SMPS power supply ...

[CNCpatriot]

Hi ,

Iam trying to build a DIY CNC router.
Iam using stepper motors, whose drivers need 24V power supply & controller needs 5V supply.

Iam wondering whether I can connect 2 SMPS supply from standard PC in series and get my 24V for the stepper drivers.
PC powersupplies are very cheap, and give enough amps needed for steppers, but give only 5V & 12V.

Looks like the COM of SMPS voltage is connected to the chasis.
Iam thinking of isolating the COM from the SMPS chasis and using 2 SMPS in series.

Have any one tried this?

regards

[RomanLini]

There a re a few threads discussing this in the "General electronics" and "Stepper motor drivers" sections of the forum.

I think it might be possible but there are some tricks.

[wireb]

Most PC supplies have output ground connected to frame ground internally.

Easy way to check for it is to take a multimeter and check for a short between the case and the ground wires with the supply unplugged.

If they are connected you can't put them in series since you will sort one supply out via the ground wire in the power cords.

Some supplies you can crack open and remove this connection but if your not used to working with electronics and the safety hazards that go with SMP power supplies it is not recommended.

Quite a few of the standalone SMP supplies have fully issolated outputs and can be stacked in series with no issues (within reason. Rule of thumb is no more than 3 in series and they all must be of the same output ratings and make sure the remote sense is hooked up at the supplies output terminals).

[zeed]

make sure the remote sense is hooked up at the supplies output terminals).

Could you clarify this please?
I used 2 pc supplys on my last cnc machine and everything was OK.
This is the thread I followed to put them together:
http://www.cnczone.com/forums/genera..._computer.html

[wireb]

That chain is using PC supplies from the looks of things. They typically don't have remote sense. If you had a supply like this one

http://www.omron247.com/doc/pdfcatal.nsf/2124205D3A15F13585257049006FFC9A/$FILE/D17S8VM0606.pdf

Note the S+ and S- terminals. in this supplies case you want to tie them directly to V+ and V- right at the supply then stack them in series.

[CNCpatriot]

Hi,

Thanks for the replies.
I went thru the "http://www.cnczone.com/forums/general_electronics_discussion/17548-daisychaining_2_more_atx_computer.html" forum and the link
"http://www.procooling.com/index.php?func=articles&disp=52&pg=2".

Only one thing is still not clear to me: Why do we need to connect smallOhmHighWattage resistors across the 5V , as suggested in the procooling site?
Does this mean we cannot use the 5V from the SMPS?
I was hoping to use the 5V also from the SMPS for my CNC controller.
(Ofcourse 5V from the SMPS whose COM is still connected to the chasis and 24V from the SMPS whose COM is disconnected from the chasis).

Have anyone used both the 5V & 24V( 12+12) from the SMPS?
Is this resistor across the 5V really necessary?

regards

[zeed]

Not realy sure about resistor and his purpose,I did not it install it on my previous daisychained PS and it was OK.
Also now I use pc psu for powering my drivers logic part with 5V,and some fans with 12V,without resistor placed on it...

[CarveOne]

Hi,

Thanks for the replies.
I went thru the "http://www.cnczone.com/forums/general_electronics_discussion/17548-daisychaining_2_more_atx_computer.html" forum and the link
"http://www.procooling.com/index.php?func=articles&disp=52&pg=2".

Only one thing is still not clear to me: Why do we need to connect smallOhmHighWattage resistors across the 5V , as suggested in the procooling site?
Does this mean we cannot use the 5V from the SMPS?
I was hoping to use the 5V also from the SMPS for my CNC controller.
(Ofcourse 5V from the SMPS whose COM is still connected to the chasis and 24V from the SMPS whose COM is disconnected from the chasis).

Have anyone used both the 5V & 24V( 12+12) from the SMPS?
Is this resistor across the 5V really necessary?

regards

The only thing that I can think of is that some of these PC power supplies need a load resistor on the output or they will not regulate properly with a zero to some small load current value. The resistor value and wattage is chosen to cause regulation to happen even with nothing connected to the +5vdc output terminals. If the +5vdc output does not regulate, neither does the 12vdc outputs. These are called pre-load resistors.

CarveOne

[wireb]

That resistor is just there to put the minimum load needed to get the 5V supply to regulate (usually a amp or two of draw). If not the 12V output will be low due to the power supply trying to get the 5V output down to spec (it regulates all the outputs based on the 5V output).

Even with the resistors you can use the 5V output. Pick the supply closest to the motor ground so 5V and "24V" are at the same ground levels.