Download Regulated Power Supplies

Regulated Power Supplies
The next step in developing our power supply is to turn the pulsating DC,
unregulated, in to a specific, well regulated, current limited voltage. Most logic circuits
run on +5 Volts. This must be within 5% of 5 Volts under varying load conditions.
Unregulated
If we measured the output of a power supply that just had rectified AC, we would
find that how much voltage we measured changed with how much current we drew from
the supply. As we draw more current from the transformer, we lose voltage in the
windings of the transformer, itself. At no load, we may have 10 V out. At half the rated
current it could supply we may get 9 V out. At full rated current we may only get 8 V out.
This is typical of an unregulated supply. In real circuits, as various parts of the circuit
work, it draws different current from the power supply. An unregulated supply would
change output voltages. This is not acceptable for most logic circuits.
In order to get a stable, predictable, output voltage under any predictable current
level we need to add a regulator. In today’s circuits, this is usually a simple three-leaded
component. The voltage regulator puts out a specific voltage over a certain range of
operating current.
The LM7805T is a very popular device in the gaming industry. It provides a
regulated +5 Volts at any current level from about 0.050 Amps to over 1 Amp. The part
number can be broken down to make sense of the device. “LM” is the manufacturer’s
prefix for analog devices, and is an industry standard. Other manufacturers may use
another prefix. The 78xx family of devices are all positive voltage regulators. 7805 is a 5
Volt device. 7812 is a 12 volt device. 7815 is a 15 volt device. 7824 is a 24 volt device.
The T suffix indicated the case the device is built in, and therefore the current that we
may draw from the regulator. T indicates a TO-220 device, rated at 1 Amp. “K” indicates
a TO-3, rated at 1.5 Amp. “L” indicates a TO-92 device, rated at 100 mA, (0.100 Amp).
We can read the part number and tell what the device does, and its ratings.
The 79xx family is a similar device made for negative voltages.
These devices provide a regulated output voltage, as well as current limiting. If
we try to draw more than the rated current value the device shuts itself off, preventing
circuit damage and blown fuses. An excessive current drain just turns the device off.
The internal operations of these devices will be demonstrated as we get into
electronic components.