Regulator IC voltages

 

Q. I am an electronics hobbyist. I like to build circuits for different applications. I recently built a regulated 5V power supply for experimental use.

I used a 7805 regulator chip, which is supposed to supply 1A of output current. I tested it by putting different power resistors across the output to get different amounts of load current. I then measured the output voltage.

I found that the output voltage started dropping off at much less load current than 1A. Apparently the regulator went into shutdown, because when I disconnected the load, the output voltage slowly came back to 5V. What is going on?

 

A. Most regulator ICs contain an additional circuit that shuts them down when the IC gets too hot.

The 1A current rating applies only when the IC is at 25°C (77°F). To keep the IC cool, you need to attach it to a heat sink. This is a chunk of aluminum with cooling fins. Heat sinks are rated in degrees Celsius per Watt. In other words, this is the temperature rise per Watt above the temperature of the air in the device’s environment. You also have to take into account the degrees per Watt of the interface between the device and the heat sink, and add this to the heat sink rating.

Most regulator ICs go into thermal limiting at about 1250€ (257°F). The ratings for the IC you are using are listed in its spec sheets.

It also helps to use the minimum possible voltage for the unregulated supply. The regulator IC is basically connected in series with the load, so all the load current, plus a small amount for the regulator itself, flows through the IC. The Voltage across the IC is, from Kirchhoff’ s voltage law, the difference between the unregulated voltage and the regulated voltage.

The power the chip must handle is thus about equal to this voltage times the load current. Keeping the drop across the regulator chip low helps minimize the power it must handle.

Regulator ICs sometimes fail, and some types do so by shorting between the input and output. This places the unregulated voltage across the load. Since you are building a 5V supply, your load is probably TTL chips. If the unregulated voltage is 7V or more, if the regulator fails, it will destroy the TTL chips, too. There are ways to prevent this, such as using a “crowbar” circuit across the output, which you may want to investigate.

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