Wiring and Power Supply Help Please

[ashjam]

Hope someone can possibly help me with the following. Forgive me for possible use of wrong words here but I am a bit of a newbie to all this CNC stuff.

I have converted a PC Power supply, forgetting all the other voltages out other than 5V, -12V and 12V, I need 24V for the stepper controller. I have used an LM338K regulator circuit and managed to get 22V out. This consists of -12V & +12V in out gives me the 22V.

The question, my stepper controller board requires 5V - Grd - 24V in, the 5V will come from the 5V out from the main power supply, the 24V from the LM338 board. Now the confusing bit, do I take a NEG from the main power supply and the NEG from the LM338 board, connect together and use as the Grd on the stepper board.

Years since I have messed with this stuff and dont just want to assume as I dont want to blow the stepper board up.

Thanks in advance guys for any advice.

Steve

[Al_The_Man]

You cannot use the same common reference for each as the 24v supply you have setup is referenced from a different point in the supply, this means that you cannot use it on any equipment that uses a 5v and 24v which both use the same reference point.
IOW, you cannot set up two different reference points using the same supply and then connect them together.
Al.

[ashjam]

Hi Al, thanks for the reply, that has gone a tad over my head.

So given that I have a power supply with the relevent voltages as well as 24V what actually do I do with the equipment I have available, are you saying I cannot use this method or when you refer to reference point are you refering to Neg/Grd?

From this equipment available to me how can i get the 5V/grd/24v, sorry to ask this but I am a bit of a newbie, once i have figured out powering the board everything else is simple(ish).

Thanks again.

Steve.

[TarHeelTom]

What Al has said is that you can use the existing power supply to get all of the voltages you need, except the 24 volts.

You'll need to add a second power supply for the 24 volts.

Or, you might possible find another power supply which gives to 5, and 24 volts.

Keling, for one, makes a power supply with a huge toroid transformer and an unregulated power supply in various voltages. It also has mounted on the same chasis, a regulated 5 volt power supply. Look at their KL- 3815, for instance. This isn't the power supply you need, but one similar to this would work.

There are many ways of solving this. Do you happen to know a friendly local ham radio operator, or someone else well versed in electronics?

You also need to know how much current you need from the 5 volt side. Probably not much.

Tom

[ashjam]

Ahh thanks Tom, that sort of makes more sense. So, if i am understanding this right I can use the 5V supply from the existing modified ATX power supply, if I then use another modded ATX supply and use the -12 and +12 though the voltage regulator board to get the 24V as I have done already, is this the same thing, two separate supplies one with 5V the other with 24V and a common grd?

As for knowing a Radio Ham, I was licenced 30 years ago (G6KPW) but have forgotton almost everything I ever learned.

Thanks for the response.

Steve

[TarHeelTom]

Yes, that would work.

However, it's kinda overkill, depending on how much current you need from the 5 volt supply.

Understand that the supply is using a center-tapped 24 volt winding to get +12 and -12. That center tap is also the ground for the 5 volt portion of the supply.

MIGHT be possible, if you can identify the leads from the transformer, to disconnect that center tap from ground and connect the -12 volt lead to ground instead.

Another method would be to use the +24 volts to feed an additional 5 volt regulator, perhaps one of the three pin types.

But it's hard to design something by remote control without being able to see in and stick my vom leads into its guts. Don't you know any currently active electronic nerds?

Tom

[ashjam]

Hi Tom, have access to loads of ATX power supplies so this is probably the easiest option, they take about 15mins to modify.

I do know a few electronic nerds so will check with them.

Thanks for the assistance on this as well as the quick responses.

Thanks again

Steve

[TarHeelTom]

There used to be (may still be) a class at our local drag strip called "Run what cha brung".

If you've got access to lot of power supplies, then "run what cha brung".

Tom

[ashjam]

Will do, if I kill a few in the process, its no great loss and if I run out they are about £10 ($13) ish.

Incidently we have a "run what cha brung" at Santa Pod Raceway just down the Motorway (freeway) from me.

[MrWild]

Originally Posted by TarHeelTom Yes, that would work. However, it's kinda overkill, depending on how much current you need from the 5 volt supply. Understand that the supply is using a center-tapped 24 volt winding to get +12 and -12. That center tap is also the ground for the 5 volt portion of the supply.

Isn't the -12v a low amp circuit? If you try to mix a low amp -12v and a high amp +12v, I bet you get smoke when you push it.

What I believe was meant in reference to a second supply was piggy backing them as demonstrated in the sticky in the Electronics forum. You put the +12v of one in series with a second. The second needs it's ground clipped at the circuit board so it floats. The first one can also provide 5v. You do NOT use the second with -12 and +12 for 24v. You use it like second battery for more power/volts.

[ashjam]

Hi thanks for the reply, nothing wrong with a bit of smoke...

Seriously though, just had a look at what you suggested in the sticky, think I understood it, need some alcohol though now to rest my brain.

Will give it a whirl.

Steve