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-12 vdc Series Regulators 79xx

In the Voltage Regulator section a zener diode was used in combination with an PNP transistor to provide greater current output than the Zener Regulator section. The previously referenced section discussed circuits that were built with discrete components such as a zener diode and an PNP transistor.

In this section, the aforementioned components are replaced by an Integrated Circuit or IC, more commonly known as a chip. This is what the world of Microelectronics has brought to us. It should be noted that IC’s or chips are very reliable and cheap relative to a circuit constructed from individual discrete components. This section discusses the -12 volt Series Regulator, Part Number 7912. It is part of the 79xx group of regulators which are comprised of the following: 7905, 7906, 7909, 7910,7912, 7915, 7918, 7920, and 7924 where the last two digits of the part number reflect the output voltage.

See Typical Circuit Diagram below:

Two capacitors are used one at the input (C1 = o.1 uf) and one at the output (C2 = 0.1 uf) for filtering purposes. It is possible that the regulator will work without these. The 7812 chip will require a heat sink if we expect a 1 amp output current. More about that later. One of the nice features of the 7812 chip is that it has built-in short circuit protection and over heating protection (thermal overload). As per datasheets available on the internet, the chip input can vary between Vin = -14 to-35 vdc.  However, you’d better have a large heat sink and fan, if you are serious about a -35 vdc input.

Actually good design dictates that the input voltage Vin should be 2 to 3 vdc higher than the expected -12 vdc output.

See Diagram Below for physical pictorial of 7812 chip.

How does one select the proper heat sink?

The purpose of a heat sink is to transfer heat away from the integrated circuit. The heat sink is rated by its ability to transfer heat. This quantity is known as Thermal Resistance and is measured in degrees centigrade divided by watts.

If the thermal resistance of the heat sink is very low, then it is a good heat sink.

To determine the heat sink rating required, perform the following.

(1) Determine the power to be dissipated (Pd) as follows:

Pd = (Vin – Vout) x Output Current where Vin = -14 vdc; Vout = -12 vdc; and Output Current = 1 amp

Refer to previous -12v Regulator Sketch

Pd = (14 – 12) x 1amp = 2 x 1 = 2 watts

(2) As per the 7912 datasheet, the maximum Temperature rating is + 125 deg centigrade.

(3) Using this formula for Fahrenheit to Centigrade conversion to translate a room temperature of 72 deg F.

C = 5/9(F – 32) = 5/9(72-32) = 5/9 (40) = 22 deg centigrade

Note: If our circuit were in a case, we would have to know what the internal case temperature is.

(4) Use this formula to find the maximum thermal resistance of the heat sink.

Rth max = (Max Temp Rating of IC – Room temp) divided by Pd = (125-22)/2 = 103/2 = 52 deg centigrade per watt

(5) Carrying out the above formula a little bit further, 52 deg centigrade per watt times 2 watts = 104 deg centigrade. Now, we add room temperature to that and we get 104 + 22 =126 deg centigrade which is beyond the 125 deg centigrade specification. Therefore, we need a heat sink. Reference 7812 datasheet.

(6) Now, we add room temperature to that and we get 52 + 22 = 74 deg centigrade which is safely below the 125 deg centigrade specification.

P.S. If you want to learn about positive voltage Series Regualtors visit the following: + 12 vdc Series Regualtors 78XX

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics

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