Making a 1.8kW DIY CC/CV Lab Bench Power Supply from ATX Power Supplies

Thread Starter

-live wire-

Joined Dec 22, 2017
911
I plan on doing some projects with high-torque stepper motors in the future. Due to many annoyances, a 1 ohm 1 amp stepper, for example, may need WAY more than 1V and 1W. From what I have heard, they may require up to 10 TIMES the rated voltage to work. And it would become an even bigger problem if you need to power multiple motors at the same time. So, I need to get a high-power power supply. I am considering going with this one, which could be useful for many future projects, too.

https://www.amazon.com/YCCTEAM-Quie...&sr=8-1&keywords=very+high+power+power+supply

You never know when you may need 100s of watts for something. . However, the power supply only supplies 12V, which is fairly low for many motors and other things. So to get the same current and twice the voltage, you could hypothetically connect them in series. However, there are problems with isolation. Given that it IS grounded, and there is often a connection between earth ground and -, how do I avoid a high-power short? And because they can supply 18 amps, they WILL get destroyed if they are connected improperly. So how would I combine them in series to get 24V? I have heard of people using diodes in between the supplies. Does that work, or should I try something else? And I may even combine 3 in series, if it is not too difficult (MORE POWER!).

P.S. I may even add a high power buck converter with current limiting/cc if connecting them in series works well. Essentially I might make a crude DIY .4/.6 kW bench power supply. If it gets extremely impractical/costly with that much power, I will just go with a less powerful commercially available LBPS.
 

Thread Starter

-live wire-

Joined Dec 22, 2017
911
My bad. It looks like the power supply is not grounded. So no isolation issues? Also, what could I do if it was? I may encounter those issues with other power supplies in the future, and it would be good to know how to deal with them.
 

Danko

Joined Nov 22, 2017
927
Connect batteries in series. Two, three or more. And put between them isolation (cardboard, plastic, etc.).
https://www.batterystuff.com/kb/articles/battery-articles/battery-bank-tutorial.html
AC feeding DC power supplies can be connected in series too, if their OUT terminals are not grounded. If one from OUT terminals is connected to body frame, then prevent body frames from contact, using isolation separator.
For preventing power supplies from damage by reverse current, connect to OUT terminals of every PS diode in polarity: anode to "-", cathode to "+".
 
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dendad

Joined Feb 20, 2016
3,517
I personally would not run supplies in series. All could work well until something goes into current limit. You may get away with it, particulaly adding the reverse diodes across the power supply outputs as Danko mentions. Also you may need to ensure the voltage rating of the pass transistors and other parts are good enough to stand a bit of a strain.
A simple power supply to drive a stepper motor is not that hard to do. Your stepper driver should control the phase current so to a certain extent the power supply voltage is not that critical. Just make it higher than the motor voltage and have your drivers capable of dissipating any excess heat. The supply probably does not need to be regulated. Just a transformer, bridge rectifier and filter cap.
Have you decided on the motors you are to use and the motor drivers?
 

Thread Starter

-live wire-

Joined Dec 22, 2017
911
So from my understanding I need to have it like this for multiple power supplies:
ps1 +, ps1 -, diode, ps2 +, ps2 -, diode, ... ps... +, ps... -, diode,

Due to the annoying forward voltages, could I use a power MOSFET as a replacement for a diode? I heard of people doing this, as apparently they block reverse current from source to gait, with minimal losses. And if I wanted to draw, say, close to 18 amps on the output, how much power would the gate need? I heard if you need to switch high power it gets more demanding. But at the same time, it is for reverse polarity protection, not PWM. So what value resistor is wise to use at the gate? Or do I use a shotky instead? I do not want it to consume too much power.

And you say you would not run them in series because of when it gets close to the current limit. Why do you say that? What could happen, and what could I do to avoid it? And is the reverse polarity protection to avoid problems with mainly inductors (like a motor)? Also, what additional problems might I encounter with such a 12V 18A power supply? Is one like that intended to be continuously operated at close to 18 amps, or will it not be happy about that? I feel like there could be something I am missing that might lead to the power supplies getting damaged.
 

Danko

Joined Nov 22, 2017
927
Without diodes and one PS is OFF or current limits, this PS will immediately damaged by reverse voltage 36V from the other PSs .
Diodes - 200V, 25A.
Drawing2.png
 
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crutschow

Joined Mar 14, 2008
24,938
Due to the annoying forward voltages, could I use a power MOSFET as a replacement for a diode?
The diodes are across each output, not in series with the output, so they cause no voltage drop.
They are to prevent damage to the power supply if one is off and the other is on, so reverse current won't be driven through the off supply (which could damage it) but instead goes through the diode.
It also protects a supply going into current limit when the others aren't.
 

Thread Starter

-live wire-

Joined Dec 22, 2017
911
Without diodes and one PS is OFF or current limits, this PS will immediately damaged by reverse voltage 36V from the other PSs .
Diodes - 200V, 25A.
View attachment 148652
Ok. But where exactly is the diode? That drawing is pretty confusing. Also, wouldn't that kill the diode if you connect it in parallel to your load with no resistor or anything? Or at least draw WAY too much current? It would help to see a more proper schematic for what you are describing.
 

Danko

Joined Nov 22, 2017
927
Diodes connected in parallel with each power supply.
Diodes connected in reverse polarity.
When all power supplies are ON, we have not current through diodes, only current through R load.
When some power supply becomes OFF or going into current limit, then current goes through diode in parallel with that power supply.
Diodes are 25A, therefore 18A is not big current for them.
I will draw schematic for you.
Drawin1.png
 
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Thread Starter

-live wire-

Joined Dec 22, 2017
911
Oh. Ok. Sometimes conventional current gets me. Can you reccomend a good diode that's at least 25A 50V and under $2 per diode (no more than $10 total)? I looked on amazon and only found tvs diodes.
 

Danko

Joined Nov 22, 2017
927
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Danko

Joined Nov 22, 2017
927
In two bridges you have 8 diodes.
Single - means one phase bridge, not 3 phase.
See schematic in post #12
 
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Danko

Joined Nov 22, 2017
927
What means FBR ?
I do not know, what is FBR...
OK, I got it.
Yes, it is Full Bridge Rectifier.
 
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