.

A current source is only hard to think about if you limit yourself to thinking you can't force current into a load.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Ok, a river is a perfect analog. Current is the flow and voltage is the height drop. If a heavy rainfall drops water that makes the river flow 10x more the water will flow no matter. The river will rise until the rate is reached. In the circuit the voltage on the load will rise until the required current is reached.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Try envisioning a very high valued resistor powered from a very high voltage source. For very small perturbations in a circuit receiving this current, the current is constant. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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             I'm looking for work... see my website. 

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

That may be hard to do.

A constant current source would be a variable voltage source with a resistor in series with it. When the load resistance changes, then the voltage will go up or down dependin if you need more or less current.

Maybe a bycycle rider going up and down hills. He peddles with a constand pressure on the peddles. As he goes up a hill, the transmission shifts gears and he will have to speed up his peddle pushing to get up the hill.

Another may be a pully and rope system. Say a man can only pull with force of 10 pounds. you put a box on the ground an a pully in a tree. he pulls it up . The box then weighs 20 pounds and you add more pullies so he can still pull the box up. To get it up in the same ammout of time , he will have to pull fster.

I highly recommend you never try teaching.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

It should be easy enough to visualise the single transistor current source.

Its essentially a self compensating common emitter stage with no bypass capacitor on the emitter resistor.

The base bias divider sets a standing base voltage, if you don't intend to feed it any kind of signal; you can use some convenient voltage reference device.

Collector current is set by the standing base voltage and the emitter resistor. An increase in current increases the voltage across the emitter voltage - which conspires to cancel out Vbe.

The text book 2 transistor type is common in linear PSUs. The first transistor is the original emitter follower series pass voltage regulator - the 2nd transistor has its B/E junction across a current sensing resistor in series with the output. When the volt drop across the resistor biasses the sense transistor, its collector shunts away some of the base bias to the emitter follower.

A river, as rickman said, is a good example.

A flood may be another - the water (current) will flow its course no matter what, an if you stand in its way (resistance), it will wash you away anyway, rising (voltage) as needed in the process.

Dimitrij

Consider it this way:

A constant voltage source is a water fall of a fixed height (voltage) that has a varying amount of water (current) flowing through it.

A constant CURRENT source adjusts the height of the waterfall (the voltage) to keep the same amount of water (current).

RwP

Imagine two metallic spheres. Standing on one of them is a small demon who, once a second, grabs an electron from near her feet and hurls it at the other sphere, as hard as it takes to hit and stick. She is a current source, 1.6e-19 amps.

The voltage between the spheres gradually increases, as one gets depleted of electrons and the other gets too many. She has to pitch harder and harder as time goes on, to get electrons to go where there are too many already. The numbers are fun.

She looks like this:

Good arm.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Howabout a pitcher with water pouring from the spout? Works for me. Variants: coffeepot, bucket, Niagara Falls.

ok.

If you were standing next to a small river and you placed a floatable object in it to measure the speed of movement, you can think of this as voltage. Now, get your hip boots on because we're going to explore this river now, taking with us a 4 x 4 sheet of plywood. Now slowly start to put that sheet of plywood in the river vertically as you hold it. you'll notice now you have pressure that is a measure of the surface area times the water speed (Voltage). This pressure is current and the source of the current is the speed x volume.

Now lets assume you have put the board in as far as it can go and the remaining water in the river is left to move on, this would be the voltage reserve for larger currents if needed and to get larger currents with this voltage (speed), you need more surface area to block the water which will create more push, hence a wider board.

Lets take this from a pipe view.

You are standing infront of a drainage pipe, a large one but only 25% of it has water coming out and it is at a slow pace. Place that board on it and see how much pressure it produces..

Now do that with a garden hose, even if you crank up the valve for a nice tall tail of water(voltage), you can stop it with your hands, that is low current but the nice tall water stream can be looked at as voltage and may sting a little.

I like this because if high pressure exhausting water (Voltage/Speed) will feel like you got hit with a few needles. The same will happen with a HV arc on your skin, like a static arc. That is HV but low current/ Not enough mass to cause problems..

So a current source is something that is moving (voltage) and the volume of movement (current). So Speed x Volume..

Years ago I use to teach boy scouts simple programming and basic electronics. They understood that fine.

Have fun.. Btw, Pay no mind to Bill Slow-Man.. He's one of those that puts lipstick on a pig. or was he putting it on him? Who knows.

Jamie

Chris, You received a lot of suggestions, some good, some not so good. If you wish to continue to study electronics you need to be able to envision a voltage that will change to maintain a constant current as the load changes.

If a voltage source is an immovable object, then a current source is an irresistable force. As can be deduced neither exists in an ideal form in the real world.

To anthropomorphise it. a real-world current source tries to pass a certain current, how hard it tries depends on how good it is.

How you should try to see them? If you use the hydraulic analogy, where voltage is pressure and current is flow, a current source is like an elephant (or a strong slow engine) operating a displacement pump.

In electronics design most circuits are voltage driven, and what are drawn as current sources don't actually produce a current, but instead restrict the available current, behaving somewhat like a magic resistor.

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

Hey thanks, man that's a lot clearer now!

OK, many thanks to everyone who chipped in here with this thread. Got it now!

Just to add to the noise...

A voltage source is like keeping your foot in the same spot on the gas pedal, and letting the car go whatever speed it goes.

A current source is like cruise control, where the car does what it needs to do to go the same speed, regardless of terrain.