# Using a voltage regulator to get positive and negative DC voltages

• posted

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?

• posted

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 | |

• posted

** 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

• posted

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.

• posted
** DO NOT TOP POST !!!

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

** Wrong again.

Basic means just that.

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

** It "contributes" a warning to others.

Google Gropers are a breed apart.

** 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.

** 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

• posted

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?"

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---->

| '-' --- .--|-/ |To RF Amp | | --- | |/| | | .-. | | -Vee | | 39K| | === | | | | | GND '----------' | '-' | | | -Vee (created by AACircuit v1.28.6 beta 04/19/05

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

• posted

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 :)

• posted

"David L. Jones Fucking Tenth Wit "

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

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

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

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

- you verminous POS.

Go take some rat bait.

........ Phil

• posted

related to what I posted.

right place

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

LMAO!

Dave :)

• posted

"David L. Jones Fucking Tenth Wit "

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

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

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

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

- you verminous POS.

Go take a lot of rat bait.

........ Phil

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