h-a-l-9000 wrote:

Actually power supplies with -5V have a 7905 built in.

True...
I have been hunting for a solution with that chip.
Something about a coil, a diode bridge, some caps and stuff.
The result will be much more expensive than I want it to be.

Found here:
http://www.frankshospitalworkshop.com/electro … wer_supply.html

The bottom part would work, as a separate diagram, if the diode bridge magically woul work as a polarity reverser.
As I read it and understand diode bridges, it can not be done.

Yes the circuit you posted is already a power supply itself. It has an AC transformer so it takes mains AC in and steps it down and isolates to safe low voltage levels.

The diode bridges rectify the AC into ripply pulses of one polarity which is smoothed to ripply DC with the cap. So again in this circuit the only thing the 7905 does is that it takes in about -12 volts and regulates it into smooth -5 volts.

So the diode bridges or transformers do not magically convert +12 volts to any negative voltage.

Depending on how much current you need, you could try the ICL7660A, which is a switchmode-inverter-on-a-chip, needing just 2 external capacitors to produce a -12v rail from +12v, or -5v from +5v.

Using it successfully to run a SB16 in my Teradrive, which only provides +5 and +12 from the PSU.

What about modding a 7905 onto the sound card?

SquallStrife wrote:

Depending on how much current you need, you could try the ICL7660A, which is a switchmode-inverter-on-a-chip, needing just 2 external capacitors to produce a -12v rail from +12v, or -5v from +5v.

Using it successfully to run a SB16 in my Teradrive, which only provides +5 and +12 from the PSU.

If I am reading the datasheet right, does that deliver max 20ma? I am aiming at 1.5 amp +. I know 1.5 is overkill, though I need to make shure it does not require heatsink because of heat.

PhilsComputerLab wrote:

What about modding a 7905 onto the sound card?

Theoretically yes.... I see two drawbacks.

One is that -12volt Line is weak. So is there enough left from that line, to power the -12 to -5?

The other is that I would feel bad, about modding such old vintage hardware.

I would rather go with some universal solution, that works on every setup. The biggest problem of all is to convert positive to negative voltage.

I feel that poaitive 12 volt to negative 5 volt is the way to go, as positive 12 volt is heavy these day's.

The answer is to use an inverting regulator. A buck-boost regulator IC would allow you to deliver -5V (this can be adjusted) using a BGA chip and a few passive components.

For example:
http://www.linear.com/product/LTM8049

brostenen wrote:

Theoretically yes.... I see two drawbacks. […]
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PhilsComputerLab wrote:

What about modding a 7905 onto the sound card?

Theoretically yes.... I see two drawbacks.

One is that -12volt Line is weak. So is there enough left from that line, to power the -12 to -5?

The other is that I would feel bad, about modding such old vintage hardware.

I would rather go with some universal solution, that works on every setup. The biggest problem of all is to convert positive to negative voltage.

I feel that poaitive 12 volt to negative 5 volt is the way to go, as positive 12 volt is heavy these day's.

Well if AT power supplies had max -5V output of 0.3A to 0.5A, I really don't see the point of making 1.5A and more available.

And do you know how much current does your current PSU provides -12V, and how much your system consumes now?
Modern ATX supplies will provide up to 0.8A.

Jepael wrote:

Well if AT power supplies had max -5V output of 0.3A to 0.5A, I really don't see the point of making 1.5A and more available. […]
Show full quote

brostenen wrote:

Theoretically yes.... I see two drawbacks. […]
Show full quote

PhilsComputerLab wrote:

What about modding a 7905 onto the sound card?

Theoretically yes.... I see two drawbacks.

One is that -12volt Line is weak. So is there enough left from that line, to power the -12 to -5?

The other is that I would feel bad, about modding such old vintage hardware.

I would rather go with some universal solution, that works on every setup. The biggest problem of all is to convert positive to negative voltage.

I feel that poaitive 12 volt to negative 5 volt is the way to go, as positive 12 volt is heavy these day's.

Well if AT power supplies had max -5V output of 0.3A to 0.5A, I really don't see the point of making 1.5A and more available.

And do you know how much current does your current PSU provides -12V, and how much your system consumes now?
Modern ATX supplies will provide up to 0.8A.

I want to make or be part of the making. Of an universal converter, that can deliver enough extra (nothing like a bit of overkill), and be easy for every user to make and be avaliable to everyone that need such a device. Just like I initially wrote when I started this tread.

gdjacobs wrote:

The answer is to use an inverting regulator. A buck-boost regulator IC would allow you to deliver -5V (this can be adjusted) using a BGA chip and a few passive components.

For example:
http://www.linear.com/product/LTM8049

Allmost fried such a device.

brostenen wrote:

gdjacobs wrote:

The answer is to use an inverting regulator. A buck-boost regulator IC would allow you to deliver -5V (this can be adjusted) using a BGA chip and a few passive components.

For example:
http://www.linear.com/product/LTM8049

Allmost fried such a device.

With butter and onions? Seriously, tales of magic smoke are always good to share.

Yeah.... I had one such device hooked up, reversing polarity. And connected it to a gnd wire, that led straight into a mobo. High pitch noise and some sparks momentary. It became unbareable to touch and yet it survived. Still stable and holding voltage beautifully. Such a tough little device. 😜

lm741 darlington

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse;

osh park link to gerbers;

https://oshpark.com/shared_projects/r59Tj1Ia

feed +12/GND in, get +5, GND, -5 out. throw some 10uf caps or something on there to smooth it out.

I should probably throw an LED on the +5V output so there is a minor load on the vreg....

I'm surprised nobody mentioned this before, but If you're into old arcade games, you might know about a product called the Negatron.
http://www.dimensionengineering.com/products/negatron

It can provide a -5V rail at 500MA for $20.

BloodyCactus wrote:

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse; […]
Show full quote

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse;

osh park link to gerbers;

https://oshpark.com/shared_projects/r59Tj1Ia

feed +12/GND in, get +5, GND, -5 out. throw some 10uf caps or something on there to smooth it out.

I should probably throw an LED on the +5V output so there is a minor load on the vreg....

That's not how 7905's work.

SquallStrife wrote:

BloodyCactus wrote:

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse; […]
Show full quote

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse;

osh park link to gerbers;

https://oshpark.com/shared_projects/r59Tj1Ia

feed +12/GND in, get +5, GND, -5 out. throw some 10uf caps or something on there to smooth it out.

I should probably throw an LED on the +5V output so there is a minor load on the vreg....

That's not how 7905's work.

Exactly, that's going to burn or something.
Haven't we stated here already many times that an 7905 cannot make -5 from +12, but it can make -5 from -12.
And I think the 7905 pinout is wrong too, 7805 is in-gnd-out and 7905 is gnd-in-out.

BloodyCactus wrote:

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse; […]
Show full quote

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse;

osh park link to gerbers;

https://oshpark.com/shared_projects/r59Tj1Ia

feed +12/GND in, get +5, GND, -5 out. throw some 10uf caps or something on there to smooth it out.

I should probably throw an LED on the +5V output so there is a minor load on the vreg....

I think the 7905 will end up shorted out of circuit as ground is common with your "-5V" rail. Your regulator stage needs to be floated for this to work.

Have a look at the buck-boost regulator I linked. I think it or something similar will suit your purpose.

SquallStrife wrote:

BloodyCactus wrote:

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse; […]
Show full quote

I knocked out a 7805/7905 schematic. I have not tested it yet ofcourse;

osh park link to gerbers;

https://oshpark.com/shared_projects/r59Tj1Ia

feed +12/GND in, get +5, GND, -5 out. throw some 10uf caps or something on there to smooth it out.

I should probably throw an LED on the +5V output so there is a minor load on the vreg....

That's not how 7905's work.

its this schematic basically, except with 12vdc and '05 chips isntead of '15.

Yep, and that won't work.

The regulator will only give you -15v with respect to its ground pin, which is at +24v.

24 - 15 = +9v (with respect to circuit ground)