Using a voltage regulator to get positive and negative DC voltages

[Matt]

I am trying to build a circuit that sets a few small DC outputs. Some
of these will need to be tuned from negative to positive, pretty much
over the range from -1 V to 1 V. I figured a variable voltage
regulator and a good pot would be able to do the trick, as we have both
lying around, and a +/- 5V supply I have on order.

On the data sheet of the National LM317 series they have a "0-30V"
circuit with the pot connected between a 1.2V Zener reference and a
resistor which goes to a -10V supply. Could I just hook up a normal
pot without the zener and resistor? In my circuit I will be hooking up
another part between ground and 5V, so I'm not sure if I can treat the
negative rail as a virtual ground. I can't find any zero-crossing
circuits in a couple of books I've looked at, or in data sheets, so I
am suspicious I am overlooking something that will blow my circuit.

It seems to me that a zero-crossing voltage source is useful in
general. Why aren't there more of these in the market, or diagrams on
the net or in books?

Thanks for your help.

 

[John Popelish]

I am trying to build a circuit that sets a few small DC outputs. Some
of these will need to be tuned from negative to positive, pretty much
over the range from -1 V to 1 V. I figured a variable voltage
regulator and a good pot would be able to do the trick, as we have both
lying around, and a +/- 5V supply I have on order.

On the data sheet of the National LM317 series they have a "0-30V"
circuit with the pot connected between a 1.2V Zener reference and a
resistor which goes to a -10V supply. Could I just hook up a normal
pot without the zener and resistor? In my circuit I will be hooking up
another part between ground and 5V, so I'm not sure if I can treat the
negative rail as a virtual ground. I can't find any zero-crossing
circuits in a couple of books I've looked at, or in data sheets, so I
am suspicious I am overlooking something that will blow my circuit.

It seems to me that a zero-crossing voltage source is useful in
general. Why aren't there more of these in the market, or diagrams on
the net or in books?

Thanks for your help.

You haven't said how much current each of the variable
voltage outputs must provide. If it is a few mA or less,
you can make a variable source with an opamp and a pot
connected to a single regulated voltage, as long as the
opamp is supplied from a source somewhat more positive and
negative than the required output range.
Here is the general concept:

___ ___
+5 ---+-|___|---+----|___|-+
| | |
| 10k | 10k |
| | +10 |
| | |\| |
| +-|-\ |
.-. | >-----+-- +5 to -5
50k | |<---------|+/
| | |/| +--
'-' -10 |
| ===
=== GND
GND

Only the +5 needs to be regulated.

 

[Phil Allison]

** Groper Alert !

It seems to me that a zero-crossing voltage source is useful in
general. Why aren't there more of these in the market, or diagrams on
the net or in books?

** There are a huge number of them - but under a different name.

They are called amplifiers.

The majority of audio amplifier circuits, with split +/- supply rails, can
be operated in direct coupled mode and so become variable voltage sources
within that supply range. It s only necessary to feed an adjustable DC
voltage to the input.

For low current jobs - an op-amp will do.

For medium levels of current, an op-amp boosted by transistor voltage
followers will do.

Fact is though - almost no ELECTRONIC circuit requires a supply of
voltage that changes polarity.



....... Phil

 

[Matt]

I'm not sure exactly what you mean by "Groper," but I figured
"sci.electronics.basics" was a good place to ask about basic
electronics for scientific equipment. If it is somehow related to
"newbie" that would certainly be accurate, though I 'm not sure how it
contributes anything.

I appreciate both comments on op-amps. In the resources I looked at
(various web sites, Horowitz and Hill) it talked about voltage
regulators in the "power supply" section. "Regulator" certainly
implies more focus on maintaining a stable voltage than "amplifier."
Since regulators are also a 3-terminal device, it looked like
something I could easily build, which given my skill level in
electronics is quite important.

The power supply is to drive an RF amp, increasing and decreasing
voltages as dictated by the instructions. While the manufacturer
(TriQuint) is rather sketchy on the details, I would expect the
maximum loading on the adjustable inputs to be in the tens of mA.
Unfortunately, TriQuint hasn't provided tech support when I emailed
them, so I am trying to figure it out the best I can.

I appreciate the suggestion.

 

[Phil Allison]

** DO NOT TOP POST !!!

I'm not sure exactly what you mean by "Groper,"

** Means Google Groper.

You are posting on " usenet " without using a news reader or news server.

but I figured
"sci.electronics.basics" was a good place to ask about basic
electronics for scientific equipment.

** Wrong again.

Basic means just that.

For all electronics design problems see "sci.electronics. design".

If it is somehow related to
"newbie" that would certainly be accurate, though I 'm not sure how it
contributes anything.

** It "contributes" a warning to others.

Google Gropers are a breed apart.

I appreciate both comments on op-amps. In the resources I looked at
(various web sites, Horowitz and Hill) it talked about voltage
regulators in the "power supply" section. "Regulator" certainly
implies more focus on maintaining a stable voltage than "amplifier."

** Shows how wrong you can be by interpreting words literally.

Amplifiers like the kind I described maintain output voltage every bit as
well as a dedicated regulator IC.

The power supply is to drive an RF amp, increasing and decreasing
voltages as dictated by the instructions. While the manufacturer
(TriQuint) is rather sketchy on the details, I would expect the
maximum loading on the adjustable inputs to be in the tens of mA.
Unfortunately, TriQuint hasn't provided tech support when I emailed
them, so I am trying to figure it out the best I can.

I appreciate the suggestion.

** Here are some more.

Don't top post, get a real news server and news reader.

Find out what "usenet" is and read up on the etiquette.



......... Phil

 

[Chris]

I'm not sure exactly what you mean by "Groper," but I figured
"sci.electronics.basics" was a good place to ask about basic
electronics for scientific equipment. If it is somehow related to
"newbie" that would certainly be accurate, though I 'm not sure how it
contributes anything.

I appreciate both comments on op-amps. In the resources I looked at
(various web sites, Horowitz and Hill) it talked about voltage
regulators in the "power supply" section. "Regulator" certainly
implies more focus on maintaining a stable voltage than "amplifier."
Since regulators are also a 3-terminal device, it looked like
something I could easily build, which given my skill level in
electronics is quite important.

The power supply is to drive an RF amp, increasing and decreasing
voltages as dictated by the instructions. While the manufacturer
(TriQuint) is rather sketchy on the details, I would expect the
maximum loading on the adjustable inputs to be in the tens of mA.
Unfortunately, TriQuint hasn't provided tech support when I emailed
them, so I am trying to figure it out the best I can.

I appreciate the suggestion.

Hi, Matt. Newbies belong here. Google "Gropers" are the people who
use Google Groups for newsgroups, instead of a regular newsgroup
reader (I'm one myself!). FYI, you might want to please check out
Google Groups Help Topic "What's good 'netiquette' when posting to
Usenet?"

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

It might help to avoid stepping on toes, I guess.

In answer to your original question, your input voltage to your RF amp
will probably have to source and sink current, depending on the output
polarity. The LM317 can definitely source the current you need, and
can be made to provide a negative voltage with just a pot (although
the regulation will suffer). Unfortunately, there's no way to make it
sink current, so that's a dead end.

Look closely at your RF amp specs. It doesn't sound reasonable that
your control input would require more current than can be provided by
an op amp (many can source or sink up to 20mA). You can do that quite
easily with a regulated bipolar +/-5V supply and a potentiometer like
this (view in fixed font or M$ Notepad):

|
| +Vcc
| |
| .-.
| 39K| |
| | |
| '-' VCC
| | +
| .-. |\|LM741
| 10K| |<--o-------|+\
| | | | | >----o---->
| '-' --- .--|-/ |To RF Amp
| | --- | |/| |
| .-. | | -Vee |
| 39K| | === | |
| | | GND '----------'
| '-'
| |
| -Vee
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

I hope this is of help. Again, check the manufacturers' specs on the
amp. Look for the "input impedance" or "load impedance" of the input
in question.

Good luck
Chris

 

[David L. Jones]

** DO NOT TOP POST !!!


You are posting on " usenet " without using a news reader or news server.

Get over it Phil.
Google groups is a perfectly fine web based usenet reader. Some people
prefer to use web based news readers, nothing wrong with that.

Matt, ignore Phil, he dishes out this rubbish to everyone, he hasn't
taken his medication today.
Your post is in the right place.

John's suggestion of the pot and opamp is one of the most routine ways
of doing it, or the biased LM317 as you mentioned is another way as
well.
If the module you are trying to drive has a relatively high impedance
input then you can get away with just the pot and some series
resistors on a dual supply.

Dave

 

[Phil Allison]

"David L. Jones Fucking Tenth Wit "

** DO NOT TOP POST !!!

Get over it Phil.

** Go drop dead - you pompous, know nothing arsehole.

Google groups is a perfectly fine web based usenet reader.
Some people prefer to use web based news readers,
nothing wrong with that.

** Not the actual issue, not the problem & not related to what I posted.

Jones is an utter ass, another illiterate, autistic public menace.

Your post is in the right place.

** Top posting is * NOT * posting in the right place

- you verminous POS.

Go take some rat bait.



......... Phil

 

[David L. Jones]

"David L. Jones ------- Tenth Wit "




Jones is an utter ass, another illiterate, autistic public menace.


- you verminous POS.

Go take some rat bait.

........ Phil

Regular as clockwork, just like a wind-up toy!

LMAO!

Dave

 

[Phil Allison]

"David L. Jones Fucking Tenth Wit "

** DO NOT TOP POST !!!


Get over it Phil.

** Go drop dead - you pompous, know nothing arsehole.

Google groups is a perfectly fine web based usenet reader.
Some people prefer to use web based news readers,
nothing wrong with that.

** Not the actual issue, not the problem & not related to what I posted.

Jones is an utter ass, another illiterate, autistic public menace.

Your post is in the right place.

** Top posting is * NOT * posting in the right place

- you verminous POS.

Go take a lot of rat bait.




......... Phil