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Adjustable 1.3-22V
Regulated Power Supply
Want a regulated voltage that can be adjusted
to suit your application? This Adjustable
Power Supply is small, easy to build and can
be adapted to produce a fully regulated voltage
ranging from 1.3V to 22V at currents up to 1A.
By JOHN CLARKE
T
HERE ARE MANY fixed-voltage
IC regulators available and these
can be had with 5V, 6V 8V, 9V, 12V &
15V outputs. But what if you want a
voltage output that does not fit into one
of the standard ranges or if you want
to be able to easily adjust this output
voltage? An adjustable regulator is the
answer – one that can be set to provide
the exact voltage you require.
This Adjustable Power Supply comprises a small PC board that utilises a
3-terminal regulator. It does not have
too many other components – in fact,
there are just three diodes, three capacitors, a resistor and a trimpot to set
the output voltage from the regulator.
Circuit details
Fig.1 shows the circuit details. REG1
is an LM317T adjustable regulator that
provides a nominal 1.25V between its
OUT and ADJ (adjust) terminals.
We say it is a “nominal 1.25V”
because, depending on the device,
it can be anywhere between 1.2V
and 1.3V. This doesn’t really matter
though, because we can adjust the
output voltage to the required level
using the trimpot.
Note: if you do want a regulator
that provides a better tolerance for
the 1.25V reference, then you could
use an LD1117V instead. This has a
1.238-1.262V range. However, do not
46 Silicon Chip
apply more than 15V to the input of
this regulator.
The output voltage from REG1 is
set by the 110W resistor (R1) between
the OUT and ADJ terminals and by the
resistance between the ADJ terminal
and ground. This works as follows.
By using a 110W resistor and assuming an exact 1.25V reference, the
current flow is set at 11.36mA. This
is calculated by dividing the voltage
between the OUT and ADJ terminals
(1.25V) by the 110W resistor. This current also flows through trimpot VR1.
This means that if VR1 is say 1kW,
then the voltage across this resistor
will be 1kW x 11.36mA or 11.36V.
This voltage is then added to the 1.25V
Parts List
1 PC board, code 10105071,
35 x 38mm
1 LM317T adjustable 3-terminal
regulator (REG1)
3 1N4004 1A diodes (D1-D3)
2 100mF 25V PC electrolytic
capacitors (C1,C3)
1 10mF 25V PC electrolytic capacitor (C2)
1 110W 0.25W 1% resistor (R1)
1 2kW horizontal trimpot (VR1)
4 PC stakes
reference to derive the output voltage
– in this case 12.61V.
In practice, however, the current
flow out of the ADJ terminal also contributes slightly to the final output
voltage. This current is of the order
of 100mA. So if VR1 is set to 1kW, this
can add 0.1V to the output – ie, we
get 12.71V.
If you are interested in the output
voltage equation, then it is:
VOUT = VREF(1 + R1/R2) + IADJ x R2
where VOUT is the output voltage,
VREF is the voltage between the OUT
and ADJ terminals, and IADJ is the
current out of the ADJ terminal (typically 50mA but as high as 100mA). R1
is the resistance between the OUT
and ADJ terminals, while R2 is the
resistance between the ADJ terminal
and ground.
Diode D1 in series with the input
provide reverse polarity protection.
This means that if you connect the
supply voltage around the wrong way,
you cannot do any damage.
Diode D2 protects the regulator
should the input become shorted to
ground. If that happens, D2 becomes
forward biased and conducts, effectively preventing any reverse current
flow through REG1 which could cause
damage.
D3 is also included to protect REG1.
siliconchip.com.au
Fig.1: the circuit is based on an LM317T adjustable voltage regulator. D1
provides reverse polarity protection while VR1 sets the output voltage.
Fig.2: here’s how to install the
parts on the PC board.
It does this by clamping the voltage
between the ADJ terminal and the OUT
& IN terminals in the event that one of
the latter is shorted to ground.
Finally, capacitors C1 & C2 reduce
ripple by bypassing the IN (input) and
ADJ terminals respectively. C3 prevents regulator oscillation by swamping any low-value capacitance that may
be connected to this output.
Construction
All parts for the Adjustable Power
Supply are mounted on a PC board
coded 10105071 and measuring 35 x
38mm. Fig.2 shows the parts layout.
As usual, begin by checking the PC
board for any shorts between tracks or
open circuits and make any necessary
repairs. It’s rare to find a board defect
these days but it’s easier to find any
problems that might exist now, before
any parts are mounted.
siliconchip.com.au
Fig.3: regulator REG1 can be mounted
underneath the PC board and attached
to a heatsink as shown here. Note that
its metal tab must be isolated from
the heatsink using a TO-220 silicone
washer and a Nylon screw.
You can now begin the assembly by
installing the 110W resistor (R1) and
the three diodes, making sure the latter
are all oriented correctly (the banded
ends are the cathodes). That done, capacitors C1-C3 can be installed, again
taking care with their orientation since
they are all electrolytics.
Next, install PC stakes for the IN,
OUT & GND terminals, then install
trimpot VR1. REG1 can then be
mounted. It can either be mounted on
the top of the PC board (as shown in
the photo) or underneath it as shown
in Fig.3, so that it can be fastened to
a heatsink.
Heatsinking stuff
Whether or not you need a heatsink
for REG1 depends on the output current and the voltage between the IN
and OUT terminals of the regulator.
That’s because these two values togethMay 2007 47
er determine the power dissipation
within the regulator. It’s determined
simply by multiplying the two values
together to get the power dissipation
in watts – ie, P = VI.
Generally, if the dissipation is
less than 0.25W, no heatsink will be
required. For example, if the current
drawn from the regulator is 50mA and
the voltage between the IN and OUT
terminals is 5V, then the dissipation
will be 0.25W and no heatsink will
be necessary.
However, if the dissipation is more
than this , you will need to fasten the
regulator to a heatsink to keep it cool.
For example, let’s say that the current
drawn from regulator REG1 is 250mA
and that the voltage across it is 5V. In
this case, the dissipation will be 1.25W
(ie, 5 x 0.25) and a heatsink will be
necessary.
The type of heatsink required depends on the wattage dissipated by
the regulator and the temperature
rise that can be tolerated. Typically,
a 20°C rise in heatsink temperature
is OK because this means that at a
typical room temperature of say 25°C,
the heatsink will run at 45°C which is
quite acceptable.
Most heatsinks are specified by their
temperature rise in °C per watt (°C/W).
This means that a 10°C/W heatsink
will rise 20°C above ambient when
dissipating 2W.
Note that the LM317T TO-220 package is rated at 15W maximum dissipation.
Usually, it will be necessary to electrically isolate the tab of the regulator
Fig.4: this is the full-size etching
pattern for the PC board. Check
your board for defects before
mounting any of the parts.
from the heatsink – see Fig.3. The reason
for this is that the heatsink may be connected to ground, while the regulator
tab sits at the output voltage.
To isolate the tab, use a TO-220 silicone insulating washer and secure the
assembly to the heatsink using an M3
Nylon screw and nut. Alternatively,
you can use a metal screw provided
you fit an insulating bush into the
regulator tab.
Note that capacitor C1 may need
to be increased in value if the input
voltage has a lot of ripple. In addition,
you should make sure that the input
voltage does not go above C1’s 25V
rating. Increase C1’s voltage rating to
35V if it does.
In fact, you can apply up to 35V to
the input if C1 is a 35V type.
Adjusting the output
Note that the voltage applied to the
supply must be several volts higher
than the required output voltage. This
is necessary in order for the regulator
to provide regulation.
In practice, the minimum voltage
across REG1 required for regulation
is called the “dropout voltage”. For
the LM317T, this voltage varies with
the current and is typically 1.5V for
currents below 200mA, rising to 1.7V
at 500mA and 2V at 1A.
Note that the drop across diode D1
must be added to the dropout voltage in order to calculate the required
input voltage. For example, if our
power supply draws 200mA and the
required output voltage is 6V, then the
input voltage must be 6V plus 0.7V (to
compensate for voltage across D1) plus
1.5V (for the dropout voltage) – ie, the
input voltage must be 2.2V higher than
the output voltage.
Therefore, we need to apply 8.2V
minimum to the input for regulation.
This is the absolute minimum and to
ensure correct regulation under varying loads, a 9V input to the supply
would be ideal. Note also that any
ripple on the input supply that drops
below the required voltage will cause
problems, since the supply will not
be regulated during these low-going
excursions.
Once you’ve connected the supply,
it’s just a matter of adjusting trimpot
VR1 to set the required output voltage.
Finally, note that in some applications,
you might want to replace VR1 with
a fixed resistor (eg, if VR1’s setting is
close to a standard fixed value). This
has been catered for on the PC board
– just replace VR1 with resistor R2
SC
(shown dotted).
Reliable • Simple
2007
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