Current Sources

C

Chris

Guest
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
 
In article <ooh7aa$k0v$9@dont-email.me>, cbx@noreply.com says...
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...

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
 
In article <MPG.3415f82283a14fec98a042@news.eternal-september.org>,
jamie_ka1lpa@charter.net says...
In article <ooh7aa$k0v$9@dont-email.me>, cbx@noreply.com says...

Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.

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
Click to expand...

Correction:

Since I = V/R = 3.6 / 180 = 20mA
 
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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:
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...

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
 
Chris wrote on 9/3/2017 12:46 PM:
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.
Click to expand...

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
 
On Sun, 3 Sep 2017 16:46:18 -0000 (UTC), Chris <cbx@noreply.com>
wrote:

On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.
Click to expand...

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 |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I'm looking for work... see my website.

Thinking outside the box...producing elegant & economic solutions.
 
In article <oohbkq$k0v$12@dont-email.me>, cbx@noreply.com says...
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.
Click to expand...

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.
 
Ralph Mowery wrote on 9/3/2017 1:23 PM:
In article <oohbkq$k0v$12@dont-email.me>, cbx@noreply.com says...

On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.

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.
Click to expand...

I highly recommend you never try teaching.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998
 
"Chris" <cbx@noreply.com> wrote in message
news:eek:oh7aa$k0v$9@dont-email.me...
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...

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.
 
On 03.09.2017 18:46, Chris wrote:
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.
Click to expand...

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
 
On 9/3/2017 11:46 AM, Chris wrote:
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.

Consider it this way:
Click to expand...

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
 
On Sun, 3 Sep 2017 15:32:26 -0000 (UTC), Chris <cbx@noreply.com>
wrote:

Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...

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:

https://www.dropbox.com/s/yl3kqe9mp4jhvxp/Elmo.jpg?raw=1

Good arm.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics
 
On Sunday, September 3, 2017 at 8:36:55 AM UTC-7, Chris wrote:
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...

Howabout a pitcher with water pouring from the spout? Works for me.
Variants: coffeepot, bucket, Niagara Falls.
 
In article <oohbkq$k0v$12@dont-email.me>, cbx@noreply.com says...
On Sun, 03 Sep 2017 12:30:12 -0400, M Philbrook wrote:

[....]

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.
Click to expand...

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
 
On 9/3/2017 11:32 AM, Chris wrote:
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Any guidance would be much appreciated; thanks.
Click to expand...
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.
 
On 2017-09-03, Chris <cbx@noreply.com> wrote:
Hi

Finding it real tough trying to visualise a current source. I have no
probs with voltage sources which seem very logical, but for some reason I
can't get my head around current sources. How should I try to see them?
Click to expand...

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
 
On Sun, 03 Sep 2017 22:26:49 -0400, M Philbrook wrote:

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..
Click to expand...

Hey thanks, man that's a lot clearer now!
 
On Mon, 04 Sep 2017 00:02:28 -0400, Tom Biasi wrote:

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.
Click to expand...

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.
 
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