Building a linear power supply

why not simply use an existing computer supply and have inline current limits on the outputs of choice?

The current limit circuits can be designed using power fets to pass the source voltage with current sensing to drive a comparator that drives the power mosfet. With a little hysteresis you can keep the fet out of the linear mode and force it to simply shut off if current limit is reached.

Pots on a panel can set the (I) range of each output..

Jamie

Hi Jamie

I have tried before to use a computer power supply as a high current 12V supply before, but found that unless the 5V line was loaded at around

1.5-2A, the 12V line dropped down to around 11V. I might not be using the 5V line when I want to use the 12V or 3.3V lines. The current limiting I was thinking of using, was using a 5A linear regulator wired as a current regulator.

Thanks

Andrew

Just revised my design a bit and are now thinking of using the 9V 300VA torodial, instead of the 12V one. Thinking I'll use a low dropout reg with a pass transistor to get the required current at 12V. This should make the heat production from the 5V and 3.3V a bit less, but still not sure how to get the -5V and -12V.

Thanks again

Andrew

It's going to take up a lot of bench space and produce a lot of waste heat. It'll be heavy. And it will cost you a lot. Your proposed 12V transformer won't work. You'll need an 18V transformer for the 12V 5 amp supply**. If you use the 18V transformer for all three supplies your dissipation will be about 400 watts under full load. Better would be 3 transformers, or a single transformer with

3 secondaries. You'll still produce a lot of heat, no matter what you do, if you go with a linear design. A linear supply to power a PC is a really *bad* choice. ** Regarding the voltage: 12VAC input to a bridge rectifier and capacitive filter will produce 16.97 VDC, ignoring the voltage drop in the diodes. Under a heavy load, you'll get a drop in the diodes totaling about 2.2 volts, reducing the voltage to 14.77 max. Ripple will reduce that further. The LM338 (5 Amp Adjustable Regulator) needs a minimum of 2.7 volts overhead at 5 amps, so it will drop out. In fact, it will only regulate for the briefest of moments when the AC is at peak. Say we figure about 3 volts ripple and about 2.2 volts drop in the diodes. That means we need 3 + 2.2 + 14.7 or 19.7 volts DC peak across the filter caps. That implies a 14 volt AC transformer which will produce ~19.79 DC peak across the caps. However, there will be some voltage drop in the transformer under load, and the input line voltage can vary, too. Say the line voltage is 110 instead of 120. With an 18 volt transformer fed by 110 instead of 120, we get 18*110/120 or 16.5 volts AC. We can drop to 14 volts AC and still work. If the transformer drops 10% under load, it still yields 14.85 volts AC and will charge the caps to 21 volts. Even if the ripple was 4 volts, we'd still have 15 volts input to the LM338 which is plenty of headroom.

The above is in no way intended to encourage you to build the supply you described. OTOH, I don't want to discourage experimentation/learning, but the proposed project has too many downsides to be worthwhile without some compelling reason(s).

Ed

That's not a problem, have yourself a front panel indicator lamp operating from the 5 Volt supply with a little extra load on it, that will satisfy most computer supplies and give you an idiot light, also add a 5 volt cooling fan..

Jamie

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A few months back there was a similar discussion on DIY audio and I raised the points you just made. The idea was that the audio from the PC would be quieter because of the linear power supply. Of course someone pointed out that all motherboards have switching supplies on board to get the CPU core voltages so who cares about a linear supply TO the motherboard?

Just as an FYI we had a power problem last year that caused the line voltage to drop to 67 VAC. One computer and monitor still worked so I could go online to get the power company. Problem was a missing phase about 1/2 mile away. A linear supply would _never_ work with that low an input.

G=B2

A single 16 VCT transformer can be used for all of the required = voltages. A=20 FWCT rectifier with two diodes and a big capacitor can provide the raw =

10=20 VDC for the 5V linear regulator (and 3.3V).

And another, with diodes and capacitor reversed, can be used for the -5 = VDC=20 supply, which is usually much lower current.

You can also get an adjustable buck supply (1.25-30V, 2-3 amps) on eBay = for=20 about $4-$6 including shipping:

dule-/260901219660

For the + and - 12 VDC supplies, you can use a voltage doubler made with = two=20 capacitors and two diodes for each polarity, which generates about 25 = VDC=20 open circuit, and is also inherently current limiting because the=20 transformer is capacitively coupled to the output.

And perhaps the easiest solution is to add a load to the +5 VDC output = of=20 the ATX supply. It's about 20W but for a test setup efficiency and heat = are=20 not much of an issue. Use a 5 volt fan as part of the load.

Paul=20

a y . g o e V

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Hmmm. Just so happens we use a similar set up on one of our devices. The PS is a basic computer switching PS with the 5V loaded with a big honking resistor. They put out approx. 21 amps with no issues at 12 VDC. We had a question as to how much they would do consistently and had one loaded to that figure. It hung in there all day long with no problems.