Fixed voltage regulator problem

In summary, LM7805 and LM7809 regulators have a normal variation in output voltage and are not precision devices. This variation is due to the fabrication process and can be affected by factors such as temperature and load. It is possible to adjust the output voltage with resistors or use a different regulator for more precise results. However, a 2% error in voltage is not significant for most circuits. Heat sinks can also help regulate the output voltage.
  • #1
elecz
17
0
Why does LM7805 and LM7809 give less voltage? Practically LM7805 is giving 4.9V instead of 5 and 7809 is giving 8.8V. Can anyone please tell me the reason?
 
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  • #2
That is normal. These are not precision devices. There is usually some variation from the nominal voltage.

It is possible to boost the voltage with a couple of resistors, or you could use a LM317K and adjust it to whatever voltage you like.
 
  • #3
But I want to know the exact reason why this happens. Is this because of some internal loses?
 
  • #4
It doesn't always happen. A 7805 will give an output voltage close to 5V. but some will give you a voltage slightly less and some slightly more than that. It has to do with how they are fabricated etc.
Note: if you test several 7805 and they all give a voltage slightly lower than 5V there is either something wrong with your circuit, or the voltmeter you are using.
 
  • #5
7805 is speced at 4.8 V min, 5.2 V max.
 
  • #6
What model meter are you using?

Bob S said:
7805 is speced at 4.8 V min, 5.2 V max.

True, but I've never had a 7805 be even as far off as 4.900V before.
 
  • #7
Topher925 said:
What model meter are you using?



True, but I've never had a 7805 be even as far off as 4.900V before.

I thought the same thing.
 
  • #8
maybe it's what you're feeding it?
 
  • #9
They are not precision, they vary widely. 8.8 for a 9 sounds pretty good to me! The only strange thing is they both go below. Are you drawing the max current? Do it get hot? You have heat sink on it? Make sure you have at least 3V head room.( for 7805, the input need to be something like 8V, read the spec to be sure.)

I had seen those regulator get hot even under light load. I am not exactly a fan of these regulators.
 
  • #10
4.9 volts instead of 5.000 is only a 2% error and these things are produced in millions for a few cents each.

It just reflects the difficulty of accurate doping of materials, particularly for the voltage reference inside these ICs.
It is the same reason that transistor gain varies so much for the one type of transistor.

A 2% error isn't going to make any difference for many circuits (especially digital ones).
If it will make a difference, you can get better regulators or variable regulators.

I have tried to trace oscillator drift and found it was due to the change in voltage output of these regulators with temperature. They need a heatsink, but seldom get one.
 
  • #11
true, as BOB said. its SPECED! nothing to do with the voltmeter. read the data sheet, don't complicate things
 

1. What is a fixed voltage regulator?

A fixed voltage regulator is an electronic component that maintains a constant output voltage regardless of changes in input voltage or load. It is commonly used in electronic circuits to provide a stable and reliable power supply.

2. How does a fixed voltage regulator work?

A fixed voltage regulator works by using a feedback mechanism to compare the output voltage to a reference voltage. If there is a difference, the regulator adjusts the output voltage to match the reference voltage, ensuring a constant output. This is achieved through the use of internal pass transistors and control circuitry.

3. What are some common problems with fixed voltage regulators?

Some common problems with fixed voltage regulators include overheating, voltage drops, and output voltage fluctuations. These issues can occur due to improper circuit design, incorrect component selection, or external factors such as temperature changes.

4. How can I troubleshoot a fixed voltage regulator problem?

If you are experiencing issues with a fixed voltage regulator, you can troubleshoot the problem by checking for loose connections, testing the input and output voltages, and ensuring the correct components are being used. You can also consult the manufacturer's datasheet for troubleshooting tips and recommended operating conditions.

5. Can a fixed voltage regulator be replaced with an adjustable one?

Yes, in most cases, a fixed voltage regulator can be replaced with an adjustable one. However, you will need to make sure that the adjustable regulator can handle the same input voltage and output current as the fixed regulator. You may also need to make some adjustments to the circuit design to accommodate the different characteristics of the adjustable regulator.

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