G
Gerhard v d Berg
Guest
Multi-rail DC supply from a Wall-wart
Warning - this circuit requires a little bit of electronic
knowledge and some soldering skills. Please ask a knowledgeable
person to help if you do not understand the circuit or risks as
mains voltage circuitry is involved.
The circuit provides a 'high' current output at a nominal voltage
as well as 'low' current auxiliary supplies at minus and at double
the nominal output voltage (if only one auxiliary supply is
required, the components for the other can be left off).
The 'low' current supplies have limited capability due to the
half wave charge pump technique employed (50-100mA). The main supply
should be limited to 60% of Wall-wart's rating as Wall-warts are
normally heavily over rated.
The power supply circuit normally forms part of the user's circuit.
This supply was quite popular in the late 70's. From a single
transformer winding it provided 5v DC for a microprocessor circuit
and +- supplies for analogue interface circuits and/or an RS-232
comms buffers (3-terminal regulators were used to regulate the outputs).
Start by buying one of the 4.5v to 12v DC switchable Wall-warts (<$10)
as it simplifies the selection of the correct output voltage(s).
Also ensure that the Wall-wart's case can be opened as it requires
a small internal rewire change (this might be illegal in some countries).
Next open the Wall-wart's plastic casing.
( WARNING === WARNING === WARNING )
( DO NOT PLUG WALL-WART INTO MAINS SOCKET WHILE IT IS OPEN !!!! )
( MAINS VOLTAGE IS PRESENT INSIDE WALL-WART WHILE IT IS PLUGGED )
( INTO A MAINS SOCKET !! = MAINS EASILY BLOWS HUMAN FUSE !! =
( The human fuse has proved to be very difficult to repair - and )
( replacement through cloning, is still in an experimental phase )
Unsolder DC output wires across Wall-wart's smoothing capacitor
(normally 1000uF 25v or 16v) and wire it directly across suitable
secondary transformer windings. Select windings to provide a
suitable AC (Alternating Current) voltage on output leads to
generate the most appropriate DC output levels for the circuit below.
(It is an AC voltage, so the polarity of leads is not an issue).
I prefer to wire the output permanently to the correct transformer
windings and not to the output of the voltage selction switch.
It averts the problem of people that adjusts the switch and select
a too low or high an output voltage.
Close the Wall-wart and as a pre-emptive measure
LABEL THE WALL-WART AS:
= NON STANDARD =
= AC OUTPUT =
Plugging a DC Wall-wart output into the circuit blow will do no
harm (provided the voltages are not to high) but the charge-pump,
auxiliary supplies will not function.
First test the AC output voltage with a suitable AC voltage meter.
Connect the output to the multi-rail DC supply circuit below
and test the DC output voltages (if necessary select a more
suitable transformer secondary winding tap).
All diodes are 1N400X (nominally 1N4001)
C1, C2, C3 and C5 should be 200uF or larger depending on the
output current and minimum ripple requirements of the
-V and +2V supplies.
C4 should be 2000uF or larger depending on current and minimum
ripple requirements.
C1 and C3 should have a voltage rating of 2X V +50%
All other capacitors should have a voltage rating of V +50%.
Regulators with suitable heatsinks can be used if regulated output
voltages are required. Low dropout regulators, large capacitors
and suitable transformer output voltage selection can minimise
heatsink requirements.
. Multi-rail DC supply
. C1 || +
. ,----||--------o------------o---------- +2V
Wall-wart . | || | |
. | D1 | |
. | +---|>|---' | +
. | | =====
. | | | C3
. | | D3 |
------- ---->>--o----|------o---|>|----o----|----o----- +V
)( . | | | D4 | | |
AC )( . | | |--|---|>|----' | | +
)( . | | | | D5 | =====
Mains )( . | | | '---|<|----| | | C4
)( . | | | D6 | | |
------- ---->>--|----o---o------|<|----o----o----o--o-- 0v
. | | | |
. | | | + GND
. | | D2 =====
. '----|---|<|---| | C5
. | | |
. | + || | |
. +----||---o------------o---------- -V
. C2 ||
.
This circuit can provide a +-12 volt supply (not +-15 as requested
and is a bit asymmetrical in capability), but it offers a simple and
inexpensive multi-voltage supply solution suitable for many circuits
that require more than one supply rail. You can use more than
one Wall-wart but it tends to be clumsy at best.
Gerhard van den Berg
CSIR
gvdberg@istccsir.co.za (drop the istc part)
2004-11-09
Warning - this circuit requires a little bit of electronic
knowledge and some soldering skills. Please ask a knowledgeable
person to help if you do not understand the circuit or risks as
mains voltage circuitry is involved.
The circuit provides a 'high' current output at a nominal voltage
as well as 'low' current auxiliary supplies at minus and at double
the nominal output voltage (if only one auxiliary supply is
required, the components for the other can be left off).
The 'low' current supplies have limited capability due to the
half wave charge pump technique employed (50-100mA). The main supply
should be limited to 60% of Wall-wart's rating as Wall-warts are
normally heavily over rated.
The power supply circuit normally forms part of the user's circuit.
This supply was quite popular in the late 70's. From a single
transformer winding it provided 5v DC for a microprocessor circuit
and +- supplies for analogue interface circuits and/or an RS-232
comms buffers (3-terminal regulators were used to regulate the outputs).
Start by buying one of the 4.5v to 12v DC switchable Wall-warts (<$10)
as it simplifies the selection of the correct output voltage(s).
Also ensure that the Wall-wart's case can be opened as it requires
a small internal rewire change (this might be illegal in some countries).
Next open the Wall-wart's plastic casing.
( WARNING === WARNING === WARNING )
( DO NOT PLUG WALL-WART INTO MAINS SOCKET WHILE IT IS OPEN !!!! )
( MAINS VOLTAGE IS PRESENT INSIDE WALL-WART WHILE IT IS PLUGGED )
( INTO A MAINS SOCKET !! = MAINS EASILY BLOWS HUMAN FUSE !! =
( The human fuse has proved to be very difficult to repair - and )
( replacement through cloning, is still in an experimental phase )
Unsolder DC output wires across Wall-wart's smoothing capacitor
(normally 1000uF 25v or 16v) and wire it directly across suitable
secondary transformer windings. Select windings to provide a
suitable AC (Alternating Current) voltage on output leads to
generate the most appropriate DC output levels for the circuit below.
(It is an AC voltage, so the polarity of leads is not an issue).
I prefer to wire the output permanently to the correct transformer
windings and not to the output of the voltage selction switch.
It averts the problem of people that adjusts the switch and select
a too low or high an output voltage.
Close the Wall-wart and as a pre-emptive measure
LABEL THE WALL-WART AS:
= NON STANDARD =
= AC OUTPUT =
Plugging a DC Wall-wart output into the circuit blow will do no
harm (provided the voltages are not to high) but the charge-pump,
auxiliary supplies will not function.
First test the AC output voltage with a suitable AC voltage meter.
Connect the output to the multi-rail DC supply circuit below
and test the DC output voltages (if necessary select a more
suitable transformer secondary winding tap).
All diodes are 1N400X (nominally 1N4001)
C1, C2, C3 and C5 should be 200uF or larger depending on the
output current and minimum ripple requirements of the
-V and +2V supplies.
C4 should be 2000uF or larger depending on current and minimum
ripple requirements.
C1 and C3 should have a voltage rating of 2X V +50%
All other capacitors should have a voltage rating of V +50%.
Regulators with suitable heatsinks can be used if regulated output
voltages are required. Low dropout regulators, large capacitors
and suitable transformer output voltage selection can minimise
heatsink requirements.
. Multi-rail DC supply
. C1 || +
. ,----||--------o------------o---------- +2V
Wall-wart . | || | |
. | D1 | |
. | +---|>|---' | +
. | | =====
. | | | C3
. | | D3 |
------- ---->>--o----|------o---|>|----o----|----o----- +V
)( . | | | D4 | | |
AC )( . | | |--|---|>|----' | | +
)( . | | | | D5 | =====
Mains )( . | | | '---|<|----| | | C4
)( . | | | D6 | | |
------- ---->>--|----o---o------|<|----o----o----o--o-- 0v
. | | | |
. | | | + GND
. | | D2 =====
. '----|---|<|---| | C5
. | | |
. | + || | |
. +----||---o------------o---------- -V
. C2 ||
.
This circuit can provide a +-12 volt supply (not +-15 as requested
and is a bit asymmetrical in capability), but it offers a simple and
inexpensive multi-voltage supply solution suitable for many circuits
that require more than one supply rail. You can use more than
one Wall-wart but it tends to be clumsy at best.
Gerhard van den Berg
CSIR
gvdberg@istccsir.co.za (drop the istc part)
2004-11-09