Modifying computer power supplies

[exequor]

These are some typical values for a computer's power supply:
+3.3V@28A, +5V@30A, -5V@0.3A, +12V@15A, -12V@0.8A, +5VSB@2A

How difficult would it be to change the voltages and current? For example to +30V@20A and -30V@20A.

 

[Pengwuino]

Probably not worth it. From what I know, the better power supplies are well regulated and various components might disallow the power supply to work if something was out of place or outside of the perameters set by the components to allow.

 

[russ_watters]

Well, 600w is more than most computer psus are capable of, so it probably isn't even possible. With resistors, you can probably make one generate a lower voltage, but a higher voltage - no.

 

[Pengwuino]

Don't the better PSU's have circuits that regulate the voltage though?

 

[Cliff_J]

Sure there's going to be circuits to regulate the voltages, but only within a certain range.

So even though it may be possible to swing the outputs slightly by adjusting the PWM circuit, a new transformer setup to handle the increased VA would be far more likely and also the recitfier diodes as well, plus the heatsink...

 

[FredGarvin]

I've watched some ee's sitting at their desk winding home made transformers and trying to keep count of how many turns they had. It looked pretty painful to me. Why not just get a good test lab variable power supply and then worry about the rest of the regulating components that will give you headaches?

 

[exequor]

I've watched some ee's sitting at their desk winding home made transformers and trying to keep count of how many turns they had. It looked pretty painful to me. Why not just get a good test lab variable power supply and then worry about the rest of the regulating components that will give you headaches?

Do you know how much one of those power supplies cost in relation to a computer's power supply? rhetorically...

 

[Pengwuino]

Do you know how much one of those power supplies cost in relation to a computer's power supply? rhetorically...

A good 600 watt powersupply is probably $80.

I purchased a 6-15V variable voltage 500W benchtop powersupply for about $120. I wonder how much a 30V power supply would be... hopefully not toooo much more?

 

[FredGarvin]

Do you know how much one of those power supplies cost in relation to a computer's power supply? rhetorically...

True. They aint cheap. You definitely won't get one for a hundred bucks.

 

[Integral]

http://www.uoguelph.ca/~antoon/circ/ps3010/ps3010a.html" is one result of a quick google of "30V Power supply"

Any specific reason you need 30V? If you could make do with 24V you will find a lot more and probably cheaper Power supplies. 24V is an industry standard so they are quiet common.

 

[russ_watters]

...And a lot of 24v transformers put out a nominal voltage of more like 27, and a lot of equipment has a high tolerance for variations in voltage...

 

[exequor]

Any specific reason you need 30V? If you could make do with 24V you will find a lot more and probably cheaper Power supplies. 24V is an industry standard so they are quiet common.

I need it for an amplifier system I'm building, and it is using one of those T class (or tripath-class D) chips that requires 30V.

 

[Integral]

...And a lot of 24v transformers put out a nominal voltage of more like 27, and a lot of equipment has a high tolerance for variations in voltage...

I assume that we are talking about a DC Power supply. If you were to build a linear 30V Power supply you would need to start with a transformer that puts out nearly 50VAC in order to get a rectified 30VDC output.

 

[Integral]

I need it for an amplifier system I'm building, and it is using one of those T class (or tripath-class D) chips that requires 30V.

You may want to verify that the high frequency content of a switching power supply will not effect your system.

It may be that a 24V supply will simply reduce your output...Just a hypothesis, not firm knowledge on my part. I pretty sure that you will not be able to get what you need by modifying a computer power supply.

 

[exequor]

It may be that a 24V supply will simply reduce your output...Just a hypothesis, not firm knowledge on my part. I pretty sure that you will not be able to get what you need by modifying a computer power supply.

Correct, by using 24V the output would be reduced. I was only trying to get the recommended supply for max output power.

This website:
http://www.coilws.com/Toroidal Power Tx/Toroid_Pwr_PN.php?orderby=data3&orderdir=desc
has some good toroids, but they're a bit pricy. I starting to think about why home audio amplifiers are so expensive.

 

[exequor]

What about rectifying a 110V AC input then stepping it down to 30V, would that be easier and/or cheaper?

 

[russ_watters]

I assume that we are talking about a DC Power supply.

D'oh - not paying enough attention.

 

[Averagesupernova]

If you were to build a linear 30V Power supply you would need to start with a transformer that puts out nearly 50VAC in order to get a rectified 30VDC output.

Since when? A full wave bridge rectifier being fed with a 50 VAC source will output (when filtered with a cap) about 70 volts DC.

 

[Averagesupernova]

What about rectifying a 110V AC input then stepping it down to 30V, would that be easier and/or cheaper?

You cannot use a transformer to step up or down DC. Since this has clearly escaped your grasp I would recommend you stay away from modifying anything electronic, let alone building something from the ground up until you pick up a bit more knowledge.

 

[exequor]

You cannot use a transformer to step up or down DC. Since this has clearly escaped your grasp I would recommend you stay away from modifying anything electronic, let alone building something from the ground up until you pick up a bit more knowledge.

I cannot remember using the word transformer, I know that DC cannot be transformed. One of the main advantages of AC over DC is the fact that AC can be transformed.

I was referring to a DC to DC converter (regulator).

 

[Cliff_J]

exequor - if you're planning on using linear regulation or a buck converter to step down a rectified 170V into 30V at a current level of 30A instead of the obvious solution of using a transformer to provide isolation (read safety) and far far far more efficient method, you should not be messing with mains power.

 

[Integral]

Since when? A full wave bridge rectifier being fed with a 50 VAC source will output (when filtered with a cap) about 70 volts DC.

Perhaps one of us is talking RMS and the other Peak to peak.

 

[Averagesupernova]

Perhaps one of us is talking RMS and the other Peak to peak.

When stated VAC it is almost ALWAYS RMS. RMS is .707 of peak. The filter capacitor charges to peak which is the DC voltage you get in my example.

 

[Averagesupernova]

I cannot remember using the word transformer, I know that DC cannot be transformed. One of the main advantages of AC over DC is the fact that AC can be transformed.

I was referring to a DC to DC converter (regulator).

Actually, the word stepping implies a using a transformer. DC to DC convertor usually implies chopping the DC into AC, using a transformer, and then rectifying back into DC. If you want to simply use a series regulator to get the voltage down to 30 volts at 30 amps you will have several problems. First, you will need a 120 VAC source capable of delivering at least 30 amps. Number 2, you will be wasting a HUGE amount of power in the pass transistors, and 3, you are not isolated from the line voltage as Cliff has mentioned. If you want to use a buck convertor which Cliff has also mentioned, that will get you some efficiency but you are still not isolated from the 120 VAC line. A buck convertor works similarly to a transformer in that it uses magnetism to transfer power at one voltage to a different voltage. However, they offer NO isolation. To do this safely, you WILL end up using a transformer in one scenario or another.

 

[exequor]

Actually, the word stepping implies a using a transformer. DC to DC convertor usually implies chopping the DC into AC, using a transformer, and then rectifying back into DC. If you want to simply use a series regulator to get the voltage down to 30 volts at 30 amps you will have several problems. First, you will need a 120 VAC source capable of delivering at least 30 amps. Number 2, you will be wasting a HUGE amount of power in the pass transistors, and 3, you are not isolated from the line voltage as Cliff has mentioned. If you want to use a buck convertor which Cliff has also mentioned, that will get you some efficiency but you are still not isolated from the 120 VAC line. A buck convertor works similarly to a transformer in that it uses magnetism to transfer power at one voltage to a different voltage. However, they offer NO isolation. To do this safely, you WILL end up using a transformer in one scenario or another.

Thanks for the clarification.

Now I understand why home audio amplifiers are so expensive, compared to car amplifiers... powering the home amps is tricky.

 

[saad ahmed khan]

i don't know about increasin the voltage. but i have another problem. in my country most computers are not earthed. that is not a problem but if someone touches the power supply casing or anythin metal it is connected to than you get a real good electric shock
any idea how this problem can be removed without earthing the power supply.
any help is always appreciated:)

 

[Cliff_J]

exequor - home amps are much cheaper than car amps, the power supply design is much less complicated. Add the correct transformer, and you can easily have +-30V rails or many other combinations. What car amplifier will give you 5 channels of real 100W output and a 6 source switcher and an FM tuner with a pretty display for $200?

A car amp has a power supply that must take a very stereo unfriendly 12V and try to step it up with a pulsed DC sent through a transformer and then rectify this back to DC. Sure the PWM chip and the mosfets are cheap to pulse the DC and using a high switching frequency of 50kHz or higher makes the part difference fairly low, but still needs a lot of current because the voltage is so low.

Now, when you want a separate home amplifier that is only made in small quantities, now the price goes up because the economies of scale drop quickly because of the limited market. The advent of the 'receiver' with multiple channels of output, preamp, tuner, switcher, etc all wrapped in one and mass-produced cheaply changed the market.

Then when you can market to audiophiles who believe this or that sounds different, now you have the big dollar Krells or Mark Levinsons that have great components (that may or may not help) and hefty profit margins for the companies willing to risk the R&D money and marketing, and botique shops selling the stuff.

Its a neat hobby, but you need to understand why things cost what they cost. Most audiophiles are big into burr-brown this or that when the big reason some of those burr-brown chips are so spendy is that they are made in small quantities. Someone like the CD manufacturer Richard Clark debunks much of this junk, he has setup 10 cheap 741 op-amps at unity gain and had people try to discern the sonic difference between them and a piece of wire, no one has won and he offered $5,000. Of course, maybe the wire needed to be cryo treated and on ongoino wood blocks (or whatever they're called).