current limiter / voltage regulator question

[panfilero]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated,
thank you
Joshua

 

[David L. Jones]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

Yep, it is a standard (adjustable) constant current building block
circuit used in a zillion applications.

Dave

 

[jasen]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

what would happen if you connected the 0 - 5V directly to your 15 ohm load ?

"all" you need to do is scale the 0-5V up to 0 - 30V the 15 ohm resistance
will convert the voltage to 0-2 A

perhaps using a single-rail op-amp and a NPN power darlington in common
emitter configuration.

Bye.
Jasen

 

[panfilero]

"all" you need to do is scale the 0-5V up to 0 - 30V the 15 ohm resistance
will convert the voltage to 0-2 A

My load changes non-linearly with the voltage across it, that's when
i'm trying to steer towards a steady current. My load is a Shape
Memory Alloy wire.

thanks

 

[TimPerry]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated,
thank you
Joshua

you only need a constant current source if you have a varying load such as a
battery. with a fixed load a variable voltage supply will be fine .

 

[Brian Ellis]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated,
thank you
Joshua

Instead of using the LM117 "Precision Current Limiter", use the LM150
"Precision Current Limiter". It goes up to 3 amps. Yes it is a variable
constant current source. You set whatever constant current you want, and as
the load varies, it will maintain that current (provided the voltage
supplying the constant current regulator is high enough).

 

[Jon Slaughter]

My load changes non-linearly with the voltage across it, that's when
i'm trying to steer towards a steady current. My load is a Shape
Memory Alloy wire.

Then use a meter to measure the current. Surely the load doesn't change with
a fixed voltage applied to it? Essentially you will be building a constant
current source anyways(ultimately its a voltage regulator anyways and you
can find schematics to build constant current sources in many of the voltage
regulators datasheets).

That is, even if your load is non linear as long as your load does vary with
time then it should be good enough.

i.e., by Ohms law V = F(I). As long as F isn't to complicated you can solve
for I in terms of V. If we assume F is injective on some bounded interval
then we can find the inverse of F.

So how to do this easily in practice? If you know the bounds on your load
then you can plug in a voltage and measure the current and then adjust V
until you get the current you want. Essentially you yourself is the constant
current source.

Say your load registers 1A at 10V but you want to supply 1.5A's. What to do?
Try increasing V to 11V and see how much your current changes and which
direction(although its most likely in the same direction as the change in
current).

Doing this you can put any current you want through the load and as long as
the load doesn't depend on t to much then it will be pretty accurate.

Essentially you can map out the V-I curve doing this and get the dynamic
resistance of your load. I don't know about memory alloys but as long as
they don't get to hot then it will work fine. It really depends on your
application though. You'll just get a different V-I graph for different
temperatures.

If you plan on doing a lot of work with it you will be better suited to get
a constant current source though. It would be much easier to turn then dial
to the current you need instead of measuring the current and voltage.
(although it wouldn't be so bad with two multimeters, one measuring current
and the other voltage.)

Ofcourse there are some issues with my method above as there is no current
regulation. If the load changes with time it could cause some serious
problems. A constant current source is just a voltage source that will
change the voltage to that it can deliver a constant current. Essentially my
quick and dirty method above is where you become the control feedback
circuit. Not the best way but works in a pinch.

Jon

 

[DaveM]

Instead of using the LM117 "Precision Current Limiter", use the LM150
"Precision Current Limiter". It goes up to 3 amps. Yes it is a variable
constant current source. You set whatever constant current you want, and as
the load varies, it will maintain that current (provided the voltage supplying
the constant current regulator is high enough).

What the last poster says is true, and recommendation of the LM150/350 is good,
but in context of the original question, there are problems with this approach.
(1) The OP stated that he needed to supply a fixed resistance with 0-2 amps.
The "Precision Current Limiter will not go to zero amps. It can get close, but
never to exactly zero amps.
(2) The current limit pot that controls the current must be able to handle at
least 2 amps... not an easy task for a common potentiometer.
I suggest that the OP consider the circuit titled "Adjustable Current Regulator"
in the LM150 datasheet for full realization of his requirements. The additional
regulator and negative voltage source in that circuit lets the output current go
down to zero amps, while still giving good control up to full output current.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

 

[Brian Ellis]

What the last poster says is true, and recommendation of the LM150/350 is
good, but in context of the original question, there are problems with
this approach.
(1) The OP stated that he needed to supply a fixed resistance with 0-2
amps. The "Precision Current Limiter will not go to zero amps. It can get
close, but never to exactly zero amps.
(2) The current limit pot that controls the current must be able to handle
at least 2 amps... not an easy task for a common potentiometer.
I suggest that the OP consider the circuit titled "Adjustable Current
Regulator" in the LM150 datasheet for full realization of his
requirements. The additional regulator and negative voltage source in
that circuit lets the output current go down to zero amps, while still
giving good control up to full output current.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters
in the address)

Some days you're the dog, some days the hydrant.

Very good observations, Dave. When you don't go through the numbers, it is
easy to overlook some of the details.

Item (1): That may or may not hurt the Op, based on what he is using it for.

Item (2). That would probably be a problem.

The "Adjustable Current Regulator" would be okay, if he has a negative
supply. At least according to the drawing, he would need a -5 to -10 volt
supply, too (even though a very low current negative supply). Or, am I
reading it wrong?

I drew a diagram up, look at http://www.fncwired.com/TwoAmpSupply/

 

[jasen]

What the last poster says is true, and recommendation of the LM150/350 is good,
but in context of the original question, there are problems with this approach.
(1) The OP stated that he needed to supply a fixed resistance with 0-2 amps.
The "Precision Current Limiter will not go to zero amps. It can get close, but
never to exactly zero amps.

IIRC he had a 0-5V input, not a pot input.

here's one way to do it using an LM350

+33.5V --[RL]----[LM350A]---[0.1]--- 0V
| 5W
|
Vin (0-5) --[2K4]---+--[100R]--- -1.3V

The silly looking 33.5V supply is needed to accomodate the 30+3+0.2V
drop under full load and stat under the 35-1.25V maximum under no load.

it works on paper with ideal parts but 1% drift in the part translates to a
6% drift in output,,,,


Bye.
Jasen

 

[Paul E. Schoen]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated,
thank you
Joshua

[limited crossposting to two]

I made two circuits that may do what you want, using just two transistors
and some resistors and diodes. The first one uses bipolar transistors, and
the second uses MOSFETs, which seem to be more linear. You may want to use
devices that are better rated for the purpose than what I found in the
LTspice library. In each case, the voltage input varies from Vbe or VgsOn
to about 2 volts above that, to vary the output from zero to two amperes. I
tried it with loads from 1 ohm to 15 ohms and it stays close to 2 amperes
(or wherever you set the current). You need a raw supply of 35 to 40 volts
to get the 30 volts into 15 ohms. I used a one ohm sense resistor, which
drops 2 volts at full current, but you might get by with 0.5 ohms or less.
Temperature stability will be an issue with this circuit, but it might do
the job. Of course, an op-amp could be used with current feedback to
achieve near perfect regulation. The LTspice files follow:

Paul

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[Chris]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Then I heard about adjustable voltage regulators. And looking through
the datasheet of an LM117, I noticed it had a curcuit example it
called: "Precision Current Limiter" where the output current is
controlled by a potentiometer and can be varied from 0 to 1.65 Amps.

I'm not sure if this is a variable constant current source, but it
kinda sounds like it to me, is anyone familiar with this circuit?

In the "Precision Current Limiter" circuit the current is controlled
by a potentiometer, but I need to control the current with a variable
voltage (between 0 - 5 V), is there such a thing as a voltage
controlled resistor? So that I could use that in this circuit?

any help is greatly appreciated,
thank you
Joshua

Hi, Joshua. You're saying you need to provide 0-2ADC to a 15 ohm load
(30V compliance) which is controlled by a voltage source. The
simplest way to do this is with a floating load (neither side of the
load connected to circuit common). Given that, you can do the whole
thing, including perfboard, with Radio Shack parts like this (view in
fixed font or M$ Notepad):

' + VCC
' | Iout = 0-2A
' | ---> +
' '------o
' VCC
' + .------o
' Vin | | -
' 0-2V ___ |\| |
' o-|___|-o----o-------|+\ ___ |/
' 750 | | | >--o--|___|--| TIP120
' .-. |.1uF.--|-/ | 220 |>
' 220| | --- | |/| --- |
' | | --- | | --- |
' '-' | | === | .01uF |
' | === | GND | ___ |
' | GND '--------o--|___|----o
' | 4.7k |
' | .-.
' | 0.22 ohm | |
' | 5 Watt | |
' | '-'
' |______________________________|
' |
' ===
' GND

(This cookbook circuit is courtesy of the estimable Mr. Winfield Hill
at s.e.d.)

With a 30V supply, you'll be seeing maximum power dissipation at the
transistor when I(out)=1A and V(load) = 15V. That means the other 15V
will be dissipated by the darlington transistor, giving you a
transistor power dissipation of 15 watts. Be sure to provide a really
good heat sink for your transistor if you don't want to let the magic
smoke out.

The op amp is another consideration. You'll need an op amp which can
accept input voltages down to the negative supply rail to accept 0V
input. You could do worse than use 1/2 of an LM358. But be careful
not to have the supply voltage for the op amp exceed 32V.

Hope this has been of help.

Good luck
Chris

 

[Andrew McMeikan]

Hello

I need to deliver a variable current between 0-2 Amps to a small load
(15 Ohms).... At first I was thinking about building a current
amplifier..... but after investigating that route again, i think, it's
too difficult to build.

Depending on just how you need to hook thiungs up, you could do this
relatively easy, using an op-amp to measure voltage accross a small
value resistor (one end ground other end your load) as compared to your
control voltage. Use output of op-amp to drive a big fet to send juice
into other end of your load.

depending on type of load take appropraite care so it can not oscilate.

hope that give some ideas.

cya, Andrew...