PC Power Supply

[NorthernLightX]

Hi,

I just disassembled an old 286 computer, and I noticed that the AT powersupply looked useful for my MidiBox. It's basucally just a PCB with the required parts, no casing or other useless metal. It is equipped with a noise filter.

It has the standard +5V -5V +12V -12V and GRND leads, but is this PSU useable for a MidiBox?

(The -12V is probably useful for my LCD.)

[LO]

I think you may have some problems as it is probably a switching powersupply? And I think this is bad! but someone else may be able to correct me?

;)

[NorthernLightX]

OK, what is the difference between a "normal" PSU and a switching PSU? How can you tell the difference? And why is a switching PSU bad?

[d2k]

I think a switching psu needs a load before it'll run, and as the mbhp stuff doesnt require that much juice it's prolly not the way to go... unless u stick a lamp or summin on ya box....i could be wrong tho...

[NorthernLightX]

Number . 5 . needs . more . input . . .  ;D

So how does that work, requiring a load before it'll work? It's a PSU from a 286, I estimate it's from around 1993 as the CPU was a 12 MHz AMD. Did they use switching PSU's that time?

[Guest Ilmarinen]

Switching PSU were introduced in the ATX form factor. Some pre ATX big company machines had them like Compaq etc, at least I've pulled those kind of things apart in the past.

The old PSU requires a physical swich to turn it on. Not one of these new stateless ones.

Excuse my really bad jargon, I have no idea how to express myself in electrical terms in english (or in ANY language for that matter).

[smashtv]

Most switching supplies need a minimum load to start up, but this can be as simple as a "ballast" resistor from +5vdc to ground.  I usually use a high wattage ceramic resistor for this since it will be releasing quite a bit of energy as heat.

The 286 supply is most likely a switcher, they have been used in everything (that can be run on noisy power) since the 70's.  

I built a board a few years ago that generates PWM to control a pair of R/C Servos, on a switching supply the circuit has all kinds of positional error and the servos "jitter" even when they were supposed to be stopped and holding position.  Run the same circuit from a linear, and it works fine.  (yes I used bypass/filter caps)

About the differences between linear and switcher, this post from usenet says it best:

A switching power supply uses semi-conductor switches to

switch voltage on and off very quickly (PWM), to supply an

AVERAGE output. The output is filtered to smooth out all

the ON/OFF cycles. They are efficient but put out a lot of

electrical noise and tend to have some ripple in the output.

A linear power supply uses semiconductors (power transistor)

as a variable resistance, to 'leak' current through to provide the

right output (analagous to the faucet on your sink where you

can adjust flow). Since it acts like a resistor, it wastes a lot of

power and generates a lot of heat. They are inefficient and

produce a very 'clean' output. Because of all the heat generated,

they are typically used only for small power requirements.

A linear power supply is called a linear power supply

because the power transistor is operated in its linear region,

where it behaves like a variable resistor rather than an on/off

switch.

The "ripple in the output" is simply a small amount of AC on the DC output.  This is bad, but is a fair trade for efficiency.  You can measure the amount of ripple with -most- meters by setting your meter to read AC and checking the DC output.

If memory serves acceptable amount of ripple for digital circuits is no more than 1% of the output voltage (1% of 5VDC is .05VAC on the meter).  Can anyone confirm this ????

Have fun!

SmashTV

[smashtv]

Switching PSU were introduced in the ATX form factor. Some pre ATX big company machines had them like Compaq etc, at least I've pulled those kind of things apart in the past.

The old PSU requires a physical swich to turn it on. Not one of these new stateless ones.

Excuse my really bad jargon, I have no idea how to express myself in electrical terms in english (or in ANY language for that matter).

Ilmarinen is talking here about how the supply is turned on and off, not the type of technology/design used in the supply.  Software power management (press a keyboard key to turn on the machine instead of flip a toggle switch) started with the ATX supplies, But has nothing to do with whether a power supply is linear or switching.

The original IBM PC (the first ones) had a switching supply in it, and I doubt that you will find any computer equipment with a linear supply inside (even the old stuff when it was new!)

Have fun!

SmashTV

[TK.]

Hi,

I built a board a few years ago that generates PWM to control a pair of R/C Servos, on a switching supply the circuit has all kinds of positional error and the servos "jitter" even when they were supposed to be stopped and holding position.  Run the same circuit from a linear, and it works fine.  (yes I used bypass/filter caps)

I can confirm this. Thomas (who built a mega-midibox with a lot of motorfaders) had the same problem.

Best Regards, Thorsten.

[Steven_C]

I dunno if its switching or linear type, but I powered an analogue audio mixer project from an old Amiga PSU, which was very quiet for audio.  :) it was handy, cos it has +/- 12v and 5v outputs ;)

[smashtv]

I dunno if its switching or linear type, but I powered an analogue audio mixer project from an old Amiga PSU, which was very quiet for audio.   it was handy, cos it has +/- 12v and 5v outputs  

I'm not sure if it was an A2000 or A3000 Amiga (Video Toaster rig) that I serviced many years ago but it had a switching supply in it.

Switchers can be used in any application but the output MUST be filtered to suit.

My QSC power amps and Yamaha board have switching supplies in them, but correct filter design makes the noisy power a non issue.

Here are some major differences between linear and switching power supply designs:

Linears always have large transformers, switchers use small ones.  I have rarely seen a normal (+5,+12) linear P.S. with a transformer small enough to mount on the PC board with the rest of the power supply circuit.

Linears use large filter capacitors, again usually too large to board mount.

Linears throw tons of heat.  They are really really ineffiecient compared to a switcher.

So if you are looking at the board in the P.S. and it takes AC line voltage in and does not have a transformer that is at least half the size of that board, it's probably a switcher.  :)

There are exceptions to these rules, but what you find at the electronics dealer or in a finished product will likely meet one of the above descriptions.

Switcher P.S. design is a throwback to the cold war tech race, smaller more efficient P.S. designs were needed "for soldiers to carry".  Many consider Peter Chou to be the father of the modern switching power supply design, if you are looking at a +5 & +12 from a computer it is either his design or a clone of his design.  Unfortunately this design (flyback style) is the most noisy.  :)

If you can't figure out what you have, Take some pics and let me know, I'll help.  Now if someone would help me with some real world knowledge of the filtering......the stuff in the books (I have) just does not cover this. :)

Sorry for the long post!

SmashTV

[NorthernLightX]

The transformer is a the large black thing I guess, well it takes up about one sixth of the board if I remember correctly. The Caps on the other hand where not so big... I'll take a picture this weekend and try to post it.

Please tell me more about the filters to use a switching PSU on "demanding" equipment.