2x24V switchmode power supplies = 48V?...

[server]

Hi. I have two identical 24V/10A switching supplies from a photocopier. Can I join them together somehow to make a 48V power supply? Something tells me there\'s more complexity to this than just \'yes\' or \'no\'.
If not (a good idea), could you suggest how I might otherwise go about it? Maybe there\'s a favourite charge pump circuit or something that would give me the -24V, or...?
If this is a dumb question please humour me!
Thanks.
-Dan.

 

[Phil Allison]

digital...@gmail.com wrote:

============================

I have two identical 24V/10A switching supplies from a photocopier. Can I join them together somehow to make a 48V power supply? Something tells me there\'s more complexity to this than just \'yes\' or \'no\'.

--------------------------

** Should be possible to isolate the ground from the output of one supply and have it sit 24V above.

If there is any \"devil\" it is in details you have not supplied us.


..... Phil

 

[server]

On Thursday, July 30, 2020 at 1:47:10 PM UTC+10, Phil Allison wrote:

digital...@gmail.com wrote:

============================

I have two identical 24V/10A switching supplies from a photocopier. Can I join them together somehow to make a 48V power supply? Something tells me there\'s more complexity to this than just \'yes\' or \'no\'.

--------------------------

** Should be possible to isolate the ground from the output of one supply and have it sit 24V above.

If there is any \"devil\" it is in details you have not supplied us.


.... Phil

Thanks Phil. How would I do this isolation?

 

[Jeff Liebermann]

On Wed, 29 Jul 2020 20:58:54 -0700 (PDT), digitaltrousers@gmail.com
wrote:

On Thursday, July 30, 2020 at 1:47:10 PM UTC+10, Phil Allison wrote:
digital...@gmail.com wrote:

============================

I have two identical 24V/10A switching supplies from a photocopier. Can I join them together somehow to make a 48V power supply? Something tells me there\'s more complexity to this than just \'yes\' or \'no\'.

--------------------------

** Should be possible to isolate the ground from the output of one supply and have it sit 24V above.

If there is any \"devil\" it is in details you have not supplied us.


.... Phil

Thanks Phil. How would I do this isolation?

You can start by supplying the maker and model number of the power
supply so we can see if both output terminal are not connected to case
ground and how any sense wires are arranged. If there is no model
number, take an ohmmeter and check for continuity between each output
terminal and the case ground. If there is continuity, see if there is
a bridging clip on the output terminal strip to case ground. Remove
it and see if you still have continuity. Lastly, apply AC power with
no load, and see if there is any voltage between either output
terminal and case ground. There might be some leakage voltage, but it
shouldn\'t be the full 24VDC. If you see the full 24VDC between either
output terminal and ground, it won\'t work.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Don Y]

On 7/29/2020 8:30 PM, digitaltrousers@gmail.com wrote:

Hi. I have two identical 24V/10A switching supplies from a photocopier. Can
I join them together somehow to make a 48V power supply? Something tells me
there\'s more complexity to this than just \'yes\' or \'no\'. If not (a good
idea), could you suggest how I might otherwise go about it? Maybe there\'s a
favourite charge pump circuit or something that would give me the -24V,
or...? If this is a dumb question please humour me! Thanks. -Dan.

That depends on the design of the power supplies as well as the characteristics
of the load. Having said that, I pulled a pair of 70A 12VDC power supplies
from a server and stacked them to form a 24V source for an electric wheelchair.

You\'ll likely have to isolate the \"0V\" output (\"-\" or \"GND\") from the chassis
(\"Earth\") before you can do so. Otherwise, stacking them will directly short
out one of the supplies!


XX
X+....................
XX
X-.........
XE .
. .
. .
. YY .
. Y+....
. YY
. Y-...............
. YE
. .
......

(forgive the crappy ASCIIART)

Note that Xe is internally tied to X-.
Likewise, Ye is internally tied to Y-.
X- is externally tied to Y+.
Xe is tied to Ye in the power cord.
So, Xe is tied to X- which is Y+ *and* Y-!

You\'ll have to disassemble the power supplies to determine where
the bond between \"gnd\" and \"earth\" occurs and then physically
sever that connection. In my case, the PCB was mounted to a METAL
boss with a metal screw that tied some \"gnd\" land around the screw
to the chassis.

I removed the metal boss(es) and replaced with nylon standoff(s).
Then, passed nylon screw through the standoff to nylon nut to
secure the board in place.

Once finished and reassembled, an ohmmeter should verify there is
no longer a direct short from the earth pin on the power inlet
to the 0V/gnd terminal on the power supply.

(note that the same reasoning applies if the \"+\" terminal had been
bonded to earth)

 

[boB]

On Wed, 29 Jul 2020 21:17:24 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

On Wed, 29 Jul 2020 20:58:54 -0700 (PDT), digitaltrousers@gmail.com
wrote:

On Thursday, July 30, 2020 at 1:47:10 PM UTC+10, Phil Allison wrote:
digital...@gmail.com wrote:

============================

I have two identical 24V/10A switching supplies from a photocopier. Can I join them together somehow to make a 48V power supply? Something tells me there\'s more complexity to this than just \'yes\' or \'no\'.

--------------------------

** Should be possible to isolate the ground from the output of one supply and have it sit 24V above.

If there is any \"devil\" it is in details you have not supplied us.


.... Phil

Thanks Phil. How would I do this isolation?

You can start by supplying the maker and model number of the power
supply so we can see if both output terminal are not connected to case
ground and how any sense wires are arranged. If there is no model
number, take an ohmmeter and check for continuity between each output
terminal and the case ground. If there is continuity, see if there is
a bridging clip on the output terminal strip to case ground. Remove
it and see if you still have continuity. Lastly, apply AC power with
no load, and see if there is any voltage between either output
terminal and case ground. There might be some leakage voltage, but it
shouldn\'t be the full 24VDC. If you see the full 24VDC between either
output terminal and ground, it won\'t work.

What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

Cathode to the positive and anode to the negative terminal. Of course
the diode will have to be rated for at least 24V and be able to take
the current you would be drawing at 48V

 

[Rob]

boB <boB@K7IQ.com> wrote:

What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.

 

[Phil Allison]

booB wrote drivel:

=================

What I would do is to wire them in series,

** Not so smart if the outputs are both supply grounded.

add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

** Makes no sense - WTF is \" unbalanced\".

However the diodes are a good idea in case one supply stops working.

Cathode to the positive and anode to the negative terminal. Of course
the diode will have to be rated for at least 24V and be able to take
the current you would be drawing at 48V

** Drivel.

...... Phil

 

[boB]

On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.

Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

 

[Lasse Langwadt Christensen]

torsdag den 30. juli 2020 kl. 20.40.36 UTC+2 skrev boB:

On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.


Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it\'ll be isolated but the output might be grounded

 

[boB]

On Thu, 30 Jul 2020 12:24:58 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

torsdag den 30. juli 2020 kl. 20.40.36 UTC+2 skrev boB:
On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.


Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it\'ll be isolated but the output might be grounded

If it is, then unground it

In those cases, there is usually a strap that can be undone.

I don\'t think he said how much current this was supposed to supply ?

 

[Rob]

boB <boB@K7IQ.com> wrote:

On Thu, 30 Jul 2020 12:24:58 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 30. juli 2020 kl. 20.40.36 UTC+2 skrev boB:
On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.


Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it\'ll be isolated but the output might be grounded


If it is, then unground it

In those cases, there is usually a strap that can be undone.

First find that out and make the mod.
There are supplies where it is easy, and there are those where it is
not so easy. It depends on the original purpose and design of the
supply.

> I don\'t think he said how much current this was supposed to supply ?

It is irrelevant. The same method works for 100mA and 100A.

 

[boB]

On 31 Jul 2020 09:06:16 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
On Thu, 30 Jul 2020 12:24:58 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 30. juli 2020 kl. 20.40.36 UTC+2 skrev boB:
On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.


Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it\'ll be isolated but the output might be grounded


If it is, then unground it

In those cases, there is usually a strap that can be undone.

First find that out and make the mod.
There are supplies where it is easy, and there are those where it is
not so easy. It depends on the original purpose and design of the
supply.

I don\'t think he said how much current this was supposed to supply ?

It is irrelevant. The same method works for 100mA and 100A.

The theory is the same but the bypass diode is not the same and would
have to dissipate quite a bit if called upon at 100A

 

[Rob]

boB <boB@K7IQ.com> wrote:

On 31 Jul 2020 09:06:16 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
On Thu, 30 Jul 2020 12:24:58 -0700 (PDT), Lasse Langwadt Christensen
langwadt@fonz.dk> wrote:

torsdag den 30. juli 2020 kl. 20.40.36 UTC+2 skrev boB:
On 30 Jul 2020 08:55:52 GMT, Rob <nomail@example.com> wrote:

boB <boB@K7IQ.com> wrote:
What I would do is to wire them in series, like you want to do but add
a bypass diode across each supply\'s output so it doesn\'t get a reverse
voltage across it if they should be unbalanced.

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is \'floating\', i.e. not connected
to the case/ground. When he is not able to accomplish that for whatever
reason, there is no need to go on to this step.


Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it\'ll be isolated but the output might be grounded


If it is, then unground it

In those cases, there is usually a strap that can be undone.

First find that out and make the mod.
There are supplies where it is easy, and there are those where it is
not so easy. It depends on the original purpose and design of the
supply.

I don\'t think he said how much current this was supposed to supply ?

It is irrelevant. The same method works for 100mA and 100A.

The theory is the same but the bypass diode is not the same and would
have to dissipate quite a bit if called upon at 100A

The reply already said \"Of course the diode will have to be rated for at
least 24V and be able to take the current you would be drawing at 48V\".
So that is well covered.

It is also dependent on the characteristics of the supplies and the load
if this is at all required. For a resistive load it isn\'t required.
For a load that can spike above the rated output of the supplies, or
for supplies that have strange V/I characteristics like \"foldback current
limiting\" (instead of a current source at max-I) it is certainly advisable.

However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

 

[Don Y]

On 8/1/2020 4:55 AM, Rob wrote:

However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

You can ALWAYS isolate the supplies from ground/earth/neutral.
The question is one of how costly it is to do so (e.g., insert
an isolation transformer into the mains feed of one -- or both).

 

[Rob]

Don Y <blockedofcourse@foo.invalid> wrote:

On 8/1/2020 4:55 AM, Rob wrote:
However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

You can ALWAYS isolate the supplies from ground/earth/neutral.
The question is one of how costly it is to do so (e.g., insert
an isolation transformer into the mains feed of one -- or both).

That would not be relevant for the usual DC supply which is already
isolated mains-to-DC to begin with. And even after installing your
isolation transformer it would still have the presumed DC-to-case ground
connection that has to be broken.

 

[server]

On Saturday, August 1, 2020 at 10:06:34 AM UTC-7, Rob wrote:

Don Y <blockedofcourse@foo.invalid> wrote:
On 8/1/2020 4:55 AM, Rob wrote:
However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

You can ALWAYS isolate the supplies from ground/earth/neutral.
The question is one of how costly it is to do so (e.g., insert
an isolation transformer into the mains feed of one -- or both).

That would not be relevant for the usual DC supply which is already
isolated mains-to-DC to begin with. And even after installing your
isolation transformer it would still have the presumed DC-to-case ground
connection that has to be broken.

Even with DC-case ground, fine as long as hot and neutral don\'t. As long as you don\'t hook up cases together.

 

[Don Y]

On 8/1/2020 10:42 AM, edward.ming.lee@gmail.com wrote:

On Saturday, August 1, 2020 at 10:06:34 AM UTC-7, Rob wrote:
Don Y <blockedofcourse@foo.invalid> wrote:
On 8/1/2020 4:55 AM, Rob wrote:
However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

You can ALWAYS isolate the supplies from ground/earth/neutral.
The question is one of how costly it is to do so (e.g., insert
an isolation transformer into the mains feed of one -- or both).

That would not be relevant for the usual DC supply which is already
isolated mains-to-DC to begin with. And even after installing your
isolation transformer it would still have the presumed DC-to-case ground
connection that has to be broken.

Even with DC-case ground, fine as long as hot and neutral don\'t. As long as you don\'t hook up cases together.

Exactly. It\'s not hard to ensure two cases don\'t touch.
\"Cannot be made isolated\" just doesn\'t apply.

\"Earthed\" supplies can be isolated by removing the \"earth\"
connection in the power inlet.

In the case where neutral is tied to an output, the xformer
does the trick (usually, the output is only tied to neutral
via the \"earth\" connection -- back at the fuse box)

[and, in practical terms, you can almost always remove a conductive
mounting fastener or cut some lands to isolate from case]

The bigger problem connecting supplies like this is ensuring the
supplies can handle the possibility of current flowing INTO
their outputs. E.g., highly reactive loads (cuz most higher
voltage DC applications tend to not be purely resistive -- motors,
etc.)

 

[Rob]

edward.ming.lee@gmail.com <edward.ming.lee@gmail.com> wrote:

On Saturday, August 1, 2020 at 10:06:34 AM UTC-7, Rob wrote:
Don Y <blockedofcourse@foo.invalid> wrote:
On 8/1/2020 4:55 AM, Rob wrote:
However, again, all of it is irrelevant when the supplies cannot be
made isolated to ground.

You can ALWAYS isolate the supplies from ground/earth/neutral.
The question is one of how costly it is to do so (e.g., insert
an isolation transformer into the mains feed of one -- or both).

That would not be relevant for the usual DC supply which is already
isolated mains-to-DC to begin with. And even after installing your
isolation transformer it would still have the presumed DC-to-case ground
connection that has to be broken.

Even with DC-case ground, fine as long as hot and neutral don\'t. As long as you don\'t hook up cases together.

You would have problems mounting the cases and it would also cause a
short when the mains input has a safety ground connected to the case.

 

[Rob]

Don Y <blockedofcourse@foo.invalid> wrote:

The bigger problem connecting supplies like this is ensuring the
supplies can handle the possibility of current flowing INTO
their outputs. E.g., highly reactive loads (cuz most higher
voltage DC applications tend to not be purely resistive -- motors,
etc.)

The type of load has not even been mentioned, so the poster would
first have to inform us about that.