Now I am going to show you a couple of simple circuits we can make using 7912 and I will also tell you about its important features. The circuits I am sharing are actually very simple and you don’t need to have an advanced knowledge of electronics.
7912 Basic Circuit Diagram
7912 Basic Circuit Diagram:
I am adding an image on this page which shows the most basic circuit diagram using 7912 for voltage regulation purposes. As you can see, apart from 7912, only two bypass capacitors are used. These capacitors should be placed as close to 7912 as possible. You can omit the input capacitor if 7912 is placed within 3 inches of the main supply capacitor but output capacitor is highly recommended to avoid distortion in the output voltage. Input capacitor is 10uf/50v and output capacitor is rated at 1uf / 50V but you can use film capacitor of 105PF rating as well here.
LM 7912 Input Voltage:
Input range for LM 7912 is from minimum 14.5V to 35V maximum. Since you are going to input raw voltage to LM 7912 you should first stress test your power supply by putting a 1.5A load on it to see how low the voltage goes. Your power supply voltage should not drop below 14.5V in order to get a fully regulated -12V output from 7912 IC. Also make sure to check your power supply’s unloaded voltage. They should not exceed 35V otherwise it can burn the IC.
7912 Load and Heat Dissipation:
Theoretically you can put up to 1.5A of continuous load on 7912 but under my experience it works best with loads up to 1A after which it tends to heat up and sometimes causes issues due to built-in thermal protection. Also, you need to keep an eye on all the heat it dissipates because if you are keeping your circuit inside a small container, it can make it hot enough to exceed the safe operating temperature for the other components in the same circuit. Also, one more reason to avoid over loading the IC is the drop in the regulated output voltage which is directly linked to the load we put on it. The more load you put on it, the less accurate the 12V supply will be.
A 7912 at full load can drop up to (4%) half volt which can be critical for sensitive circuits. If your load is continuously at 1.5A or a bit above then you should consider using more than one 7912 in parallel using the circuit below.
Being a linear voltage regulator, 7912 is not very efficient and it wastes a lot of energy as heat. The more the gap between input and output voltage, the more energy it will waste by generating heat. You can use following formula to calculate how much energy is being wasted as heat.
(Input Voltage – 12) x Output Current
If you are using a 24V power supply as input source for your 7912 and your load is pulling 1 Amp from the 7912 then following is how we will calculate the energy being wasted.
(24 – 12) x 1
=12 x 1
You can see that you will be wasting an additional 12W as heat energy for using 12W for your source. In addition to this loss of energy, you will also need to dissipate that heat using a large heatsink plate. To avoid this issue, 7912 input voltage should stay as close to 14.5V as possible but at the same time the input voltage should never drop below 14.5V otherwise the output will become unregulated. I recommend to use 18V power source for this purpose.
Parallel Use of 7912
Parallel Use of 7912:
Theoretically, no two 7912 can deliver exactly same output voltage due to the micro differences in their dies from the manufacturing process. The voltage difference is usually only a few millivolts that can be covered by slight load balancing. To achieve this, we can use resistors. I have added a circuit diagram in which we are using two LM 7912 in parallel to achieve an output close to 3A. In case of continuous load, you can comfortably put 2A to 2.5A load on it which can even spike to 3A for small intervals. This circuit is pretty much self-explanatory, we have two 7912 in parallel with a 0.47Ohm resistor attached to the output of each 7912. Please note that both of these resistors must be at least 15W otherwise they will burn at the peak load. At the combined input and output, we have two capacitors of 10uf / 50V and 1uf / 50V respectively.
7912 Safety Features
LM 7912 has inbuilt overload and short circuit protection, a safe area protection and on the top of that it also has a thermal shutdown so it looks like a great safety package. These safety features may come handy in some certain situations and will most likely protect the circuit you are powering from severally burning.
Posted or Updated On:Nov 18, 2020