voltage version — LM-317H — that
can go to 60 V). Note that this is
technically the voltage differential
between input and output, not
from input to ground. If the output is
maintained above ground, then the
input can be higher.
However, for a general-purpose
power supply you must assume the
output will be shorted to ground from
time to time. This means that the input
voltage must be limited to about 37 V
because there will be about 1.4 V
dropped by the bridge diode and
an additional 2. 5 V dropped by the
Also, there is no reason to provide
two amps of primary DC current if the
maximum output is rated at one amp.
(Note that the rating is less than the
absolute maximum the circuit can provide.) The third reason is for regulator
heat control. This is also the reason
for the added switch which will be
discussed in more detail below.
The main filter capacitor has been
reduced from 10,000 μF to 4,700 μF
because the power supply current
specification has been reduced from
two amps to one amp.
The adjustable current regulator is just three parts: an LM-317, a
fixed resistor, and a variable resistor.
The fixed resistor is necessary to limit
the current through the resistor string.
Obviously, as the resistance gets
lower, more current flows. With a 1.25
V differential between the output and
adjustment pin, the one ohm fixed
resistor limits the current to 1.25 A.
If this resistor was eliminated and
the adjustable resistor was set to 0.1 Ω
for example, the current would be
(theoretically) 12. 5 A (using Ohm’s
Law). This would either destroy the IC
or cause it to shut down from thermal
overload. Neither is good.
Also, the power through that
small resistance would be 15. 6 W
(power is equal to current squared
times resistance). This would burn
the variable resistor. By forcing a
minimum of 1.0 Ω in the circuit,
the current is limited to a maximum
1.25 A and the variable resistor is
The worst case power dissipated
by the fixed resistor is 1.25 W. A
three watt resistor (rather than a
wo watt device) is chosen to be
conservative. The worst case power
requirement of the variable resistor
occurs when it is equal to the fixed
resistor or 1.0 Ω.
In this instance, there will be
about 0.4 W of power through it. This
means that a one watt potentiometer
is adequate, but a two watt unit will
be used to be very conservative.
Remember, it’s better to spend a little
bit extra on the power supply than to
spend a lot on ruined circuits.
The voltage regulator section is
simply the design from the beginning
of this article. The only change is to
use a variable resistor (R6) instead of a
fixed resistor (Radj).
Heatsinking is an important issue.
The voltage regulator must be able to
dissipate up to 32 W of power, worst
case. This creates a significant
problem. The typical TO-220 case is
limited to 15 W as per the spec sheet.
One poor solution is to reduce the
maximum power rating to about 0.5 A
for lower voltages. This is changing the
specs to match the design instead of
designing to match the specs. There is
another approach that requires a bit
of creative engineering that will be
Curiously, the current regulator is
very efficient. The voltage drop is
typically 2. 5 V (worst case) so the
maximum power dissipated at one
amp is 2. 5 W. A simple and inexpensive clip-on heatsink is sufficient here.
Fixing the Heat Problem
As noted, there is too much heat
to be dissipated at low voltages. The
problem comes from the large voltage
drop from the input to the output of
the regulator. The solution is to use
a center-tapped transformer and
switch between taps. Center-tapped
transformers are common and
generally cost no more than non
When the switch is connected to
the top end of the transformer, the full
voltage is applied to the rest of the
circuit and the full range of voltages is
possible. As long as the current draw
is kept below about 0.5 A, there will
be no heat problem. However, if the
power supply needs to provide a high
current at a low voltage, the switch is
used to connect to the center-tap and
reduces the input voltage by 50%.
Thus, only about 18 volts is applied to
the rest of the power supply.
So, if five volts at one amp is
needed, then only 13 W of power
must be dissipated by the regulator as
heat (instead of about 30 W). This
allows the standard TO-220 style regulator to be used within the specified
limits. (If you want to get fancy,
you could monitor the adjustment
voltage and use a separate circuit to
automatically switch between the
Fixing the Minimum
1.2 Volt Problem
Most people want a power supply
to go all the way to ground. It turns
out that there is an easy way to
accomplish this: just add two diodes
in series with the output. Each diode
drops the voltage by about 0.7 V
(depending on load). So, two diodes
will reduce the output by 1.4 V. In
actual operation, only about 1.0 V is
dropped at no load. This makes the
minimum output no load voltage 0.2
V. This is virtually zero.
If you really want to, you can
add another diode in series to drop
the output further, but it seems
unnecessary. The resistor to ground is
included to drain off any leakage
through the diodes. Without the
resistor, the output can float up to the
voltage applied to the diodes.
It should be noted that you can
pull the output of the regulator to
ground if you apply a negative voltage
to the adjustment pin. This is somewhat complicated if a negative voltage
is not easily available (as in this case).
The addition of two diodes is a quick
and easy fix.
Minimum Current Discussion
As noted on the schematic
shown in Figure 4, the minimum
current limit is about 5 mA. For
virtually all applications, this is not a
problem. Very few components will
January 2008 63