24V from a PSU?

Willsonman

[H]ard|Gawd
Looking at a solution for my case mod issue and am looking at a part that requires 24 volts. is there any way to place a series circuit to get this? Any other way to get 24V?

You could be very very lazy and do a probably bad for everything involved ganging together of two 12V sources from the PSU. Other than that, I believe there are voltage multiplying/doubling circuits (rectifiers maybe), but I haven't gotten that far yet in my studies.

One of the other guys who usually eat these questions alive will probably post a better answer. (Mohonri, cpenna, the other EE/CEs/electric guys).

ATX power-supplies output +12V and -12V... just take the diff between these AS long as the resultant object didn't need to be reference to 0V AND you don't exceed the current-rating of the relevant rail (-12V seems to always have lower power capability)

well, It will be powering a linear actuator... so not in use all the time. I imagine it will operate off 12V but at a decrease in lifting capacity. How would you wire up the +12V and -12V to get 24V?

There is 24 VDC between the blue (-12V) and yellow (+12V) PSU wires. Your circuit cannot rely on the 24VDC being referenced to Ground however, and is only good for 0.5 amps maximum.

You could be very very lazy and do a probably bad for everything involved ganging together of two 12V sources from the PSU.
Putting two molex 0V & 12V pairs in series in an attempt to get 24V is guaranteed to cause a bang.

What current do you need? As said, the -12V rail is a low rating, read the PSU label for your figure.

Another option which I have sucessfully used is a 12V DC-DC converter with an isolated output, powered from PSU 12VDC. If you need 24VDC (referenced to ground) connect the negative voltage output to your PSU +12VDC, and your load between PSU ground and the DC-DC converter positive output.

No offense intended, but what movax is suggesting is a very good way to (at a minimum) blow a fuse in your power supply.

Using the -12V rail from an older PSU is also not recommended, because the current rating is very low for that rail. The -12V rail was only used for negative signals on things like serial ports, not for actually powering anything.

IMO, your best bet is to actually get a real 24V power supply that can source the kind of current you need.

No offense intended, but what movax is suggesting is a very good way to (at a minimum) blow a fuse in your power supply.

Using the -12V rail from an older PSU is also not recommended, because the current rating is very low for that rail. The -12V rail was only used for negative signals on things like serial ports, not for actually powering anything.

IMO, your best bet is to actually get a real 24V power supply that can source the kind of current you need.

i agree, most -12v rails on powersupplies are an afterthought at best... i wouldent really recommend using it for anything that is actually going to draw any current...

just get yourself a small 24v switching supply from an electronics surplus place and hide it away somewhere and you should be good to go

Up convert with a DC to DC converter.
IT's the only way if your actuator draws more then .25a or so... I wouldn't trust it otherwise.

What movax suggested will cause a major problem... so steer clear.

I tried this once with USB ports... wanted to get 10v... It only works with batteries, and seperate sources of power.

Since almost all 12v lines come from the same place in the PSU (and all grounds go back to common) you will get a big spark, and a fire.

alright, I'm fine with a dc-dc converter. Anyone know where to get one that I would need?

digikey.com and mouser.com are good sources for DC-DC converters. You'll want one that has an input voltage range that your PC can supply (typically rated 9V-18V), and has an output current rating suitable for what you are going to power (it's best to have some headroom... it you need 1 amp, buy one that has 1.5 amp output), and MOST IMPORTANT... isolated output.

As I touched on earlier, you would connect it like this for 24VDC referenced to PC power supply ground:
DC-DC converter + input to PSU +12V
DC-DC converter - input to PSU Ground
DC-DC converter + output to whatever you are powering
DC-DC converter - output to PSU +12V
Ground for whatever you are powering to PSU ground

That will give you 24VDC to whatever you are powering.

You'll want some filtering too or the output tends to oscillate. Put a 10uF capacitor across the input and 0.47uF across the output.

What exactly prevents the OP from putting two 12V molex connectors into a series and then hookup to the load?

You mean two 12V molex from a standard ATX power supply? The problem would be connecting yellow on one to black on the other, thus shorting out the power supply.

You mean two 12V molex from a standard ATX power supply? The problem would be connecting yellow on one to black on the other, thus shorting out the power supply.
Yeah I guess it wouldn't work because they have a common ground unlike a battery for example. But what about mains inside your house? You know how you have 220v for your appliances? IIRC they take two 120v lines and combine it into one so I guess it could work that way.

Yeah I guess it wouldn't work because they have a common ground unlike a battery for example. But what about mains inside your house? You know how you have 220v for your appliances? IIRC they take two 120v lines and combine it into one so I guess it could work that way.
There's a world of difference here between your computer PSU and your house mains. As in Westinghouse vs. Edison difference--the power in your computer is DC, while the mains is AC. You're right about how they get 240VAC in your house--there's typically actually two 120VAC lines and a neutral coming into your house. The two AC lines are 180 degrees out of phase, so the voltage difference between the two is 240V.

But that's completely unrelated to what we're working with here. Except that we're still pushing electrons through metal wires....

There's a world of difference here between your computer PSU and your house mains. As in Westinghouse vs. Edison difference--the power in your computer is DC, while the mains is AC. You're right about how they get 240VAC in your house--there's typically actually two 120VAC lines and a neutral coming into your house. The two AC lines are 180 degrees out of phase, so the voltage difference between the two is 240V.

But that's completely unrelated to what we're working with here. Except that we're still pushing electrons through metal wires....

Yes I know house mains are are AC while computers are DC but that doesn't mean you can't add up electrical sources in order to increase amperage or voltage.

Yes I know house mains are are AC while computers are DC but that doesn't mean you can't add up electrical sources in order to increase amperage or voltage.
It depends on how the electrical sources are built. Batteries and (depending on phase) AC power, yes. Wall-connected DC power supplies...it depends on how their wired. If they're properly isolated, then yes. But I wouldn't count on that being the case for a computer PSU, since the ground on the PSU is often connected to the case ground. It *might* be possible to get 24V using two PSUs, but I wouldn't count on it. I guess you could get a couple of el-cheapo PSUs and try it out. Just wear safety glasses .

It *might* be possible to get 24V using two PSUs, but I wouldn't count on it.

I have tried it... you get sparks.

The DC-DC converter idea works though. I used a few when I needed -12V at 2.5 amps for some computers at work.

I have tried it... you get sparks.

The DC-DC converter idea works though. I used a few when I needed -12V at 2.5 amps for some computers at work.
Yeah because they share a common ground.

My idea would be to emulate that of house wiring where they have 1 common ground and 2 "hot".

So I guess you'd hook up the two 12v molex connectors and then finish the circuit out to the ground.

My idea would be to emulate that of house wiring where they have 1 common ground and 2 "hot".

So I guess you'd hook up the two 12v molex connectors and then finish the circuit out to the ground.
The reason you can "stack" the two circuits in your house is that one is the opposite voltage of the other. For AC, that's the same as being 180 degrees out of phase. The difference (sin(x) - (-1*sin(x) = 2sin(x))gives you twice the voltage difference.

We don't have that same luxury with a DC PSU. If the second rail were -12V, then sure, we could connect the two grounds together and get 24VDC out. But both of the rails are +12V with respect to the same ground. If I'm understanding you correctly, you're suggesting that you could tie the 12V line from one molex connector to the GND line of another molex connector, and that the "12V" line on the second connector would then be at 24V relative to the GND of the first connector. Is that what you're suggesting? If so, please don't try it. Since the 12V lines from both molex connectors come from the same solder pad inside the PSU, and the GND lines also come from a common solder pad, if you connect one 12V line to a different GND line, you're shorting out the PSU.

The reason you can "stack" the two circuits in your house is that one is the opposite voltage of the other. For AC, that's the same as being 180 degrees out of phase. The difference (sin(x) - (-1*sin(x) = 2sin(x))gives you twice the voltage difference.

We don't have that same luxury with a DC PSU. If the second rail were -12V, then sure, we could connect the two grounds together and get 24VDC out. But both of the rails are +12V with respect to the same ground. If I'm understanding you correctly, you're suggesting that you could tie the 12V line from one molex connector to the GND line of another molex connector, and that the "12V" line on the second connector would then be at 24V relative to the GND of the first connector. Is that what you're suggesting? If so, please don't try it. Since the 12V lines from both molex connectors come from the same solder pad inside the PSU, and the GND lines also come from a common solder pad, if you connect one 12V line to a different GND line, you're shorting out the PSU.
You say that you can do that with AC because they're 180* out of phase, but wouldn't they be perfectly within phase if they both come from the same panel? My electrician says you take two 10gauge lines, combine the hot and then you'd have 220v. But since they both come from the same panel, I don't see how they'd be out of phase like you're saying.

You say that you can do that with AC because they're 180* out of phase, but wouldn't they be perfectly within phase if they both come from the same panel? My electrician says you take two 10gauge lines, combine the hot and then you'd have 220v. But since they both come from the same panel, I don't see how they'd be out of phase like you're saying.
They may come from the same panel, but they're not the same source. In order to get both 120V and 240V, you have to run three wires into the house: a common neutral and two AC lines, which are 180 degrees out of phase with one another. Those two 120V lines then feed different circuits for most things in your house. Around half the circuits will be on one and half on the other, all of them connected to 120VAC on one end and the common neutral at the other. Where there is a need for 240V, the plug is wired to one 120VAC line on one side and the other 120VAC line on the other. The difference between those two lines gives you 240VAC.

I don't know exactly what words your electrician said, but you might be misunderstanding them. In a way, yes, you're combining the two "hot" wires, I guess.

You say that you can do that with AC because they're 180* out of phase, but wouldn't they be perfectly within phase if they both come from the same panel? My electrician says you take two 10gauge lines, combine the hot and then you'd have 220v. But since they both come from the same panel, I don't see how they'd be out of phase like you're saying.

if you look at your panel you will see that there are two hots, and the "dual" breakers for your 240v lines are taking one from each hot...

you cant take two lines from the same hot and get 240... you would get 0 between them actually, since they are in phase...