11
Capacitors
For this power supply circuit the capacitor has two main roles and one side effect. The first role
for the capacitor is to dampen the voltage and remove any ripple leftover. The second role for the
capacitor is to act as a decoupling capacitor to remove noise from another circuit element by acting
as a shunt and reducing the effects throughout the entire circuit. The issue that is primarily derived
from the capacitor is equivalent series resistance. The capacitors resistances comes from the leads,
furthermore it generates transient spikes that could damage your components which are current
sensitive at the output of the power supply. To remedy this problem a serial resistor can be used or
a protective parallel resistor.
The capacitors that are included in this kit are radial electrolytic with their voltage ratings. The
orientation for the capacitors are important as the wrong orientation could cause the capacitors
to exploded or leak. The composition of the polarized capacitor allows for an affordable and
inexpensive material, however do the design method the anode film and cathode film cannot be
reversely charged or else it could burn the film.
Voltage Regulator
Linear Voltage Regulators are used commonly in many power regulating circuits in the form of a
T0-220 package. A liner voltage regulator is responsible for supplying a constant output voltage
regardless of the change in input to the IC. The circuitry inside the Regulator attempts to hold
the value set by the users demand or the adjust pin. The maximum current in the input is the
maximum current that can be outputted. Essentially the system will act as a proportional, integral
and differential system (PID). When the output voltage is higher or lower than demand, the current
will adjust to meet the output required. If you are scratching your head trying to understand how
this is done, recall ohm’s law V=I*R. Increase the current and keep the same resistance, voltage
goes up. Decrease current and keep the same resistance the voltage does down.
Due to this constant regulation of voltage if a load is introduce at the output that is active and
requires current it gets harder for the regulator to do its job, especially when the voltage output
requested is low. To remedy that, a heat sink is introduced to cool the circuitry within the packaging;
aluminum is a good thermal conductor and that why you shouldn’t touch the silver back side. This
allows users to attach heat sink that are screwed onto the back of the IC or in this case clipped on.
The LM317 like any device has its limits; the maximum it can regulate is 37 Volts. The maximum
current it can handle is 1.5 amps. If you are within these operating temperatures and forgot to
place a heat sink on the regulator the chances that it might work again after cooling down is very
likely. The LM317 has a protective circuity to shut down the regulator if the temperature reaches a
very high point, that around 125°C! You can safely dissipate 0.25 watts with the LM317.
Here is the formula for finding your thermal dissipation.
Dissipated power (watts) = (Vin-Vout) * iL (Load current)
So the power supply outputting 1 volts would be (18v – 1v) * 1 Amp = 17 watts. THAT IS A LOT!
You may be wondering, when I measured the output of my power supply the maximum current
of 5 led(s) at the output was a total of 0.15 amps not 1 amp! Don’t panic, this is perfectly normal.
You will never exceed 0.5 Amps, since the regulator is responsible for regulating it only outputs
the current you need for the voltage you want and thus you may end up with a lower current.
The power supply is designed to handle 1 amp so if you did do a future project that required such
amperage, please place an appropriate heat sink.
Summary of Contents for AK-10
Page 16: ......