Lm 317 as negative by switching ground and positive

[atticmike]

Hey there,

I recently got a board from silentarts, designed for +V. When I asked whether I could run it at v- by swapping something, he advised me to simply swap ground and positive at the output.

My question now is what the downsides are since I'm loosing track of why there's actually a dedicated negative regulator if I could simply do that with the Lm 317?

Mike

 

[PRR]

> why there's actually a dedicated negative regulator

To make regulated +/- power from un-regulated +/- power.

If you have one battery, you can connect it either way.

If you have two batteries, pre-wired with a common center-tap, you regulate the + side with a positive regulator but must regulate the - side with a - regulator.

The most popular source of raw +/- power is a single-winding (usually center-tapped) transformer which yields center-tapped + and - raw output.

 

[atticmike]

> why there's actually a dedicated negative regulator

To make regulated +/- power from un-regulated +/- power.

If you have one battery, you can connect it either way.

If you have two batteries, pre-wired with a common center-tap, you regulate the + side with a positive regulator but must regulate the - side with a - regulator.

The most popular source of raw +/- power is a single-winding (usually center-tapped) transformer which yields center-tapped + and - raw output.

yeah but what's the downside of using the 317 and swapping zero and ground to get -V?

 

[mjrippe]

If you are powering a piece with a single voltage rail (say -24VDC for a germanium preamp) you can use the 317 and wire it as you described.  If you are talking about a dual voltage supply (like +/- 15VDC for 500 series modules) you will want to use an LM337 for the negative rail.

 

[Rob Flinn]

If you are talking about a dual voltage supply (like +/- 15VDC for 500 series modules) you will want to use an LM337 for the negative rail.

Not necessarily.  If your mains transformer has 2 secondaries you can make up 2 identical 15v regulated supplies using LM317's  & stack them like you would with 2 x 9v batteries to make a +9v/0/9v.  This is how AMEK did some of their console supples but with LM338K's

 

[mjrippe]

Aaaah yes, you are correct Rob!  I didn't consider two secondary windings.

 

[JohnRoberts]

Back in the early days of IC regulators (anybody remember 723 or LM100?) it was pretty common to stack a positive regulator from ground down to make a negative supply. No need to any more with proper negative regulators available for decades and cheap.

JR

 

[[silent:arts]]

... but what's the downside of using the 317 and swapping zero and ground to get -V?

why should there be any?

 

[JohnRoberts]

... but what's the downside of using the 317 and swapping zero and ground to get -V?

why should there be any?

The regulator ICs are designed anticipating 0V on the ground leg. I can imagine some subtle differences perhaps in decopling PS caps strategy.

I would look for old voltage regulator app notes that probably show recommended typical circuits. I don't recall ever using one this way, but it was done, so it shouldn't create a black hole and kill us all. Seems a little unnecessary in this century. 

JR

PS: Kind of like in that theivinin thread, i was wondering how much a 1.5V battery would look like a voltage source with current coming into it...  8)

 

[[silent:arts]]

John, you are right - not necessary now days with costs and availability.
(Except you want more than 1A on the negative rail ...)

but even below 1A I haven't ever seen any PSU or PCB with one single "negative output"

and - we are not talking about new designs for mass production, just an existing PSU / design with switched polarity.

 

[3nity]

Is this a psu you need to put out -?

 

[RuudNL]

I had the same thoughts when I needed a + / - 15 V power supply with LM350's.
Obviously it is a strange 'twist' in your (=my) mind, that you can't use a positive regulator for a negative* voltage.
(AFAIK there is no negative equivalent of a LM350.)
But there are no theoretical objections, and the power supply that I built works perfectly!

*: Of course you will need separate tranformer windings for the + and - voltage!

 

[Rob Flinn]

If you guys re read my post above.  Amek used an lm388k for the neg of their desk psus.  Thats 5a or there about.  I copied it for the strident I had.  No problems at all.

 

[PRR]

> Amek used an lm388k for the neg

"*IF* your mains transformer has 2 secondaries..."

Not all do. It needs two rectifiers as well. Sometimes it is simpler to use one winding&rectifier to make bipolar raw voltage, already commoned. Then you "need" a neg reg. (Or some clever workaround.) Also when you need several neg voltages off one supply.

Getting back to Mike: sure, when it's just ONE supply output, and nobody has strapped the case to one side of the output, you can wire it either-way just like a battery.

 

[Rob Flinn]

> Amek used an lm388k for the neg

"*IF* your mains transformer has 2 secondaries..."

True, but it's not massively difficult to find a transformer with 2 secondaries nowadays.

 

[pstamler]

I seem to remember Walt Jung did a +/- regulator with LM317s in both legs, and a dual-secondary transformer. He justified it by saying that positive regulators were cheaper -- which, back then, they were.

Peace,
Paul

 

[atticmike]

Hey guys, thanks so much for all your input.

I'm about to hook up my frankenstein box, consisting of the following voltages:
First transformer -> 16 0 16 V and 24 0 24, creating on the powerstation 1 x +/- 16V , 1 x +/- 18V as well as 1 x -24V on a small seperate PSU.
Second transformer -> 6 0 6 V, creating +5V on a small, seprate PSU.

Now the culprit is how am I gonna get the "1 x -24V on a small seperate PSU" working without touching ground which I obviously can't? It has a dedicated winding without any other board using it on the transformer, yet sitting in the same box.

I'd hugely appreciate it, if you could tell me how to proceed from there on to get my -24V working.

 

[[silent:arts]]

connect one 24V AC winding (24 / 0) to AC at your POSPSU.
all connectors labeled GND on the POSPSU are now your -24V.
the connector labeled POS is your new 0V.
easy, isn't it 

 

[atticmike]

but isn't the cycle closed if I don't ground it to the star point?

 

[gemini86]

You ground the 'pos' terminal to star ground, then the '0' terminal becomes A negative voltage rail, because it is more negative than ground or 0v.