Project psu: dedicated 12v or cheap ATX?

[burnttoastnice]

So I'm looking to get into a few low power 12v projects and a power supply to drive these is a no-brainer.

In the past I've used a couple old ATX supplies, before realising they were garbage and scrapping them for parts. At the moment I'm using a 48W 12v power brick, but it's not holding up well and gets ridiculously hot running just one of my 12v halogen lights (Waiting for my led strip to arrive ). So I'm looking to get something that has a high power handling capacity, at least 6 amps, while still running reasonably quiet. These are what I had in mind:

 

• An old free computer from an upcycling website
• A cheap ATX psu from Amazon
• A purpose built 12v 300W psu

 

Spoiler

Cheap ATX psu - https://www.amazon.co.uk/dp/B00647L7H8/

 

Spoiler

Purpose built 12v 300W psu - https://www.amazon.co.uk/dp/B00MAC9MO6/

 

My budget is basically anything under £30. Any recommendations/suggestions?

[werto165]

I used an old xbox PSU for my 3d printer that draws probably 100W or so. certainly a cheap option but not a quiet one I can tell you that. 

[mariushm]

If you want something really low noise and quality, you could always make yourself a linear power supply.

All you need is a transformer, a bridge rectifier, some capacitors and a linear regulator.

The transformer will be the most expensive part, and you need rated for

 

[burnttoastnice]

3 hours ago, werto165 said:

I used an old xbox PSU for my 3d printer that draws probably 100W or so. certainly a cheap option but not a quiet one I can tell you that. 

Funny enough I never considered getting a PSU from an old games console or anything like that, probably something I'll look into. Let's hope my ps2 doesn't die suddenly, it's afterlife will be very productive indeed

 

2 hours ago, mariushm said:

If you want something really low noise and quality, you could always make yourself a linear power supply.

All you need is a transformer, a bridge rectifier, some capacitors and a linear regulator.

The transformer will be the most expensive part, and you need rated for

 

I used a linear regulator for one of my bike lights (converted a standard led ceiling bulb to run from 9.2v) but it ran quite hot to the touch even when the lights were pulling only around 300mA. Managed to reduce the heat output considerably by lowering the input voltage to the regulator though.

I think they're better suited to low current applications, but if you know any linear regulators that can handle high current please let me know

[mariushm]

Eh.. i thought I cancelled that post... seems part of it got through.

 

I wrote a long post suggesting a 160-200VA 14v AC transformer (TME.eu stocks them at around 18-21 uk pounds) which gives you up to ~ 17v DC and 9A

Using 22000uF capacitor (~1.6 uk pounds) after the bridge rectifier (0.7 uk pounds) gives you a minimum of 13v DC at 8-9A at the output... at less output current, the peak voltage will go up towards  17v DC.

Then you can use two 5A linear regulators in parallel to get your 12v at 8-9A of current... problem is TME.eu and Farnell didn't have 5A regulators at reasonable prices so that's where i decided to cancel the post

 

I'm spoiled having a bunch of LT1084 regulators (adjustable 5v) from Linear - used to be able to order up to 2pcs of a part as samples and if you weren't greedy they came within a couple of weeks. Linear is expensive though, 8$ at Digikey..

 

LM1084 ("clone" made by Texas Instruments) is only 2.6$ in US at Digikey, if i remember it correctly it was around 6$ at Farnell or something like that : https://www.digikey.com/product-detail/en/texas-instruments/LM1084IT-ADJ-NOPB/LM1084IT-ADJ-NOPB-ND/363557

Anyway, point is at 9A of current the regulators will see 13-14v and with two regulators in parallel, you're gonna have ~ 4.5A on each regulator, so you're only gonna have around 8-10 watts wasted on each regulator as heat. Not a problem with a heatsink and maybe a 80-92mm fan blowing some air over the heatsinks.

At lower currents, the input voltage will be on average higher, but the current will be lower so your dissipated power will be sort of constant.

[unknownmiscreant]

8 hours ago, mariushm said:

If you want something really low noise and quality, you could always make yourself a linear power supply.

All you need is a transformer, a bridge rectifier, some capacitors and a linear regulator.

The transformer will be the most expensive part, and you need rated for

 

Don't do that. Thats a n00bs way of making a heater. Linear regulators are fiercely inefficient. For more than 500mA (tops) linear regulators are an utter waste of time in my book. Do some research into them, they basically drop the voltage by making heat. The power dissipated by a regulator is P=(Vin-Vout)*I.

 

You could do a transformer based supply with a 9vac transformer which would give ~12.7v DC output. However this would be unregulated. I have a similar system which I use to power my amplifier, it gives an okay result, but power amplifiers aren't too picky, and can accept a wide voltage range. However I use linear regulators to step the +/-35v down to +/-15v for the preamps which draw next to nothing.

5 hours ago, burnttoastnice said:

if you know any linear regulators that can handle high current please let me know

None can. They may be rated for it but it would be a terrible idea to use one for anything. The heatsink requirements would be at least the cost of the regulator itself. Likely more if you want it to be able to run at sustained load.

 

If you want a 'nice' solution, getting a decent quality purpose built supply from a company like traco power would be your best bet. But they aren't cheap. I don't think there will be much difference between a cheap ATX supply and cheap purpose built supply. Although I never use cheap power supplies myself due to coil whine and fire risks.

[mariushm]

3 hours ago, unknownmiscreant said:

Don't do that. Thats a n00bs way of making a heater. Linear regulators are fiercely inefficient. For more than 500mA (tops) linear regulators are an utter waste of time in my book. Do some research into them, they basically drop the voltage by making heat. The power dissipated by a regulator is P=(Vin-Vout)*I.

 

As opposed to a bronze efficiency 300-450w power that will be 80% at 120-150w out meaning at 120w out, it will pull 140-150w from the wall, therefore 20-30w will be wasted as heat.

In my example at 8-9A, with 13v..14v input the regulators will dissipate (14v-12v) x 9A = 18w + another 15w or so  wasted on the bridge rectifier (at peak current) so probably less or about the same amount of heat.

 

In theory you could improve it by replacing the bridge rectifier with an ideal one using LT4320 and four mosfets and then your linear power supply will be actually more efficient than a switching power supply (at this output).. but it adds to the overall cost

 

Switching power supplies are used even at low power nowadays because it saves weight and copper which is expensive and because they can be made much smaller, not necessarily because it's much better in all cases. There's also the power factor thing (you have a low power factor with old style transformers but you as a consumer it doesn't really affect you) and the only other downside is that if you have big fluctuations in the mains voltage you may have too low output on the transformer to regulate properly.

[unknownmiscreant]

On 12/12/2017 at 3:41 AM, mariushm said:

As opposed to a bronze efficiency 300-450w power that will be 80% at 120-150w out meaning at 120w out, it will pull 140-150w from the wall, therefore 20-30w will be wasted as heat.

In my example at 8-9A, with 13v..14v input the regulators will dissipate (14v-12v) x 9A = 18w + another 15w or so  wasted on the bridge rectifier (at peak current) so probably less or about the same amount of heat.

 

In theory you could improve it by replacing the bridge rectifier with an ideal one using LT4320 and four mosfets and then your linear power supply will be actually more efficient than a switching power supply (at this output).. but it adds to the overall cost

 

Switching power supplies are used even at low power nowadays because it saves weight and copper which is expensive and because they can be made much smaller, not necessarily because it's much better in all cases. There's also the power factor thing (you have a low power factor with old style transformers but you as a consumer it doesn't really affect you) and the only other downside is that if you have big fluctuations in the mains voltage you may have too low output on the transformer to regulate properly.

Well your efficiency comparison seems biased to say the least... You're getting the switchmode supply to run off mains power, yet the linear regulators are running off 13-14v. If you factor in the transformer (or similar) they will come out around the same or a bit worse. I should also mention that 1-2v drop is a bare minimum for voltage regulators to maintain fixed output voltage. Depending on how well regulated the transformer (or otherwise) you use is, 4-5v drop maybe required at idle and at least 1.5v at full load. I would also caution against placing linear regulators (or any IC/fuse) in parallel as small variations in manufacturing can mean one IC carries more current than the other, causing it to fail prematurely and the other will fail in quick succession.

 

Switch modes definitely have their downsides, power factor, switching noise etc, as do linear regulators. But both have their place, I use linear regulators for the opamps in my audio setup as they are a quick and easy way to make a relatively quiet +/-15v supply. Since its low current, and I have a fair amount of heat sinking available in the amp, heat is a non-issue.

 

Both linear regulators and switchmodes can be improved, but it gets into a cost vs return situation. You could make the linear regulators more efficient with transistors, but for the same cost, you could also step up to a higher efficiency switch mode.  Ultrasonic audio recording circuitry often runs off carefully designed switching power supplies that have lower electrical noise output than batteries (believe it or not!)

 

19 hours ago, JohnKapri said:

They do however come with the bad feeling of high power chinese electronics which probably have never seen QA. I like to stand a few steps back when I power them on for the first time. But that's just me.

Most cheap Chinese stuff has never actually been certified. At the very least it is Hi-Pot tested, not UL Listed. This means if it burns your house down, you get no insurance. I generally buy PSUs from reputable brands, as they are generally better made, have cleaner output, make less noise and have less chance of causing a fire.

 

If you wanna go cheap, a chinese one is a good way to go. Bench PSUs are good, I still need to get a proper one... Although I prefer using fixed voltage supplies in long-term projects, as then there is no chance of knocking the knob and frying something.

[burnttoastnice]

On 11/12/2017 at 10:45 AM, unknownmiscreant said:

-snip-

 

None can. They may be rated for it but it would be a terrible idea to use one for anything. The heatsink requirements would be at least the cost of the regulator itself. Likely more if you want it to be able to run at sustained load.

 

If you want a 'nice' solution, getting a decent quality purpose built supply from a company like traco power would be your best bet. But they aren't cheap. I don't think there will be much difference between a cheap ATX supply and cheap purpose built supply. Although I never use cheap power supplies myself due to coil whine and fire risks.

Silly of me to forget I'd need a fat ass heatsink for the regulator, for my use case it'd probably fry itself, and I'd have basically no real short-circuit protection if something went wrong. I've never heard of traco power before but I'll check them out

 

On 12/12/2017 at 4:23 AM, JohnKapri said:

-snip-

It depends on what you want to do with it. Do you just want to toy around lamps and LED strips or are you planning on doing something permanent (maybe a light installation of some sorts)?

Mainly messing around with lights and other electronics that run on 12v, and current guzzlers that can't run from a boost converter on my power bank.

 

On 12/12/2017 at 5:57 AM, unknownmiscreant said:

-snip-

 

Most cheap Chinese stuff has never actually been certified. At the very least it is Hi-Pot tested, not UL Listed. This means if it burns your house down, you get no insurance. I generally buy PSUs from reputable brands, as they are generally better made, have cleaner output, make less noise and have less chance of causing a fire.

 

If you wanna go cheap, a chinese one is a good way to go. Bench PSUs are good, I still need to get a proper one... Although I prefer using fixed voltage supplies in long-term projects, as then there is no chance of knocking the knob and frying something.

The fixed voltage is basically my main reason for wanting a 12v supply over a variable bench power supply, risking low voltage parts with the dreaded adjustment knob. At the moment when I need to step the voltage up or down I just use a buck or boost converter, and those are usually in low current applications (2A peak)

 

 

I think I'll look into a dedicated 12v supply, you guys have definitely put me off the cheap ATX option since I don't want an arc welding show plugging it in

[KaminKevCrew]

On 12/10/2017 at 9:11 PM, burnttoastnice said:

it ran quite hot to the touch

That means you were using a linear regulator. Get a switching regulator, and you'll be able to easily and efficiently regulate a wide range of voltages down to 12V. It's a way better way to go.

 

https://www.gearbest.com/multi-rotor-parts/pp_695137.html?currency=USD&vip=760153&gclid=CjwKCAiAvMPRBRBIEiwABuO6qWDL6gCaLIu5WmfRdMLanRfZ0KTjcd5_P1o-bn57KIj1CSNzxUXf0hoCYLUQAvD_BwE

 

Hell, grab two, three, four, or more of these and run them in parallel, then use whatever power source you want between ~16 and ~22 volts.