Current Sources

Started by September 3, 2017
```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.

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

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.

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.

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.

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/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.

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.

I highly recommend you never try teaching.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998
```
```
news:ooh7aa\$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.

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.

```