Name of widget to control amps?

[Kris]

Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up in
my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

- Kris

 

On Saturday, June 11, 2016 at 11:30:05 AM UTC-10, Kris wrote:

Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up in
my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

- Kris

A potentiometer can work. With half the power to the load, 6 volt output can be a goal.

V = IR

6 volts = 3 amps 2 ohms

Power = VI = 18 watts

 

Rheostat, 18 watts

 

[Dan Coby]

On 6/11/2016 2:30 PM, Kris wrote:

Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up in
my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

- Kris

To answer your question, we really need more information. To us, your
question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R
where V = voltage (measured in volts)
I = current (measured in amps)
R = resistance (measured in ohms)

This equation can also be rearranged as:
or I = V / R
or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then the
voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the current
will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then the
current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it, then
the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to your
12 volt source that you also want to limit to 3 amps? There are two
possible answers:

1) The current will be limited to 3 amps. This means that the voltage
will not be held at 12 volts, instead, it will have to drop to 9 volts.
This contradicts your statement that the voltage remains at 12 volts.

2) The voltage will be held at 12 volts. This means that the current
will increase to 4 amps which contradicts your desire that the current
is limited to 3 amps.


We can give a better answer if you really describe what the application
is that you have in mind.


Dan

 

[Kris]

Dan Coby <adcoby@earthlink.net> wrote in
news:55mdnSToxfV_E8HKnZ2dnUU7-RHNnZ2d@earthlink.com:

Thank you, Dan,

I understand the V to I to R relationship in thory. THe practical
application I'm looking at is doing some small-scale metal etching using a
12VDC source using the emthod described here:
http://byo.com/stories/issue/item/3113-etch-your-kettle-projects
although I'll be ethching much smaller items than kettles.

It's my understanding (which is likely to be entirely wrong...) that I need
to maintain the Voltage steady at 12VDC, while being able to dial-down the
Amperage.

If that's the case, I can't order the Amp-control-widget-thing because I
don't know what it is called - and unfortunatley, there is no listing at
Digikey for "Amp-control-widget-thing", Heh!

Hopefully, that clarifes why I'm asking about what I need to vary Amperage


THanks again!
- Kris

On 6/11/2016 2:30 PM, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris

To answer your question, we really need more information. To us, your
question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R
where V = voltage (measured in volts)
I = current (measured in amps)
R = resistance (measured in ohms)

This equation can also be rearranged as:
or I = V / R
or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then
the voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the current
will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then the
current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it, then
the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to
your 12 volt source that you also want to limit to 3 amps? There are
two possible answers:

1) The current will be limited to 3 amps. This means that the voltage
will not be held at 12 volts, instead, it will have to drop to 9
volts. This contradicts your statement that the voltage remains at 12
volts.

2) The voltage will be held at 12 volts. This means that the current
will increase to 4 amps which contradicts your desire that the current
is limited to 3 amps.


We can give a better answer if you really describe what the
application is that you have in mind.


Dan

 

[Kris]

Thank you Omni,

I added additional details in my reply to Dan, so I'll refer to that rather
than write the same thing here

THanks!

- Kris

omnilobe@gmail.com wrote in
news:91893c7c-b413-4f03-bebc-1bd815dbbfde@googlegroups.com:

On Saturday, June 11, 2016 at 11:30:05 AM UTC-10, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris

A potentiometer can work. With half the power to the load, 6 volt
output can be a goal.

V = IR

6 volts = 3 amps 2 ohms

Power = VI = 18 watts

 

[Nobody]

On Sat, 11 Jun 2016 19:24:03 -0500, Kris wrote:

It's my understanding (which is likely to be entirely wrong...) that I
need to maintain the Voltage steady at 12VDC, while being able to
dial-down the Amperage.

You want a constant-current power supply; the voltage will vary (up to
some limit) depending upon the resistance.

These are often used for driving LEDs, but the current tends to be fixed
(and they're likely to be designed for a stable load).

Bench power supplies often have a constant-current mode, and the current
and (maximum) voltage can be varied; but they're somewhat more expensive
than a wall wart or battery pack.

 

[Kris]

Dan Coby <adcoby@earthlink.net> wrote in
news:55mdnSToxfV_E8HKnZ2dnUU7-RHNnZ2d@earthlink.com:

On 6/11/2016 2:30 PM, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris

Hi again Dan,

OK, I *thought* I understood V:I:R but after reading your post again, and
reading subsequent posts, I did not understand it =

I should print all of these posts out and study a bit in the morning when
my mind is fresh

THanks!

- Kris

To answer your question, we really need more information. To us, your
question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R
where V = voltage (measured in volts)
I = current (measured in amps)
R = resistance (measured in ohms)

This equation can also be rearranged as:
or I = V / R
or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then
the voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the current
will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then the
current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it, then
the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to
your 12 volt source that you also want to limit to 3 amps? There are
two possible answers:

1) The current will be limited to 3 amps. This means that the voltage
will not be held at 12 volts, instead, it will have to drop to 9
volts. This contradicts your statement that the voltage remains at 12
volts.

2) The voltage will be held at 12 volts. This means that the current
will increase to 4 amps which contradicts your desire that the current
is limited to 3 amps.


We can give a better answer if you really describe what the
application is that you have in mind.


Dan

 

[Tim Wescott]

On Sat, 11 Jun 2016 16:30:01 -0500, Kris wrote:

Hello,

This is the "Basics" group, and yeah, I have a question that's most
definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the voltage
constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up
in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

It's called a "magic box", because the load itself will determine how
much current will flow for a given voltage.

Think about a resistor: E = IR means that the voltage is _always_ the
resistance times the current. Conversely, the current is _always_ the
voltage divided by the resistance. So a 10 ohm resistor on a 12 volt
supply will flow 1.2 amps -- the only way to change the current is to
change the voltage.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!

 

[Rheilly Phoull]

On 12/06/2016 5:30 AM, Kris wrote:

Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up in
my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

- Kris

For the basic theory of the device mentioned google "current source with
lm317.

 

[amdx]

You want a constant current regulator.
> https://www.google.com/search?q=constant+current+redulator&ie=utf-8&oe=utf-8#q=constant+current+regulator

However what you said here will not be true,
" I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3."

The output voltage will vary to keep the current constant.
But that's OK, that's the way it works.

This instructables link may be what you want.

> http://www.instructables.com/id/Adjustable-constant-current-source-4mA-to-3A/

Mikek






On 6/11/2016 7:24 PM, Kris wrote:

Dan Coby <adcoby@earthlink.net> wrote in
news:55mdnSToxfV_E8HKnZ2dnUU7-RHNnZ2d@earthlink.com:

Thank you, Dan,

I understand the V to I to R relationship in thory. THe practical
application I'm looking at is doing some small-scale metal etching using a
12VDC source using the emthod described here:
http://byo.com/stories/issue/item/3113-etch-your-kettle-projects
although I'll be ethching much smaller items than kettles.

It's my understanding (which is likely to be entirely wrong...) that I need
to maintain the Voltage steady at 12VDC, while being able to dial-down the
Amperage.

If that's the case, I can't order the Amp-control-widget-thing because I
don't know what it is called - and unfortunatley, there is no listing at
Digikey for "Amp-control-widget-thing", Heh!

Hopefully, that clarifes why I'm asking about what I need to vary Amperage


THanks again!
- Kris



On 6/11/2016 2:30 PM, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris

To answer your question, we really need more information. To us, your
question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R
where V = voltage (measured in volts)
I = current (measured in amps)
R = resistance (measured in ohms)

This equation can also be rearranged as:
or I = V / R
or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then
the voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the current
will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then the
current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it, then
the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to
your 12 volt source that you also want to limit to 3 amps? There are
two possible answers:

1) The current will be limited to 3 amps. This means that the voltage
will not be held at 12 volts, instead, it will have to drop to 9
volts. This contradicts your statement that the voltage remains at 12
volts.

2) The voltage will be held at 12 volts. This means that the current
will increase to 4 amps which contradicts your desire that the current
is limited to 3 amps.


We can give a better answer if you really describe what the
application is that you have in mind.


Dan

 

[Kris]

Nobody <nobody@nowhere.invalid> wrote in
newsan.2016.06.12.02.21.40.401000@nowhere.invalid:

On Sat, 11 Jun 2016 19:24:03 -0500, Kris wrote:

It's my understanding (which is likely to be entirely wrong...) that I
need to maintain the Voltage steady at 12VDC, while being able to
dial-down the Amperage.

You want a constant-current power supply; the voltage will vary (up to
some limit) depending upon the resistance.

These are often used for driving LEDs, but the current tends to be fixed
(and they're likely to be designed for a stable load).

Bench power supplies often have a constant-current mode, and the current
and (maximum) voltage can be varied; but they're somewhat more expensive
than a wall wart or battery pack.

Thanks! Yeah, I really need to study all the info. I thought I could be
varied while keeping V constant OR V veried with I constant, but it doesn't
seem to work that way at all.

I think I need to go to the page about steel etching and just *exactly*
follow the directions for using the 9V battery, and take a lot more time to
think about adapting the wall wart.

THanks for the information!

- Kris

 

[Kris]

amdx <nojunk@knology.net> wrote in news:njif1e$ffl$1@dont-email.me:

You want a constant current regulator.
https://www.google.com/search?q=constant+current+redulator&ie=utf-8&oe
=utf-8#q=constant+current+regulator

However what you said here will not be true,
" I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3."

The output voltage will vary to keep the current constant.
But that's OK, that's the way it works.

This instructables link may be what you want.

http://www.instructables.com/id/Adjustable-constant-current-source-4mA
-to-3A/

Mikek

OH! THanks, Mike,

So amps cannot be controlled in and of themselves. Hmm.
I think I need to print out these comments (and the suggested links!) and
give a good close read in the morning when my mind is fresh

- Kris

On 6/11/2016 7:24 PM, Kris wrote:
Dan Coby <adcoby@earthlink.net> wrote in
news:55mdnSToxfV_E8HKnZ2dnUU7-RHNnZ2d@earthlink.com:

Thank you, Dan,

I understand the V to I to R relationship in thory. THe practical
application I'm looking at is doing some small-scale metal etching
using a 12VDC source using the emthod described here:
http://byo.com/stories/issue/item/3113-etch-your-kettle-projects
although I'll be ethching much smaller items than kettles.

It's my understanding (which is likely to be entirely wrong...) that
I need to maintain the Voltage steady at 12VDC, while being able to
dial-down the Amperage.

If that's the case, I can't order the Amp-control-widget-thing
because I don't know what it is called - and unfortunatley, there is
no listing at Digikey for "Amp-control-widget-thing", Heh!

Hopefully, that clarifes why I'm asking about what I need to vary
Amperage


THanks again!
- Kris



On 6/11/2016 2:30 PM, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of
3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris

To answer your question, we really need more information. To us,
your question is somewhat confusing. Let me explain why.

A basic equation of electricity is: V = I * R
where V = voltage (measured in volts)
I = current (measured in amps)
R = resistance (measured in ohms)

This equation can also be rearranged as:
or I = V / R
or R = V / I

If I have a resistor that is 12 ohms and I run a 1 amp current then
the voltage across it will be 12 ohms x 1 amp = 12 volts.

If I have 4 ohm resistor and I put 12 volts across it, the the
current will be 12 volts divided by 4 ohms = 3 amps.

Now if I have a 3 ohm resistor and I put 12 volts across it, then
the current will be 12 volts divided by 3 ohms = 4 amps.

If I have a 3 ohm resistor and I run a 3 amp current through it,
then the voltage across the resistor will be 9 volts.

Now, what do you want to happen when we connect a 3 ohm resistor to
your 12 volt source that you also want to limit to 3 amps? There are
two possible answers:

1) The current will be limited to 3 amps. This means that the
voltage will not be held at 12 volts, instead, it will have to drop
to 9 volts. This contradicts your statement that the voltage remains
at 12 volts.

2) The voltage will be held at 12 volts. This means that the current
will increase to 4 amps which contradicts your desire that the
current is limited to 3 amps.


We can give a better answer if you really describe what the
application is that you have in mind.


Dan

 

[Kris]

Rheilly Phoull <rheilly@bigslong.com> wrote in
news:RN6dnTr8QeZoXsHKnZ2dnUU7-YXNnZ2d@westnet.com.au:

On 12/06/2016 5:30 AM, Kris wrote:
Hello,

This is the "Basics" group, and yeah, I have a question that's
most definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the
voltage constant but control the *amperage* from 0 to a maximum of 3.
I'm having a difficult time pinning-down what it's called - I
*think* what I need is a "varistor" or a "potentiometer", but the
info I've turned up in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name
so I can order the right thing =

- Kris


For the basic theory of the device mentioned google "current source
with lm317.

Sounds good!, thanks! I'll add that to the other info and do some studying.

- Kris

 

[Kris]

Tim Wescott <seemywebsite@myfooter.really> wrote in
news:iKmdneo5oM4EWMHKnZ2dnUU7-NmdnZ2d@giganews.com:

On Sat, 11 Jun 2016 16:30:01 -0500, Kris wrote:

Hello,

This is the "Basics" group, and yeah, I have a question that's most
definitely basic...

I want to purchase a small widget for a 12V DC setup to keep the voltage
constant but control the *amperage* from 0 to a maximum of 3. I'm
having a difficult time pinning-down what it's called - I *think* what I
need is a "varistor" or a "potentiometer", but the info I've turned up
in my searches sems contradictory.

Many thanks to any kind soul who will let me tknow the correct name so I
can order the right thing =

It's called a "magic box", because the load itself will determine how
much current will flow for a given voltage.

Think about a resistor: E = IR means that the voltage is _always_ the
resistance times the current. Conversely, the current is _always_ the
voltage divided by the resistance. So a 10 ohm resistor on a 12 volt
supply will flow 1.2 amps -- the only way to change the current is to
change the voltage.

Oh....! OK, Thank you, Tim!, I had thought one could be varied while
keeping the other constant....

I'll be collecting all the info that the group members have generously
given, and print it out for study in the morning - and meanwhile, I think
I'd better just *exactly* follow the instructions at the website for the
salt+vinegar steel etching using a 9V battery, and make sure I'm *much*
more clear on all of this before I even think about fooling around with a
wall-wart =

Thanks again!,

- Kris

 

[Robert Roland]

On Sat, 11 Jun 2016 21:14:49 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

a 10 ohm resistor on a 12 volt
supply will flow 1.2 amps -- the only way to change the current is to
change the voltage.

....or the resistance.
--
RoRo

 

[amdx]

On 6/12/2016 5:24 AM, Robert Roland wrote:

On Sat, 11 Jun 2016 21:14:49 -0500, Tim Wescott
seemywebsite@myfooter.really> wrote:

a 10 ohm resistor on a 12 volt
supply will flow 1.2 amps -- the only way to change the current is to
change the voltage.

...or the resistance.

And, the resistance will change depending on the electrolytic solution
and the amount between the electrode and the kettle.

Mikek

 

[Bob Engelhardt]

On 6/11/2016 8:24 PM, Kris wrote:

... THe practical
application I'm looking at is doing some small-scale metal etching using a
12VDC source using the emthod described here:
http://byo.com/stories/issue/item/3113-etch-your-kettle-projects
...

That link does not talk about current, so I suppose that your 0 - 3 amps
was found somewhere else?

Anyhow, in that application, the load resistance is going to vary a lot,
depending upon the electrolyte and your technique. So, I would say that
your need is to regulate the current. You can use your 12v source, but
the voltage doing the etching will vary as the load resistance varies.
And you don't care if it does! Your current will be constant at
whatever value you set. In the end, it is the current that is doing the
etching.

HTH,
Bob

 

[Bob Engelhardt]

On 6/12/2016 4:59 PM, Bob Engelhardt wrote:

On 6/11/2016 8:24 PM, Kris wrote:
... THe practical
application I'm looking at is doing some small-scale metal etching
using a
12VDC source using the emthod described here:
http://byo.com/stories/issue/item/3113-etch-your-kettle-projects
...

That link does not talk about current, so I suppose that your 0 - 3 amps
was found somewhere else?

Anyhow, in that application, the load resistance is going to vary a lot,
depending upon the electrolyte and your technique. So, I would say that
your need is to regulate the current. You can use your 12v source, but
the voltage doing the etching will vary as the load resistance varies.
And you don't care if it does! Your current will be constant at
whatever value you set. In the end, it is the current that is doing the
etching.

HTH,
Bob

Here is a widget that might do your job:
www.ebay.com/itm/321921444650

I don't know how one can adjust both voltage & current, but maybe
someone here can explain that.

 

[Ralph Mowery]

In article <njkkar01dr7@news4.newsguy.com>, BobEngelhardt@comcast.net
says...

Anyhow, in that application, the load resistance is going to vary a lot,
depending upon the electrolyte and your technique. So, I would say that
your need is to regulate the current. You can use your 12v source, but
the voltage doing the etching will vary as the load resistance varies.
And you don't care if it does! Your current will be constant at
whatever value you set. In the end, it is the current that is doing the
etching.

HTH,
Bob

Here is a widget that might do your job:
www.ebay.com/itm/321921444650

I don't know how one can adjust both voltage & current, but maybe
someone here can explain that.

As everyone knows atleast two of the 3, volts, current, resistance, has
to change. Assuming you want to keep a constant current when the
resistance is changing, you set the voltage to the maximum you want
without a load connectd. YOu then adjust the current for the maximum
value with the otuput shorted or into a very low resistance. The
current will stay constant as long as the voltage is set high enough.
The voltage will automatically be reduced to whatever it takes to
provide the constant current.

You can also set the current to a high value and the voltage will remain
constnt as long as the current is under the set value when the load
changes. When the load gets to the current limit , the voltag will
automatically drop.