Power supplies in series

Hello !

In order to speed up the heating process of my heated bed, i have plugged two 12V power supplies output in series to get 24V.
Knowing that both of my PSU are identical and rated 30A : is there any security/technical concern having :
- the heated bed (24V) and the hotend (12v) plugged on psu1 ?
- the Ramps plugged on PSU2 ?

Should i install any other component ? Like a diode ?

Thank you for your help

Would think it should be OK.

As the power supplies you are using have a isolated frame and neutral ground, further isolating output ground from AC Ground.

The circuit will look like any 24vdc Supply with a Center Tap of 12vdc.
It appears that you have one Mosfet connected to 12vdc and the Other connected to 24Vdc. Is this Correct.
So long as you have enough Drive from the Controller to fire Up the External Mosfet driving 24 volts, I see no problem using the Power Supplies in this design...

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Computer Programmer / Electronics Technician

If your powering a heated bed that is a 12 or 24 v switchable bed, this will not help. They are designed so he wattage is the same for 12v and for 24v systems.

With supplies is series you are still limited to the current of the lowest current supply

yes to diodes see below.

All about power supply in parallel and in series. [www.nextys.com]

Edited 1 time(s). Last edit at 02/17/2019 07:45PM by Dust.

Dust is referring to the circuit below





Power supplies in series should have a reverse polarity diode connected across the output terminals so that if operated in series with other supplies,
damage will not occur if the load is short-circuited or if one supply is turned on separately from its series partners.

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Computer Programmer / Electronics Technician

thank you so much guys for this great help !

concerning the diode : would a 1N4001 do the job ?
If i understand well, I have to wire the diode cathode with the positive side of the PSU output and the anode on the negative side ?

With 2x30A power supplies in series : How many amps will I be able to draw from my device ? 30A ?
Do you know how many Amps is drawn by a 24V heated bed ?

have a great day !

Thomas

no 1N4001 is a 1amp device, you need something larger than the max current your going to use.
"an anti-parallel diode (rated to the maximum voltage of the system and to with a peak surge current at least equal to the nominal current)"
This is why I linked the page vs just grabbing the image...

30a is your maximum that can be supplied (according to the label)

For the bed you need to know the resistance

using standard ohms law



you want to know I = V/R
v is 24 volt and R you measure on the bed (while unplugged)

Thank you so much for sharing...This is very valuable info !

Therefore I have measured my unplugged 24V heatbed : I get 6 Ohm at the end of the wire (where I connect the MOSFET). Therefore 4amps

According to reprap wiki : "Turn off your printer completely, and measure the resistance of your heatbed using a multimeter: it should not be higher than 1.2 Ohm. If it is higher than 1.2 Ohm, your heatbed will not get enough current and will not dissipate enough heat to reach the required temperatures."

I have made a temperature test : my bed reaches 60°C in approx 1:30min.

Does this sound ok for you ?

Have a great day !

According to reprap wiki : "Turn off your printer completely, and measure the resistance of your heatbed using a multimeter: it should not be higher than 1.2 Ohm. If it is higher than 1.2 Ohm, your heatbed will not get enough current and will not dissipate enough heat to reach the required temperatures."

the above is for 12v systems
ie a 12v bed of 1.2 ohms is 120watts (W = V^2/R)

your system

24v of 6 ohms is 96 watts... so this should be slower to heat up than a regular 12v heated bed.
a 120 watt 24v bed would be 4.8 ohms

But since your bed heated up fine and quite quickly, I suspect your meter doesn't read low resistance very accurately. (this is very common)

Have you already connected your PSU’s in series ? And it is OK ?

If so, then you can ignore the following

All I know about putting PSU’s in series is that some sorts of PSU’s /some types have the negative output ‘tied’ to earth/ground

So the moment you connect the positive output of the first PSU to the negative of the second, you are connecting the positive output to ground..... instant ‘bang and flash’ unless PSU 1 has some sort of short circuit protection.

This info is coming from my background of creating strings of PSU’s from PC power supplies, where to get that to work, you first must strip the PSU, and cut tracks etc to make the output ‘floating’


As I said to start with maybe your PSU’s have floating output , but not all PDU’s will

Quote
Roberts_Clif

As the power supplies you are using have a isolated frame and neutral ground, further isolating output ground from AC Ground.

This Design of Power Supplies do not have the 120VAC ground connected to neutral or to the 12vdc ground terminals all three are isolated grounds.
This means that they can be wired in series.

If you are unsure check continuity between the three grounds, there should be no continuity between the three grounds.
Further testing you can connect both power supplies up to 120VAC measure the voltage of each at 12vdc.
Then connect the Meter leads to one Power supplies 12vdc and the other Supplies Hot 12vdc then 12vdc Ground there should be no voltage between them.
This would prove they can be wire in series.


P.S. I just did it with my Spare 24vdc supplies. making a 48vdc supply.

See Image




Edited 1 time(s). Last edit at 02/20/2019 07:34AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Ah yes, American 120VAC
Sorry I had it in my mind UK 240VAC standard domestic wiring, where many ( most) mains plugs are 3 pin. Then if the house wiring is PME ( Protective Multiplr Earthing ? I think) where from memory earth is bonded to neutral.

I am using a 3 Prong plug with Ground, Neutral and Line just 60hz 120vac, it still should not matter as the supply has isolated grounds.
The supplies are designed this way so one supply can be used in every country without have stock for the two standards. Thous the 120vac to 230vac switch.

While yours would be safety Ground, Neutral and Line 50hz 230vac, and our 240vac is safety Ground, neutral, line, line where the two lines are 120vac in the opposite phase.
One Positive 120vac the other negative 120vac making 240vac.

Edited 8 time(s). Last edit at 02/20/2019 07:57AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Yes

I only remember this as I used to have a string of about 15 hacked 500W PC PSU’s and Meanwell’s series’d up to charge LiPo packs

There's normally some tie between - and ground to reduce noise and dump AC voltage to ground. I could see an issue where tying multiple units in series could push the capacitors out of there voltage spec. That said, I imagine they are speced to dump mains AC voltages.