Fixed voltage regulator problem

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Discussion Overview

The discussion revolves around the performance of fixed voltage regulators, specifically the LM7805 and LM7809, focusing on their output voltages being lower than expected. Participants explore reasons for this behavior, including manufacturing variations, internal losses, and the impact of circuit conditions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that variations in output voltage are normal for these regulators, as they are not precision devices.
  • Others seek to understand the exact reasons for the lower output voltages, questioning whether it is due to internal losses or fabrication differences.
  • It is mentioned that the LM7805 is specified to have a minimum output of 4.8V and a maximum of 5.2V, suggesting that the observed voltages are within acceptable limits.
  • Concerns are raised about the conditions under which the regulators are used, including input voltage, load current, and the necessity of heat sinks.
  • One participant emphasizes that a 2% error in output voltage is generally acceptable for many applications, particularly in digital circuits.
  • There is a suggestion that the observed voltage discrepancies could be influenced by the voltmeter used for measurement.
  • Some participants express skepticism about the reliability of the LM7805, citing personal experiences with its performance.

Areas of Agreement / Disagreement

Participants generally agree that variations in output voltage are expected for the LM7805 and LM7809, but there is no consensus on the specific reasons for the observed lower voltages or the implications of these variations.

Contextual Notes

Participants mention factors such as temperature effects, load conditions, and the specifications of the regulators, indicating that these may influence the output voltage but do not resolve the underlying uncertainties.

elecz
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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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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.
 
But I want to know the exact reason why this happens. Is this because of some internal loses?
 
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.
 
7805 is speced at 4.8 V min, 5.2 V max.
 
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.
 
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.
 
maybe it's what you're feeding it?
 
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
 

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