Let's talk to newbies on proper bypassing and surge suppression first.
This is a very common problem for newbies when it comes to using a regulator chip. There are four common problems that cause your regulator to overheat.
1) Improper pin connections.
2) Missing bypass capacitor(s).
3) Excessive dissipation. (Excessive input voltage or excessive current draw.)
4) Insufficient heat sinking.
1) Improper pin connections
Let's take item one. Proper pinout. There are two pin configurations for the standard TO-220 case. One for positive ground regulators and one for negative ground regulators. Figure 1 shows the pinout for the negative ground regulator. Figure 2 shows the connections for a positive ground regulator. Hooking it up wrong and applying power the first time usually damages the device beyond use.
For TO-92 case devices (Example 78L05 and 79L05) the pinouts are totally different too. The following shows the proper connections:
2) Missing bypass capacitors
Often times the bypass capacitors are left out of the regulator circuit. They must be in place in order to keep the regulator from oscillating and overheating. At a minimum, a U10 (0.10uf) capacitor must be placed on the input to ground pin as close as possible to the device and with as short leads as possible. This is all that is really necessary to stabilize the device. This does not have any bearing on the value of the filter capacitor on the input. The value of the filter capacitor should be large enough to handle the load without ripple in the input. This web page will not go into detail with the calculations. However, as a general rule, the larger the value, the better. Do not attempt to put a large value filter capacitor on the output of the regulator. It is not necessary and could damage the regulator if the input voltage drops quick and below the output voltage. If it is imperative that you use a large value on the output, a 1N4001 (CR2) across the input and output of the regulator with the cathode to the input as shown below.
3) Excessive dissipation
Proper design of the regulator is vital when using the regulator. The datasheet for the typical 7805 states it can regulate up to 1 amp or 5 watts. This is an absolute maximum rating and cannot be exceeded. It does not mean you can use a 24 volt power supply and expect to get 5 volts at 1 amp out of it. Let's see how they come up with the stated specification. In order to provide good regulation, you must have at least 3 volts above the regulated output. With an 8 volt power supply and a 1 amp load, and using the formula P=E*I, the regulator will dissipate 3 watts.
.... The high temperature on the regulator is caused by excessive power dissipation of the device. A standard TO-220 device can dissipate up to five watts and must be adequately heatsinked. Five watts dissipation in free air is actually too hot to touch.....
Let's look at a typical regulator circuit and see how much the regulator dissipates:
The above regulator circuit shows a 5 volt regulator with a maximum load of 100 mA (0.1 amps). The following is a diagram showing the relationship of voltage, current and power dissipation using the formula P=E*I.
As you can see, the regulator is dissipating 400 milliwatts (0.4 watts). The regulator will become warm to the touch. Next we will see how much power the regulator will dissipate if the power supply is increased to 12 volts.
As you can see, the regulator is now dissipating 700 milliwatts (0.7 watts). Now the regulator is getting hotter. Let's increase the power supply to 15 volts.
The regulator is now dissipating 1 watt. The regulator is now too hot to touch, and will require a heatsink in order to release some of the heat built up on the package. If the regulator gets this hot or hotter, the regulator may shut down in order to protect itself.
Under construction as of March 1, 2004
©2004 Rick C.