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True Zero to 10 vdc PS

V

V8TR4

Jan 1, 1970
0
Hello everyone,

I need to make a Power supply that mimicks a 0-10vdc out that is common on
industrial equipment outputs like chart recorders and stuff. I need very
little current, not more then 100ma ever, and 10ma is probably what is the
norm.

I have built one using an LM317 that then outputs to a voltage devider. The
problem was it doesn't get down to low volatge, much less zero.

Is there a slicker and not very complicated PS design out there to do what I
want? I was considering using a 555 as a PWM and then using a 2n2222 to vary
a voltage source. Any comments or ideas would be much appreicated. I have
just started playing around with op-amps and it seems at first glance that
these could be used to do what I want as well. Not sure of the details yet
but think it can be done.

This unit is for testing some instruments I have and is not for commercial
use.

Thanks,
Oliver
 
C

cpemma

Jan 1, 1970
0
V8TR4 said:
Hello everyone,

I need to make a Power supply that mimicks a 0-10vdc out that is
common on industrial equipment outputs like chart recorders and
stuff. I need very little current, not more then 100ma ever, and 10ma
is probably what is the norm.

I have built one using an LM317 that then outputs to a voltage
devider. The problem was it doesn't get down to low volatge, much
less zero.

Is there a slicker and not very complicated PS design out there to do
what I want?

One simple way is to regulate to 10.7v or so, put that across a 1k pot, then
take the wiper to the base of a small high-gain NPN transistor connected as
an emitter-follower. The final output is input less 0.7 volts, so you'll get
your zero and a fairly linear scale compared to a potential divider alone.

But I don't see why your potential divider didn't go to zero, are you sure
you didn't connect the pot as a variable resistance?
 
C

Costas Vlachos

Jan 1, 1970
0
V8TR4 said:
Hello everyone,

I need to make a Power supply that mimicks a 0-10vdc out that is common on
industrial equipment outputs like chart recorders and stuff. I need very
little current, not more then 100ma ever, and 10ma is probably what is the
norm.

I have built one using an LM317 that then outputs to a voltage devider.
The problem was it doesn't get down to low volatge, much less zero.

Is there a slicker and not very complicated PS design out there to do what
I want? I was considering using a 555 as a PWM and then using a 2n2222 to
vary a voltage source. Any comments or ideas would be much appreicated. I
have just started playing around with op-amps and it seems at first glance
that these could be used to do what I want as well. Not sure of the
details yet but think it can be done.

This unit is for testing some instruments I have and is not for commercial
use.



There is a way to make your LM317 go down to 0 Volts. Check the typical
applications on the data sheet:

http://www.national.com/ds/LM/LM117.pdf

Alternatively, you could use an op-amp configured as a buffer, -5V/+15V
rails, and a multi-turn pot from GND to 10V regulated voltage, with moving
pin to the op-amp's [+] input. Like this:

.. ___
.. .----|___|----.
.. +10V | 3k9 |
.. | | |
.. | | +15V |
.. | | |\| |
.. | '----|-\ |
.. .-. | >-----+----o 0 - 10V
.. | |<----+----|+/
.. 10k| | | |/|
.. '-' | -5V
.. | ---
.. | ---100nF
.. | |
.. | |
.. +------+
.. |
.. ===
.. GND

You can then choose an op-amp that can deliver the output current you want,
or use a pass transistor to amplify it.

Regards,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
SPAM-TRAPPED: Please remove "-X-" before replying
 
R

Roger Hamlett

Jan 1, 1970
0
cpemma said:
One simple way is to regulate to 10.7v or so, put that across a 1k pot, then
take the wiper to the base of a small high-gain NPN transistor connected as
an emitter-follower. The final output is input less 0.7 volts, so you'll get
your zero and a fairly linear scale compared to a potential divider alone.

But I don't see why your potential divider didn't go to zero, are you sure
you didn't connect the pot as a variable resistance?
I suspect the problem was that the 'target' instrument, has a small amount
of 'pull-up' behaviour. I have seen this with many instrument inputs of this
type. Assuming that the potential divider has a significant resistance, then
the output will never go to zero.
Really, I'd look at having a symmetrical power supply (perhaps +/-15v), and
using a power op-amp. Something like an LM675, can be driven from a pot
across a voltage reference, or even a D-A converter, and will give more
current than needed, and will maintain the output very close to the required
voltage, into a wide range of different loads.

Best Wishes
 
R

Roger Johansson

Jan 1, 1970
0
V8TR4 said:
I need to make a Power supply that mimicks a 0-10vdc out that is common on
industrial equipment outputs like chart recorders and stuff. I need very
little current, not more then 100ma ever, and 10ma is probably what is the
norm.
just started playing around with op-amps and it seems at first glance that
these could be used to do what I want as well. Not sure of the details yet
but think it can be done.

If you use an op-amp in voltage follower configuration, the negative
input connected to the output, you can steer the output by changing
the voltage on the positive input, for example with a linear pot.

If you have +-10Volt power to the op-amp you can put the input pot
between these power rails and get a +-10Volt output.
If you want 0- +10Volt you put the pot between +10 and ground.

Most op-amps can deliver 10mA at the output, but if you need more you
can use two transistors to buffer the output so it can deliver more
current.

Something like this:The two transistors emitters connected together
becomes the output. The negative feedback can be moved from the opamp
output to the new output.
But this buffering with transistors will reduce the maximum output
voltage, it will not reach exactly the full power rail voltage.

Not all opamps do that either, so you must choose opamp carefully if
you need to get close to the rails. It is easier to choose power rails
which are more than the maximum voltage you will need.
Power the circuit from +-15 if you want +-10 output, for example.



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created by Andy´s ASCII-Circuit v1.21 Beta www.tech-chat.de
 
B

budgie

Jan 1, 1970
0
Hello everyone,

I need to make a Power supply that mimicks a 0-10vdc out that is common on
industrial equipment outputs like chart recorders and stuff. I need very
little current, not more then 100ma ever, and 10ma is probably what is the
norm.

I have built one using an LM317 that then outputs to a voltage devider. The
problem was it doesn't get down to low volatge, much less zero.

Is there a slicker and not very complicated PS design out there to do what I
want? I was considering using a 555 as a PWM and then using a 2n2222 to vary
a voltage source. Any comments or ideas would be much appreicated. I have
just started playing around with op-amps and it seems at first glance that
these could be used to do what I want as well. Not sure of the details yet
but think it can be done.

This unit is for testing some instruments I have and is not for commercial
use.

Using the KISS principle, why not just a wirewound pot across a
regulated 10V source? That'll certainly go all the way from 10V down
to 0V even if the load has a pull-up or pull-down characteristic.
 
S

Spehro Pefhany

Jan 1, 1970
0
Using the KISS principle, why not just a wirewound pot across a
regulated 10V source? That'll certainly go all the way from 10V down
to 0V even if the load has a pull-up or pull-down characteristic.

Finally, a good answer. ;-)

Maybe put a small resistor in series with the regulated source (12V)
to limit the current through the wiper if it gets shorted. Measure the
actual voltage into the instrument under calibration/test with an
appropriate DVM.

Best regards,
Spehro Pefhany
 
J

Jeff

Jan 1, 1970
0
Refernce the pot to negative 1.2V with respect to ground. This works great
with these cheap regulators. If you have a negative voltage source avalible,
it's also really easy to do with a 1.2 V voltage refernce!
 
C

cpemma

Jan 1, 1970
0
budgie said:
Using the KISS principle, why not just a wirewound pot across a
regulated 10V source? That'll certainly go all the way from 10V down
to 0V even if the load has a pull-up or pull-down characteristic.

At the worst case 100mA draw that's a high wattage pot and very poor
linearity. Much more expensive than a transistor or two and a standard pot.
 
S

Spehro Pefhany

Jan 1, 1970
0
At the worst case 100mA draw that's a high wattage pot and very poor
linearity. Much more expensive than a transistor or two and a standard pot.

Yeah, but 100mA is just plain silly if you're at all familiar with
these things. 1mA *maximum* is more like it.

Best regards,
Spehro Pefhany
 
B

Baphomet

Jan 1, 1970
0
Spehro Pefhany said:
Finally, a good answer. ;-)

I'm sorry...that's too simple. Let H.P. design it and they'll find a way to
do the same thing with 57,000 transistors and a partridge in a pear tree ;-)

Maybe put a small resistor in series with the regulated source (12V)
to limit the current through the wiper if it gets shorted. Measure the
actual voltage into the instrument under calibration/test with an
appropriate DVM.

Best regards,
Spehro Pefhany
http://www.speff.com
 
B

budgie

Jan 1, 1970
0
At the worst case 100mA draw that's a high wattage pot and very poor
linearity. Much more expensive than a transistor or two and a standard pot.

I doubt if it will get anywhere near that current.
Source linearity is academic in any solution, as testing and/or
calibration would mandate that a suitable meter be connected at the
DUT.
 
G

gagir

Jan 1, 1970
0
There are lots of opamps that can source 100mA.

A voltage follower maybe?

Eric Girard
 
G

GPG

Jan 1, 1970
0
Eliminate diodes, put resistor between bases and emitters so that v
across will turn transistors on
 
R

Roy J. Tellason

Jan 1, 1970
0
Roger said:
Something like this:The two transistors emitters connected together
becomes the output.

You don't need the second transistor if you don't need negative voltage
output...
The negative feedback can be moved from the opamp output to the new
output.

But that's not what your little diagram shows here!
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.-. | >|--o--o o-------
| |<---|+/ | |
| | |/| V |
'-' - |
| | |<
o-----|
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created by Andy´s ASCII-Circuit v1.21 Beta www.tech-chat.de

You've got the feedback connection at the output of the op amp, which is
going to result in less voltage available at the output of the circuit...
But this buffering with transistors will reduce the maximum output
voltage, it will not reach exactly the full power rail voltage.

If 15 volt supplies are used then the output should reach 10 volts, no
problem.
 
M

mike

Jan 1, 1970
0
Roy said:
Roger Johansson wrote:




You don't need the second transistor if you don't need negative voltage
output...




But that's not what your little diagram shows here!




You've got the feedback connection at the output of the op amp, which is
going to result in less voltage available at the output of the circuit...




If 15 volt supplies are used then the output should reach 10 volts, no
problem.

If thems is diodes in the base, you ain't gonna get ANY output. I'd
call that a problem.
mike




--
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Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
R

Roger Johansson

Jan 1, 1970
0
Roy J. Tellason said:
You don't need the second transistor if you don't need negative voltage
output...

Yes, but I gave him a more general solution, just in case he wanted a
more generally useful variable voltage supply.
But that's not what your little diagram shows here!

That is why I wrote it in the text, I was too lazy to change the
schematic after adding the two transistors.
It wasn't a complete schematic anyhow, more an illustration of the
building blocks used.
 
K

Keith R. Williams

Jan 1, 1970
0
no- said:
Yes, but I gave him a more general solution, just in case he wanted a
more generally useful variable voltage supply.

Yes, and note that the second transistor is needed for any
negative currents, even when the voltage is positive. Both
transistors allow for full four-quadrant operation.

However, your diodes are going to be a problem. How are you
going to turn on the PNP transistor? You need at least biasing
resistors to get base current. I'd leave the diodes out
completely. You'll get some cross-over distortion as the op-amp
slews from +.7V to -.7V (it will be relatively quick since it's
open gain), but that shouldn't be a problem for a power supply.
That is why I wrote it in the text, I was too lazy to change the
schematic after adding the two transistors.
It wasn't a complete schematic anyhow, more an illustration of the
building blocks used.



Like: ;-)


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| +---+|-\ | |> |
.-. | >---++ | |
| |<----------|+/ | o--------+------------o
| | |/| | |
'-' | |<
| o----|
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created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-
chat.de
 
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