1mv accuracy on 7805 linear voltage regulator?

t_n_k

Joined Mar 6, 2009
5,455
That applies to AC sine wave ripple with a frequency of about 100 Hz. At high frequency, where the loop gain has rolled off, it has ZERO rejection.
The OP gave no clue as to the frequency at which the ripple appeared. So we are in the dark with respect to that. As usual we are hamstrung by poorly defined issues.
 

Thread Starter

mrclauds

Joined Jul 11, 2012
4
Thank you all for all the input

I do appologise for lack of information, but I simply do not know the details of whats involved, which makes this frustrating for me, but thats the way I learn these things...

Please be patient as I have no training on electronics except for self taught trial and error and some books...

I've been able to purchase a voltage reference and I notice that at 10ms, 50mv divisions, I have a sine wave 160mvpk-pk to 180mvpk-pk, Vavg ranges from 92mv - 101mv at a frequency 98-101Hz.

At 2.5uS its all over the place, if I bring it back to 1ms and average it out the reading to 32, I get 96.0 -96.2mv average...

I guess I need to learn alot about using the scope correctly, any advise on how I should be checking on this would be appreciated...

If i was to interface this to a microcontroller, would I simply sample the voltage 32 times and then average it out?

Thanks
Clauds
 

takao21203

Joined Apr 28, 2012
3,702
Some idiot already said that on the first page of the thread.

Sincerely:

The Idiot
Please, can we try to get along? We are busy with many things and even if we have certain qualification we can't know all details. So we use this forum (for instance).

Yes I know of temperature drift. It's not very relevant for me.
 

takao21203

Joined Apr 28, 2012
3,702
OK, I understand. You are feeding high frequency EMI from the output of a switcher into a sensitive circuit... and can't figure out why it fluctuates?

As I once tried to write an app note about when I worked at National Semiconductor...... and was threatened with death by the Marketing Director if I did:

Any linear reg (including a 7805) has a finite unity gain bandwidth of maybe 100 kHz. If you feed switcher ripple into it, the 7805 can only filter out the lower frequency content. The high frequency stuff shoots through like june bugs through a goose.
Well actually I have a coil inlined after the switcher to reduce noise.
The 78L05 is connected directly to 12V electronic transformer. But they are also noisy.

How much resolution can you really get with 10 bit A/D?

2.5 / 1024 = 2.5mV in the best case.
To get 0.1V display stable, not changing all the time you need at least to reduce ripple to 20 or 30mV. And that's totally sufficienct for my purposes.

TL494 for instance is directly supplied with 12V electronic transformer...don't really recall if there is 7805 used as reference voltage but I think so.

Additional filter coil after electronic transformer or dc/dc converter actually gives quite good results in terms of noise reduction.
 

takao21203

Joined Apr 28, 2012
3,702
I tried to make sense of that matter in relation to the OP's reported output ripple voltage - i.e. 400mV [see post #1].

Looking at Fairchild's data on the LM7805 they quote at least 65dB rejection.

Even allowing for a rejection as low as 41dB this would lead to an input ripple of around 45V which makes little sense. I suspect the OP's observations aren't necessarily related to regulator input ripple problems. I'd be looking for other explanations.
Maybe it's faulty, I'd try another...having a bag here with 80pcs 78L05.

And I don't think the switcher noise just shoots through. It depends how much the switcher or electronic transformer is loaded. Some of these work at much lower frequencies than 100 KHz. The noise reduction will decline gradually with increased frequency.
 

tracecom

Joined Apr 16, 2010
3,944
Do you have a 0.33uF cap from the input terminal to the ground terminal, and a 0.1uf cap from the output terminal to the ground terminal, with the leads as short as they can be?

Those caps are not optional. Without them, it is very easy to wind up with oscillations. Having the 0.33uF on the input and 0.1uF on the output effectively dampens out those oscillations.
You have been given some excellent advice here; check the datasheet to confirm.
 

ErnieM

Joined Apr 24, 2011
8,377
I like these SOIC (and such) adapter boards:



(There are some very cheap suppliers from China you can get thru EBay.)

I use them on 0.1" spaced pads like the other guy:



0805 caps & resistors work nicely on these boards as they drop between 2 pads. SOT23 transistors get a little angle and drop onto 3 pads.

(Dang he has a lot of flux on that board.)
 

ErnieM

Joined Apr 24, 2011
8,377
Some idiot already said that on the first page of the thread.

Sincerely:

The Idiot
Apparently some other idiot missed that post.

But we're three pages into this and not getting anywhere closer to getting a precision output voltage from a crude power regulator.

<sigh>

Sincerely,

Some Other Idiot
 

takao21203

Joined Apr 28, 2012
3,702
Lead solder is a bit easier for these jobs, also used tin solder in the past, but it has higher viscosity.

A few times I have tried this with TFQP44 but it's quite a hassle, and pins broke off. 28pin SOIC adapters are also good for SOT23 (MOSFETs etc.)
 

takao21203

Joined Apr 28, 2012
3,702
Apparently some other idiot missed that post.

But we're three pages into this and not getting anywhere closer to getting a precision output voltage from a crude power regulator.

<sigh>

Sincerely,

Some Other Idiot
I am using 78L05 to supply infrared receiver module. Extracted from old television set. The uC circuit = 3.3V.

Sometimes the LED was flashing up a little in random intervals.
Adding 0.1uF to the 78L05 only helped it a little.
Adding 1uF directly to the IR receiver component almost cured it.
Adding 2.2K in paralel to the 1uF, and this has disappeared.

Oscilloscope leads maybe are picking up noise?
It is not clear if the 7805 was loaded with some mA and had capacitor, when the measurement was done.

I am always dealing with noise here, which is picked up quite easily by wires. PICs can live with very high noise level, but to get rid of it, you don't only need to add caps as close to regulator as possible. Indeed the distance is not so important. You need to add caps to the device as well.

On the PIC chip itself, I often don't use any capacitor. Never had any trouble. And they work at max. frequency.
 
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