12V power supply

[Barry Gordon]

A lot of us are often looking for a good 12 volt PS to power many other auxiliary glue devices. One fellow mentioned taking 12 V off the PC and bringing it out.

I just bought a 500 Watt Ultra ATX power supply from directron.com for 79.95 less $30 rebate or $49.95. This supply comes with seperate cables that you plug into the supply so you only have cables you need. The supply is esthetically pleasing, has two 80mm fans and is very very quiet. It will deliver 34 amps at +12 volts but the total output of all voltages may not exceed 480 watts. Output voltages have overvoltage and overcurrent protection.

I modified the unit as follows:

Took off the cover (4 screws)

Drilled 4 holes into what I call the back, but is actually a side. This will allow me to mount it on the plywood wall and then put back on the cover.

Cut into the ATX connector lines to get at the wires for Pin 14 (green) and common (ground, any black wire). I drilled a hole in the cover and mounted a small toggle switch and connected it to the green and black wires. This is the power on off switch for the supply. I could have connected the ATX power cord and modded it, but I wanted less cords coming off the PS. The "12V" cord which normally is used for Hard Drives supplies 12V and 5V (2 blacks a yellow(12) and a red(5)). They actually supply a mess of cables to handle any configuration.

This unit is ATX specific providing +/- 12V, +/- 5V and +3.3V

When it is all mounted and connected to the Altronix Fuse block I will start taking pics and post them.

[Bill Brower]

I had always wondered about this...


A long time ago (mid 1990s) I was told not to run a PC power supply unloaded so I never experimented with this since.

Thanks, now I have that 3.3 Volt source to power my exterior security perimeter laser!

[tman1]

I do that.

Bill is correct, if you attempt to run the power supply without a load on it you will not develop the full output potential of the power supply.

There are diagrams on radio control websites about using pc power supplies to charge all of the mariad of battery powered devices we use for airplane support. I helped a buddy hook one up to charge his cell phone using the car charger and a car charger for an electric airplane. We put an automotive add-on two output cigarette lighter recepticle with a couple of led's mounted in the recepticle case to load up the power supply.

I will get the wiring for the power supply with the required led sizes to load up the output when I go over to his house tonight and help with a different computer problem. I'll post the results asap.

Later, Terry

[Barry Gordon]

Thanks I would be interested in seeing those. According to the mfg, they stated that the PSU has built in dummy loads to ensure that under a no load condition the PSU is protected.

They did indicate that I might see a drop in the 12V rail if I draw about 15 Amps or more. they indicated something like a 5% drop with no additional loads. Resistors are cheap, so I know I can load the other rails. I am sure the mfg will be able to tell me what I might need as loads on the other rails to ensure optimal operation. A 25 watt 1 Ohm Resistor on the 5 volt rail will draw 5 amps at 25 watts. 1 amp would require a 5 ohm resistor and only dissipate 1 watt. Does not sound too tough. if needed.

[zoomkat]

I'd put a meter on it and see just how stable it is. A small auto 12v bulb might be a good load. Sometimes these switching power supplies have electrical noise that might affect some micro controllers that have timing circuits in them. A big capacitor on the outputs might help smooth things out.

[Stuart]

On that same Idea. I took a 7amp 12v power supply. Then built a circuit board with 3 regulators and capacitors on it. I feed the board from the 12v and have outputs for 5,10 and 12 vdc. Temp sensors on 5v, Ocelot on 10v, and the benefit is that the internal regulators in the ocelot and the secu 16 run a lot cooler then being fed with a 12v line. been this way for well over a year.
Stuart

[Barry Gordon]

Had a discussion (bbs) with the manufacturer. First of all it was like being on the phone, immediate response. They stated:

1) The PSU has internal loads to protect it if no external loads present

2) They recommend a load on the 5V rail to draw a current that is 10% of the maximium 12V load for maximum stability.

3) The max the PSU can deliver is 34 amps on the 12V rail, this would want 3.4 amp load on the 5V rail

4) a 1.5 ohm Resistor on the 5V rail will draw 3.33 amps at 16.5 watts

5) a 1.5 ohm 50 watt Power resistor with built in heat sink is $3.19 from Mouser.com part number 241-HS50-1.5F

I guess the code for the part number is easy 241-HS(wattage)-(Ohms)F. the price goes up steep as wattage goes up. e.g. 100 watt units run about $12.

Mouser is www.mouser.com.

I love it when a plan comes together!

[Bill Brower]

And that wraps up this thread (maybe).

Barry, great follow-up on something I am sure a lot of us can use!

[swc69]

OK - maybe the thread is not quite closed.....

With my limited knowledge of electronics I followed most of what you said.

Regarding resistors to stabilize the load:

Would you just take one of the outputs on the 5V side of the PS and run it through the resistor? i.e. - 5V positive out from the PS into the resistor and back through the black wire to the PS?

I'm at the point in my HA where I need a permanent 12v source to get rid of a lot of wallwarts, and being a PC guy a PC PS seemed like a logical option.

Also - if a device uses a 300mA wallwart, or a 500mA wallwart, do I have to be worried about the PS providing too much amps?

Steve C.

[Barry Gordon]

A PS can never supply too much amperage. Let me try to explain. Think of total amperage as an available resource to combine with a fixed voltage to provide power. Power (watts) is Volts * amps. Since the voltage (baring bad or poor design) will stay fixed the amperage is what varies to provide the needed power. The load device will take as much amperage as it needs to get the power it requires to do its job.

Think of a car with a car battery. The starter motor needs a lot of power, it gets it when it needs it, but we have to supply heavier wire to allow the increased amperage to get where it needs to be with out loss. Power loss occurs in a wire according to the formula PLost=(I**2)*R. Larger (heavier, fatter) wire has less resistance per foot. We can't control I (the amperage), the load will dictate that, so we must increase the wire size to lower its resistance so we do not drop too much power in the wire (The lost power shows up as heat).

The light bulb in the glove box also takes what it needs which is a very small amount of amperage since its power requirements are very low.

Hope that helps.

As I stated before, in my situation I will place a 2 ohm 50 watt resistor across the 5 volt rail drawing 3.33 amps and dissipating 16.5 watts of power. The resistor will cost $3.19. It is wirewound with a built in heat sink. On the PSU I am using that will allow the 12V rail to supply about 30 AMPS of power with almost no voltage variation with load. I doubt if my total 12V load will draw 10 amps at this time.

Now compare that to a wall wort that supplies 500milliamps which is 0.5 amps. The device the wall wort is powering probably does not use the 1/2 amp, but uses somewhat less.

PS My house was just placed as the feature home at electronichouse.com. I managed to get in quite a few mentions of Homeseer.

[swc69]

Thanks barry.

Congrats on the article, BTW

Steve C.

[GregKing]

I had initially planned on using a spare PS to provide power, but I eventually ended up using a large (>1.0A) walwart.

I created several fused power connectors which allow for me to hook up various items. If I need more, I'll just add another walwart and connector set.

It ended up being simpler, smaller, and quieter than a PS (although it doesn't look as cool! )

gk

[jasv]

If all you need is 12v why not a cctv power supply

I was planning on using one of these cctv camera power supplies to get rid of some of my wall warts. It seems neater and easier to set up. Any downside?

http://www.video-surveillance.us/pro...12VPowerSupply

I know this web site is expensive but I found some on ebay for less than $100.

[smoothtlk]

The downside of too much amperage capacity is in energy usage. Typically a large power supply supplying 200ma will draw more power from the wall outlet than a 200ma power supply also supplying 200ma.

There might also be some more RF related issues with close proximity cables to a large power supply vs. small one.

With lower efficiency also comes more heat.

Other than that, buy big!