Thats a diagram from something else, not what I made above, just using it to show the principal. I'm not using 7915/7815. Im using 12v and 7805/7905 to give -5v. Anyway, I'm not fabbing it or spending any money on this, I have no dog in this race.

BloodyCactus wrote:

Thats a diagram from something else, not what I made above, just using it to show the principal.
I'm not using 7915/7815. Im using 12v and 7805/7905 to give -5v. Anyway, I'm not fabbing it or spending any money on this, I have no dog in this race.

And we already said this principal won't work. That circuit would appear to magically generate 30 Volt output from 24 Volt input, and it's not possible because linear regulator output voltage is always less than input voltage.

With only +12V and 0V (GND) available to that circuit, any combination of using just linear regulators like 7905 and 7805, you just can't make something that is 5 volts below 0V, because no input was below 0V to begin with.

Look up LMC7660

Should work if the current limit isn't a problem.

Jepael wrote:

And we already said this principal won't work. That circuit would appear to magically generate 30 Volt output from 24 Volt input, and it's not possible because linear regulator output voltage is always less than input voltage.

Guess its a good thing its 12v, for -5/+5 giving 10v.. 10v output < 12v input!

With only +12V and 0V (GND) available to that circuit, any combination of using just linear regulators like 7905 and 7805, you just can't make something that is 5 volts below 0V, because no input was below 0V to begin with.

have you ever changed somethings reference to ground? like series two isolated PSU? two batteries in series?

http://www.learnerswings.com/2014/07/5v-0-5v- … using-7805.html

negative voltage isnt magic, all it is, is the adjust of your reference point, which is why we feed GND into the 7905 input, and 12v as its input, changing its reference point, input - common = 0 - 12V = -12V

Ill just have to take your word for it that it work work

BloodyCactus wrote:

Guess its a good thing its 12v, for -5/+5 giving 10v.. 10v output < 12v input! […]
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Jepael wrote:

And we already said this principal won't work. That circuit would appear to magically generate 30 Volt output from 24 Volt input, and it's not possible because linear regulator output voltage is always less than input voltage.

Guess its a good thing its 12v, for -5/+5 giving 10v.. 10v output < 12v input!

With only +12V and 0V (GND) available to that circuit, any combination of using just linear regulators like 7905 and 7805, you just can't make something that is 5 volts below 0V, because no input was below 0V to begin with.

have you ever changed somethings reference to ground? like series two isolated PSU? two batteries in series?

http://www.learnerswings.com/2014/07/5v-0-5v- … using-7805.html

negative voltage isnt magic, all it is, is the adjust of your reference point, which is why we feed GND into the 7905 input, and 12v as its input, changing its reference point, input - common = 0 - 12V = -12V

Ill just have to take your word for it that it work work

Yes I totally see what you mean. I don't know if this helps but I'll try to explain.

If you take a look at the picture, all you have two 9V batteries with no reference whatsoever to each other or outside world, so of course that's 18V total over the both batteries, and when their center tap is considered as 0V (ground reference) as in the picture then you actually see that the top regulator sees only +9V from the top battery on its input and it regulates it into +5V output and the bottom regulator sees only -9V from the bottom battery in its input and it regulates it into -5V output.

Which is totally same as having 18V battery and regulating 14V and 4V from it. It definitely depends on where you put the 0V reference point, for example the black lead of multimeter when measuring voltages in the circuit.

Same thing with isolated PSUs, but unfortunately PC PSUs are not isolated, so they can't be connected like 12V batteries. And their reference cannot be changed, +12V +5V +3.3V and -12V outputs are referenced to the black 0V leads which are the reference point, for motherboard, metal chassis and connected to mains earth even.

So even if I connect multimeter black lead to +12V and multimeter red lead to -12V, the multimeter will happily give -24V reading because I can change the multimeter's point of reference, but if I need to generate -5V referenced to motherboard black 0V ground wire, I need to connect -12V and ground to 7905 to get -5V in reference to ground out.

BloodyCactus wrote:

negative voltage isnt magic, all it is, is the adjust of your reference point, which is why we feed GND into the 7905 input, and 12v as its input, changing its reference point, input - common = 0 - 12V = -12V

Right.

And while you've adjusted the reference point for the regulator, the reference point for the widget you want to power (say, the op-amp on the sound card) is circuit ground. You can't change that.

So even though your regulator's Vout is at -15 or -12 or -5 with respect to its own reference, its output is still +something with respect to circuit ground.

I'm completely lost you, people. Is anybody's saying that you can only have positive voltages in relation to GND? This is wrong. 78XX chips are designed to regulate voltages that are positive relative to GND, 79 XX are their negative counterparts. And 7660, I mentioned before can be used to "pump" electricity between positive and negative side by charging a capacitor then reconnecting it to GND with reversed polarity.
With 78xx/79xx linear regulators you can't do something like that, no matter how you connect them.
It is true that everything can be relative, but, for the purposes being discussed, there's only one GND in your system and, in order to reverse polarity relative to it, you need a switching circuit that uses a capacitor or a magnetic coil/transformer.

noop wrote:

Is anybody's saying that you can only have positive voltages in relation to GND?

Nobody is saying that.

What we're saying is that you can't get negative voltages from a positive one using linear regs, and pointing out that even though your 79xx might produce a negative voltage relative to its own "ground", it's still positive relative to circuit ground.

I tried this years ago myself, and after scratching my head trying to figure out why the LM7905 wasn't producing -5V from my +12V supply, I finally realised it doesn't create negative voltage, it can only reduce an already existing negative voltage. So you need to run it off -12V in order to produce -5V.

And of course as SquallStrife says, the output you want is -5V with respect to the chassis as GND, which means the input has to be -12V or so with respect to the chassis as well, otherwise the two GND voltages will differ. You'll get -5V out of the regulator with respect to the input GND (which is +12V) but you'll get something completely different once you try to use that with respect to the chassis GND. I'm not actually sure what voltage you would get as I don't know how to calculate it. @SquallStrife any ideas?

If you disagree, go ahead and build a circuit and you will see - the parts are cheap enough. If it works we will all buy one from you 😀

The thing is to remember that voltages don't exist in isolation. You can't have a voltage on one wire. Voltage is a measure of the difference between two different wires (that's why you need to connect both multimeter probes to measure a voltage. If you connect only one probe then there is no reading.) For PC power supplies, the second probe is always connected to chassis GND, and you make your measurements with respect to that point. The proposed circuit only produces -5V when you have moved the GND probe somewhere else in the circuit, which nothing else in the PC uses as GND, which is why the reading will change when you go back to measuring GND from the chassis again.

It's a bit like sitting on a chair in the bottom of a deep hole and saying you have the shortest chair in the world, so short that the seat is at a negative altitude. But once you bring that chair back to the dinner table, suddenly it's no shorter than the others. You only get an accurate measurement when the ground the chair is sitting on (GND) is the same for all the things you are measuring.

Malvineous wrote:

I'm not actually sure what voltage you would get as I don't know how to calculate it. @SquallStrife any ideas?

+12 + (-5) = +7

Why not build the circuit on a breadboard and measure how much actual current draw is on the -5V? It might be a totally minimal amount.

SquallStrife wrote:

Malvineous wrote:

I'm not actually sure what voltage you would get as I don't know how to calculate it. @SquallStrife any ideas?

+12 + (-5) = +7

Right, and my tip here is to just think that in normal operation, the 7905 wants to keep its output 5V below its own GND pin, so no matter where you connect the chips GND pin, the output is 5V below that, so in this case +12-5=+7.

PhilsComputerLab wrote:

Why not build the circuit on a breadboard and measure how much actual current draw is on the -5V? It might be a totally minimal amount.

Many sound cards have MC3407 or TL074 op-amps. When doing nothing, these typically take 3 to 10 mA current per chip. When they drive a load, they can sink or source in the order of 10-20mA max, but audio signals are such high impedance signals that it won't be much.

If I'd have to estimate, I'd say 50mA to 100mA for a sound card is plenty.

Backed by the fact that many sound cards that generate their own -5V from -12V, they use 79L05 which is able to provide up to 100mA.

The ICL7660A I used for the SB16 in my Teradrive is only rated to 20mA, and does the trick quite nicely.

SquallStrife wrote:

The ICL7660A I used for the SB16 in my Teradrive is only rated to 20mA, and does the trick quite nicely.

Which SB16 model you have?

I didn't know there are models that need -5V through ISA, as the ones I've seen are missing the -5V ISA bus pin and regulate their own -5V with onboard 79L05.

Though, I haven't found many pictures from the underside of the card to see which pins are used.

Once this is figured out, I'd be interested in some simple ATX PSU to AT motherboard adapters that incorporate -5v when there is none available. It'd be really nice to run vintage systems with modern PSUs that are silent and efficient, without losing the -5v rail.

Jepael wrote:

Which SB16 model you have? […]
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SquallStrife wrote:

The ICL7660A I used for the SB16 in my Teradrive is only rated to 20mA, and does the trick quite nicely.

Which SB16 model you have?

I didn't know there are models that need -5V through ISA, as the ones I've seen are missing the -5V ISA bus pin and regulate their own -5V with onboard 79L05.

Though, I haven't found many pictures from the underside of the card to see which pins are used.

Not -5, the card uses -12 to make -9. Just pointing out that there's a reason PSUs only really provided 500mA or less of either, the requirements are minute.

Ozzuneoj wrote:

Once this is figured out, I'd be interested in some simple ATX PSU to AT motherboard adapters that incorporate -5v when there is none available. It'd be really nice to run vintage systems with modern PSUs that are silent and efficient, without losing the -5v rail.

These already have existed for years, in the second post in this thread there's a link to adapter sold on Ebay.
It uses some kind of switch mode regulator or charge pump to generate -5V from +12V (black and yellow wire go into heat shrink tubing, white wire comes out).

SquallStrife wrote:

Not -5, the card uses -12 to make -9. Just pointing out that there's a reason PSUs only really provided 500mA or less of either, the requirements are minute.

Ah sorry I forgot in this case you needed the missing -12V and -5V was not needed.