THERE
are many fi xed-voltage IC regulators available, and these can be had with 5V,
6V 8V, 9V, 12V and 15V outputs. But what if you want a voltage output that does
not fi t 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 voltage 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:
The
full circuit diagram for the Adjustable Regulated Power Supply is shown in
Fig.1. REG1 is an LM317T 1.5A adjustable voltage 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 apply more than 15V to the input of this regulator.
Output voltage:
The
output voltage from REG1 is set by the 110Ù resistor (R1) between the OUT
and ADJ terminals
and by the resistance between the ADJ terminal and ground (0V).
This works as follows. By using
a 110Ù resistor and assuming an exact 1.25V
reference, the current fl ow is set at 11.36mA. This is calculated by dividing
the voltage between
the OUT and
ADJ terminals (1.25V) by the 110Ù resistor. This current also fl ows
through trimpot VR1. This means that if VR1 is set at say 1kÙ, then the voltage across this
resistor will be 1kÙ x
11.36mA, or 11.36V.
This
voltage is then added to the 1.25V reference to derive the output voltage – in
this case 12.61V. In practice, however, the current fl ow out of the ADJ terminal also contributes slightly
to the fi nal output voltage. This
current
is of the order of 100ìA.
So, if VR1 is set to 1kÙ,
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 50ìA, but can be as high as 100ìA).
R1
is the resistance between the OUT and ADJ terminals,
while R2 is the resistance between the ADJ terminal and ground (0V).
Protection:
Diode
D1, in series with the input, provides 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.
Diode D3 is
also included to protect REG1. It does this by clamping the voltage between the
ADJ terminal and the OUT and IN terminals in the event that one of the latter is shorted to ground
(0V).
Finally,
capacitors C1 and C2 reduce ripple by bypassing the IN (input) and ADJ terminals respectively. Capacitor 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, code 698, measuring
just 35×38mm. This board is available from the EPE PCB Service. The circuit
board component layout is shown in Fig.2 and the PCB copper foil master in
Fig.4. As usual, begin construction by checking the PC board for any shorts
between copper tracks or open circuits and make any necessary repairs.
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 (K)). That done, capacitors C1 to C3 can be installed,
again
taking care with their orientation since they are all electrolytic types.
Next,
install PC stakes for the IN, OUT and GND terminals, then install trimpot VR1. Regulator REG1 can also be ounted. It can either be mounted on the top of
the PC board (as shown in the photo) or underneath, as shown in Fig.3, so that
it can be fastened to a heatsink.
Heatsinking:
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.
Heatsink temperature:
The type of
heatsink required depends on the amount of power dissipated by the regulator
and the temperature rise that can be tolerated. Typically, a 20°C rise in
heatsink temperature is acceptable because this means that at a typical room
temperature of say 25°C, the heatsink will run at 45°C which is quite
tolerable.
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 power dissipation.
Isolation:
Usually, it
will be necessary to electrically isolate the tab of the regulator from the
heatsink – see Fig.3. The reason for
this is
that the heatsink may be connected to ground (0V), while the regulator metal
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 fixing hole.
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 ‘working’ 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 volage 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 the 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 (shown
dotted).
