Dual supply using 2 transformwers

J

John

Guest
Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 / LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the shelf or
using PIC, I could use a little guidance concerning how to power them e.g.
what issue can I expect as the meters will have common supply/ground with
the PSU proper and each other, what if I use a third with a shunt for
current.
Thanks.
 
John wrote:
Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 / LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the shelf or
using PIC, I could use a little guidance concerning how to power them e.g.
what issue can I expect as the meters will have common supply/ground with
the PSU proper and each other, what if I use a third with a shunt for
current.
Thanks.
Click to expand...
You cannot connect the secondaries of two transformers in series to
form a single center tapped winding to be rectified into a pair of
stacked DC supplies. This is because each of those half windings
carry DC adn this cancels only if both windings are on a single core.
A better way to use your two transformers is to use a full wave bridge
rectifier on each full secondary, and then connect the outputs of
those two bridges in series to form a positive and negative raw
supply.

--
John Popelish
 
John Fields wrote:
On Sat, 24 Jan 2004 18:35:14 -0500, John Popelish <jpopelish@rica.net
wrote:

John wrote:

Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 / LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the shelf or
using PIC, I could use a little guidance concerning how to power them e.g.
what issue can I expect as the meters will have common supply/ground with
the PSU proper and each other, what if I use a third with a shunt for
current.
Thanks.

You cannot connect the secondaries of two transformers in series to
form a single center tapped winding to be rectified into a pair of
stacked DC supplies. This is because each of those half windings
carry DC adn this cancels only if both windings are on a single core.
A better way to use your two transformers is to use a full wave bridge
rectifier on each full secondary, and then connect the outputs of
those two bridges in series to form a positive and negative raw
supply.

---
Are you sure, John?

Although the argument could be made that they're not really in series,
they're not really in parallel either and with the flux reversing in
both secondaries (since there are two oppositely wired half-wave
rectifiers in each one) It seems like this would work:

ACIN>-------- ----+--->|-------+--->+OUT
)||( | |
)||( +---|<---+------->-OUT
)||( | |
)||( | |
ACIN>-------- ----+--[+C]--+ |
| | |
+---------------->COM
| | |
ACIN>-------- ----+--[C+]------+
)||( | |
)||( | |
)||( +---|<---+ |
)||( | |
ACIN>-------- ----+--->|-------+
Click to expand...
The bridge rectifier version is certainly better than the 2 diode
rectifier version, but there is still a chance of DC in the cores if
the load is not connected only across the positive and negative bus.
Any load that flows from either bus to common produces DC current in
both transformers. I also doubt that the two transformers will share
the load properly if they are not identical transformers.

--
John Popelish
 
On Sat, 24 Jan 2004 18:35:14 -0500, John Popelish <jpopelish@rica.net>
wrote:

John wrote:

Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 / LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the shelf or
using PIC, I could use a little guidance concerning how to power them e.g.
what issue can I expect as the meters will have common supply/ground with
the PSU proper and each other, what if I use a third with a shunt for
current.
Thanks.

You cannot connect the secondaries of two transformers in series to
form a single center tapped winding to be rectified into a pair of
stacked DC supplies. This is because each of those half windings
carry DC adn this cancels only if both windings are on a single core.
A better way to use your two transformers is to use a full wave bridge
rectifier on each full secondary, and then connect the outputs of
those two bridges in series to form a positive and negative raw
supply.
Click to expand...
---
Are you sure, John?

Although the argument could be made that they're not really in series,
they're not really in parallel either and with the flux reversing in
both secondaries (since there are two oppositely wired half-wave
rectifiers in each one) It seems like this would work:


ACIN>-------- ----+--->|-------+--->+OUT
)||( | |
)||( +---|<---+------->-OUT
)||( | |
)||( | |
ACIN>-------- ----+--[+C]--+ |
| | |
+---------------->COM
| | |
ACIN>-------- ----+--[C+]------+
)||( | |
)||( | |
)||( +---|<---+ |
)||( | |
ACIN>-------- ----+--->|-------+
 
Hi Guys.
Thanks, shame, it would have been good to put them to good use. As it
happens I have also come across a pair of 12v 500VA transformers as well,
never rains etc....

"John Popelish" <jpopelish@rica.net> wrote in message
news:40131A57.7917D87@rica.net...
John Fields wrote:

On Sat, 24 Jan 2004 18:35:14 -0500, John Popelish <jpopelish@rica.net
wrote:

John wrote:

Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I
connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 /
LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the
shelf or
using PIC, I could use a little guidance concerning how to power them
e.g.
what issue can I expect as the meters will have common supply/ground
with
the PSU proper and each other, what if I use a third with a shunt for
current.
Thanks.

You cannot connect the secondaries of two transformers in series to
form a single center tapped winding to be rectified into a pair of
stacked DC supplies. This is because each of those half windings
carry DC adn this cancels only if both windings are on a single core.
A better way to use your two transformers is to use a full wave bridge
rectifier on each full secondary, and then connect the outputs of
those two bridges in series to form a positive and negative raw
supply.

---
Are you sure, John?

Although the argument could be made that they're not really in series,
they're not really in parallel either and with the flux reversing in
both secondaries (since there are two oppositely wired half-wave
rectifiers in each one) It seems like this would work:

ACIN>-------- ----+--->|-------+--->+OUT
)||( | |
)||( +---|<---+------->-OUT
)||( | |
)||( | |
ACIN>-------- ----+--[+C]--+ |
| | |
+---------------->COM
| | |
ACIN>-------- ----+--[C+]------+
)||( | |
)||( | |
)||( +---|<---+ |
)||( | |
ACIN>-------- ----+--->|-------+

The bridge rectifier version is certainly better than the 2 diode
rectifier version, but there is still a chance of DC in the cores if
the load is not connected only across the positive and negative bus.
Any load that flows from either bus to common produces DC current in
both transformers. I also doubt that the two transformers will share
the load properly if they are not identical transformers.

--
John Popelish
Click to expand...
 
John wrote:
Hi Guys.
Thanks, shame, it would have been good to put them to good use. As it
happens I have also come across a pair of 12v 500VA transformers as well,
never rains etc....
Click to expand...
You can certainly use them by the method I first described with only
an extra bridge rectifier. Not much extra cost compared to the
transformers.

--
John Popelish
 
Sorry guys, lost the tail end of the thread, can you explain to me what
effect the DC portion that you refer to is and how it gets into the core.
Am going to try the two transformers, and use two bridge rectifiers in
series and see what happens with a load! what should I be looking for if
there is an issue.

Another question I have a transformer with a 115/230 primary 15/30 secondary
(210VA), what would happen to the secondary voltage if I used the 115
primary at 230V and what would be the effect on the VA rating.
Thanks.


"John" <reeferjon@msn.com> wrote in message
news:wUVQb.4$BZ1.1@news-binary.blueyonder.co.uk...
Hi Guys.
Thanks, shame, it would have been good to put them to good use. As it
happens I have also come across a pair of 12v 500VA transformers as well,
never rains etc....

"John Popelish" <jpopelish@rica.net> wrote in message
news:40131A57.7917D87@rica.net...
John Fields wrote:

On Sat, 24 Jan 2004 18:35:14 -0500, John Popelish <jpopelish@rica.net
wrote:

John wrote:

Hi.
I have two 12v-0v-12v one 230VA other 240VA transformers, can I
connect the
two transformers in series to give me a 24v-0v-24v supply (24v from
12v-0v-12v). Then, feed these via rectifier to well sunk LM317 /
LM337T's.
Would I have any issues.
I would also like to add a couple of digital meters, either off the
shelf or
using PIC, I could use a little guidance concerning how to power
them
e.g.
what issue can I expect as the meters will have common
supply/ground
with
the PSU proper and each other, what if I use a third with a shunt
for
current.
Thanks.

You cannot connect the secondaries of two transformers in series to
form a single center tapped winding to be rectified into a pair of
stacked DC supplies. This is because each of those half windings
carry DC adn this cancels only if both windings are on a single core.
A better way to use your two transformers is to use a full wave
bridge
rectifier on each full secondary, and then connect the outputs of
those two bridges in series to form a positive and negative raw
supply.

---
Are you sure, John?

Although the argument could be made that they're not really in series,
they're not really in parallel either and with the flux reversing in
both secondaries (since there are two oppositely wired half-wave
rectifiers in each one) It seems like this would work:

ACIN>-------- ----+--->|-------+--->+OUT
)||( | |
)||( +---|<---+------->-OUT
)||( | |
)||( | |
ACIN>-------- ----+--[+C]--+ |
| | |
+---------------->COM
| | |
ACIN>-------- ----+--[C+]------+
)||( | |
)||( | |
)||( +---|<---+ |
)||( | |
ACIN>-------- ----+--->|-------+

The bridge rectifier version is certainly better than the 2 diode
rectifier version, but there is still a chance of DC in the cores if
the load is not connected only across the positive and negative bus.
Any load that flows from either bus to common produces DC current in
both transformers. I also doubt that the two transformers will share
the load properly if they are not identical transformers.

--
John Popelish
Click to expand...
 
John wrote:
Sorry guys, lost the tail end of the thread, can you explain to me what
effect the DC portion that you refer to is and how it gets into the core.
Am going to try the two transformers, and use two bridge rectifiers in
series and see what happens with a load! what should I be looking for if
there is an issue.
Click to expand...
There is no issue with that arrangement.

The problem occurs if you connect two separate secondaries (from two
cores )in series as if they were a single center tapped winding, with
a single bridge to create a positive and negative supply. If you load
either output (or have mismatched loads) each secondary is effectively
feeding a half wave rectifier, passing DC through the winding, and
risking saturation on the core.

Another question I have a transformer with a 115/230 primary 15/30 secondary
(210VA), what would happen to the secondary voltage if I used the 115
primary at 230V and what would be the effect on the VA rating.
Thanks.
Click to expand...
If you apply 230 volts to a 115 volt winding the transformer catches
fire. If you connect 115 volts to a 230 volt winding, all secondaries
produce half of their rated voltage at rated current, so the power
rating of the transformer is effectively cut in half. But it will run
very cool due to the low core magnetization and produce much lower
(less than half) stray magnetic fields.

--
John Popelish
 
Thanks John, got it now, LOL.

"John Popelish" <jpopelish@rica.net> wrote in message
news:401990C8.78A59CD8@rica.net...
John wrote:

Sorry guys, lost the tail end of the thread, can you explain to me what
effect the DC portion that you refer to is and how it gets into the
core.
Am going to try the two transformers, and use two bridge rectifiers in
series and see what happens with a load! what should I be looking for if
there is an issue.

There is no issue with that arrangement.

The problem occurs if you connect two separate secondaries (from two
cores )in series as if they were a single center tapped winding, with
a single bridge to create a positive and negative supply. If you load
either output (or have mismatched loads) each secondary is effectively
feeding a half wave rectifier, passing DC through the winding, and
risking saturation on the core.

Another question I have a transformer with a 115/230 primary 15/30
secondary
(210VA), what would happen to the secondary voltage if I used the 115
primary at 230V and what would be the effect on the VA rating.
Thanks.

If you apply 230 volts to a 115 volt winding the transformer catches
fire. If you connect 115 volts to a 230 volt winding, all secondaries
produce half of their rated voltage at rated current, so the power
rating of the transformer is effectively cut in half. But it will run
very cool due to the low core magnetization and produce much lower
(less than half) stray magnetic fields.

--
John Popelish
Click to expand...
 
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