Video review: ATX to AT power adapter review with -5V

[philscomputerlab]

Very useful for 386 and 486 machines, when you're using devices that require -5V.

In this video I am reviewing two ATX to AT power adapters. One is cheaper, but does not supply -5V, the other one is more expensive, but has a converter that adds -5V. Some sound cards, like the Roland LAPC-I, Sound Blaster 2 or Pro Audio Spectrum 16 require -5 V.

In the video I'm also showing you what these sound cards sound like if the -5V is missing.

Link to ATX to AT adapter: http://www.ebay.com.au/itm/ATX-to-A...713?pt=LH_DefaultDomain_3&hash=item25a70c4b39

and

http://www.ebay.com.au/itm/NEW-ATX-...WITH-A-DUMMY-LOAD-ON-OFF-SWITCH-/251312860753


Link to one ATX to AT adapter with -5V support:
http://www.ebay.com.au/itm/ATX-20-P...-OFF-SWITCH-DUMMY-LOAD-USA-MADE-/251025150787

Link to four ATX to AT adapters with -5V support: http://www.ebay.com.au/itm/4-PCS-AT...711?pt=LH_DefaultDomain_0&hash=item3ceaad8777

 

[GiGaBiTe]

I've come across some 486 and early socket 5/7 boards that refuse to power on if -5v is missing so there's that too.

But having a dummy load on the output makes no sense because ATX PSUs since forever have been designed to turn on without a load (and even the wofat IED models will too.) It's also not even on the correct power rail, most modern ATX PSUs are basically just mains to 12v rectifiers and have DC to DC rectifiers running off the 12v output to get +5v, +3.3v and -12v. But technically all ATX PSUs have a load on them because of the internal cooling fan.

It doesn't make a lot of sense to buy the more expensive one with the -5v rectifier when you can buy the much cheaper one without and add your own charge pump voltage inverter (like a TC7662B) and still pay less.

 

[philscomputerlab]

It doesn't make a lot of sense to buy the more expensive one with the -5v rectifier when you can buy the much cheaper one without and add your own charge pump voltage inverter (like a TC7662B) and still pay less.

It all depends. Not every one has the skill, time or equipment, or wants to make such a modification.

 

[cmc]

Thanks for this. I have been wondering the best way to upgrade to a modern PSU for my old PC.

 

[GiGaBiTe]

It all depends. Not every one has the skill, time or equipment, or wants to make such a modification.

It's a DIP8 chip. All you need is a socket, a few capacitors, some resistors and a POT. It can all be done dead bug style on the socket.

No specialized equipment is needed, just a soldering iron.

 

[philscomputerlab]

It's a DIP8 chip. All you need is a socket, a few capacitors, some resistors and a POT. It can all be done dead bug style on the socket.

No specialized equipment is needed, just a soldering iron.

Could you please make a guide / write up about this mod? I'm sure plenty of people would find this helpful.

 

[GiGaBiTe]

Could you please make a guide / write up about this mod? I'm sure plenty of people would find this helpful.

My circuit diagrams are confusing and hard to follow.

But fortunately, you don't have to look at my terrible handwriting. The datasheet for this component has several circuit diagrams that are very easy to read and implement.

http://ww1.microchip.com/downloads/en/DeviceDoc/21469a.pdf

This negative charge pump may not have enough current to run an ISA sound card though (25mA.) There are higher current parts such as:

LTC660 (100mA):
http://www.linear.com/product/LTC660

SP6661 (200mA):
https://www.exar.com/common/content/document.ashx?id=720

AT/ATX PSUs almost always only have a 500mA supply for -5v and can drive a whole board full of ISA cards, so these regulators should be just fine for one or two cards.

One thing that's concerning about that Ebay adapter is it doesn't tell you the current limits on the converted -5v rail. You'd actually have to buy the thing and look at the regulator to know.

 

[philscomputerlab]

Does this help?

http://i.imgur.com/xHCmoTD.jpg

 

[H-A-L-9000]

Why not just use a 7905 voltage regulator, as almost every power supply used for -5V?

Take:
- 7905 voltage regulator
- small heatsink
- two electrolytic capacitors 33µF/35V
- a bit of wire
- a bit of soldering
- a bit of isolation tape

And the Soundblaster 2.0 works.

 

[GiGaBiTe]

While a 7905 would technically work, it's really not a good idea for a number of reasons.

- Linear regulators are by nature very inefficient. This disparity greatly increases as the input and output voltages drift farther apart. At -12v/-5v, the 7905 will burn 10.5W of power under full load.
- The -12v rail in most AT/ATX PSUs only supplies 500mA, which the 7905 can eat into significantly and exceed in some situations.
- Linear regulators can get hot enough in some circumstances to melt plastic (ie. power wires and cause shorts.)
- Other devices in the system also use the -12v supply so you're very limited to what you can pull from the rail.

The most you can likely safely pull out of the 7905 is 2W (which will give you ~3.4W draw on the -12v rail) and still have enough for whatever is also using the -12v line.

This is why I recommend a switching regulator. The SP6661 draws from a positive supply, where more current is available, and only pulls 470mW under full load.

There's no need for a heatsink, and you only need two capacitors to get it up and running, so it's no more complicated than a 7905.

 

[H-A-L-9000]

10,5W? In our application these values seem more reasonable: (12-5)*0,3 = 2,1W.

The switching regulator also has disadvantages:
- it adds additional noise by nature. I suspect the Soundblaster 2.0 uses the -5V just because it is stabilized by a regulator. Switching stuff could be counter-productive here.
- more difficult to obtain and more expensive
- small case, SMD more difficult to solder for many people, requires PCB
- direct connection to xxA positive rail, in case the cirquit shorts because of some malfunction it can produce well burnt wires

The 7905 has over-temperature protection (125C at the chip, will be less at the heatsink). And you can arrange the heatsink in a way so it doesn't melt any cables. To protect from really bad mistakes like shorting the output to a positive rail we need a fuse, as does the switching regulator.

About the power consumption: -12V are used by a few VGA cards and serial ports -> mouse. The required current is somewhere in the XXmA range. In AT power supplies the -5V are generated in exactly the same way, by a 7905 from the -12V.

The SB 2.0 did not heat up the 7905 noticable when using headphones. Maybe it would if you connect speakers and turn up loud. Generally no huge currents flow in the negative rails.

 

[GiGaBiTe]

10,5W? In our application these values seem more reasonable: (12-5)*0,3 = 2,1W.

Yes, the 7905 will be using 2.1W for a 1.5W load on the -5v. So total power consumption will be ~3.6W, or 60% of the -12v rail. The 10.5W figure is the 7905 at full load.

The switching regulator also has disadvantages:
- it adds additional noise by nature. I suspect the Soundblaster 2.0 uses the -5V just because it is stabilized by a regulator. Switching stuff could be counter-productive here.
- more difficult to obtain and more expensive
- small case, SMD more difficult to solder for many people, requires PCB
- direct connection to xxA positive rail, in case the cirquit shorts because of some malfunction it can produce well burnt wires

The power from an ATX PSU is pretty noisy by itself. Have you ever connected an op-amp to an AT/ATX PSU without filtering chokes on it? You can hear noise from pretty much anything. It's not limited to the supply lines either, ground wires are also really noisy. Most quality sound cards have filtering and isolation circuitry on-board because of this and stray EMI bouncing around inside the case. I don't think that using a switching regulator is going to cause any problems that don't already exist.

As for expense and difficulty to obtain, they're about the same price as a 7905 and are readily available on Mouser. There are some SOIC/MSOP parts, but there are also some that come in DIP-8 packages. In either case, the legs are big enough to get solder on. They don't really need a PCB since they only need a couple of caps and can be soldered dead bug style pretty easily.

The SB 2.0 did not heat up the 7905 noticable when using headphones. Maybe it would if you connect speakers and turn up loud. Generally no huge currents flow in the negative rails.

If it works for you, that's good. I just prefer switching regulators for stuff like this.