[AllanG]

The output of the 7809 will remain constant even if the input voltage changes. If I remember correctly, the 7809 should handle input voltages from around 11 to 35 volts.

We where replying at the same time, but it looks like my answer isn't wrong

I was thinking the same thing!!

[hvalentim]

There's no doubt the regulator can handle a voltage up to 35V.

The problem as I see it is: since my supply is somewhat unstable and at times it constantly shifts in voltage this can confuse the Arduino reference values, particularly when using the 5V (I know this is the reason why Werk_AG has chosen to use the more stable 1,1V internal reference voltage instead for the Solar radiation part).

I have just tested and with an intermittent input voltage of 15,7-13,7 the regulator intermittently outputs 8,8 to 9,2 v.

Although most sensors claim to have internal voltage regulation of their own, I think that like with most anything overall my system would benefit from a more constant, stable power supply.

Interesting reference material I have found:

Voltage regulator tutorial & USB gadget charger circuit
A straight forward, clear Youtube video explaining how linear regulators work and how to connect them.

How to Connect a Voltage Regulator in a Circuit
More on the matter but in text form.

Secret Arduino Voltmeter – Measure Battery Voltage
An interesting post on how Arduinos handle voltage.

I quote:
"A common assumption when using analogRead() is that the analog reference voltage is 5.0 volts, when in reality it may be quite different (...) The fact is the default analog reference is not 5.0 volts, but whatever the current level of Vcc is being supplied to the chip. If our power supply is not perfectly regulated or if we are running on battery power, this voltage can vary quite a bit"

Arduino misconceptions 6: a 9V battery is a good power source
Argues that using a 9v power source to power an Arduino may not be the best solution.

Ideally, perhaps we could skip linear regulators all together and use something like these DC/DC converters (they can be used in the current boards/design already since they fit the TO-220 footprint) to power the Arduino from a lower voltage and either power the fan independently 24/24h or use some sort of switch to act upon an independent 12v circuit?

[AllanG]

I have just tested and with an intermittent input voltage of 15,7-13,7 the regulator intermittently outputs 8,8 to 9,2 v.

Given those figures, your regulator is working exactly as expected and within parameters - even a regulated output varies very slightly. The Datasheet for a Fairchild LM7809 specifies a minimum output of 8.6v, typical voltage of 9.0 and a maximum of 9.4v

I do not know whether a difference of ~0.2v can and will affect the accuracy of the Arduino or other components, I'm sure others will comment on that.

(Is your voltmeter properly calibrated and capable of measuring with absolute accuracy??)

[meteoestarreja]

I have just tested and with an intermittent input voltage of 15,7-13,7 the regulator intermittently outputs 8,8 to 9,2 v.

Weird, I never noticed something like that, usually I get the output as I expected.
There are regulators with lower deviation like 0,1% but even so, with cheap regulator in previous tests, never happened to me having such a deviation!

In fact, I just tested (5 minutes ago) in my labor lab, a regulator (a cheap one) between 15 and 35V, and the output was always the same without any change.
I was using a calibrated voltmeter, so I trust those measures.

[hvalentim]

My voltmeter is certainly far from perfect. But that's beside the point. Even if it's not perfectly calibrated in absolute terms (although I have reasons to believe it can be accurate up to 0,1V) the matter at stake pertains a relative variation in the 7809 output provided the voltage it is supplied constantly changes (and goes up and down) very fast.

When the supplied voltage applied to my L7809CV remains consistent (for instance 12,8v) the output is also consistent at 9v give or take a couple of 0,01 of volt, which in fact can be a voltmeter issue.

For your lab test to mimic my conditions you would need to apply a constantly shifting voltage to the input, varying say 2v up and down in less than 1 second cycles.

I would suggest the L7809 has let's call it a "response time" in regulation than does not allow him to keep up at it's peak accuracy when it has to constantly adjust, the more when sometimes it's adjusting down the others up...

Otherwise, as Allan remarked, the spec.s claim no more than a "8.64 - 9.36" accuracy under 25º conditions. So it is not performing below what it claims.

EDIT: It is worth mentioning that I am testing the L7809CV in it's bare form. Adding capacitors seems to be a suggested best practice that may help a bit. I do not know/understand exactly how this is handled in WeatherDuino but it seems we have 100nf capacitors in both lines. Perhaps I need to add those two to my equation as well.

Here's an in detail illustrated article on the anatomy of these things with a number of tips:
Reverse engineering a 7805 voltage regulator

[meteoestarreja]

For your lab test to mimic my conditions you would need to apply a constantly shifting voltage to the input, varying say 2v up and down in less than 1 second cycles.

I never imagined that the speed (frequency) was that high!
In deed, the fastest frequency that I used was around 1 to 1,5 Hz, but you said that in your case is faster than that.
The more strange to me is the fact you're using a battery/solar panel, so, it shouldn't be anything to do with ripple or something like that (even that the regulator itself tends to filter the ripple, and if you are using capacitors, definitely ripple shouldn't be the "killer" here).
I will try to reproduce later if I can find anything at home that allows to run at the same conditions ... I'm leaving the office

Just for curiosity, how did you managed to read both high and lower peak at that frequency?
You used a oscilloscope or your voltmeter do record high an low values?

[hvalentim]

if you are using capacitors

I am not. As I do not have them around right now.
I just have 0.17A fan running to provide the load.

You used a oscilloscope or your voltmeter do record high an low values?

No oscilloscope used. Just observed the voltmeter readings over a period of time and mentioned lowest and highest values observed.

The bottom line of it all seems to be the purchase of a quality more recent solar regulator is in order, although I very much doubt those cheap models sold from China are a great improvement.

[werk_ag]

The problem as I see it is: since my supply is somewhat unstable and at times it constantly shifts in voltage this can confuse the Arduino reference values, particularly when using the 5V (I know this is the reason why Werk_AG has chosen to use the more stable 1,1V internal reference voltage instead for the Solar radiation part).

You are mixing a lot of things.
This 9V (more or less) supply voltage doesn't have any effect on the ADC reference voltages used by the Arduino. This only depends on the quality of it's own 5V regulator, which you can't control (at least keeping the Arduino board untouched).

Returning to the 7809: It have some tolerance in the output voltage, but it does not float... at least in a so large range as you are reporting!

It seems that you are powering it with a problematic power source. Try adding a large electrolytic capacitor (1000uF or more) to input an then check the results.

Ideally, perhaps we could skip linear regulators all together and use something like these DC/DC converters (they can be used in the current boards/design already since they fit the TO-220 footprint) to power the Arduino from a lower voltage and either power the fan independently 24/24h or use some sort of switch to act upon an independent 12v circuit?

If your concern is about efficiency, you can spend the almost 20€ that a TR1- 2490 will cost, and replace the 7809, but it don't gonna give better accuracy on the system, just a little more power efficiency. Anyway, the same result can be obtained, spending less than 1€, with an external DC/DC converter module. Wiring it instead of a 7809 is very easy.



Most of the times, the better is keep the things simple!

[werk_ag]

The bottom line of it all seems to be the purchase of a quality more recent solar regulator is in order, although I very much doubt those cheap models sold from China are a great improvement.

I'm using one of those cheap chinese solar regulators, for more than two years, and my TX unit with a 7809 is happily working with it.

[hvalentim]

This 9V (more or less) supply voltage doesn't have any effect on the ADC reference voltages used by the Arduino. This only depends on the quality of it's own 5V regulator, which you can't control (at least keeping the Arduino board untouched).

Yes. But regardless of individual units, by design, the Arduino voltage regulator itself is a linear regulator which should perform in a similar fashion/have the same caveats as the L7809. We actually have two linear regulators applied in sequence.

It seems that you are powering it with a problematic power source.

That particularity is the core of the matter.

Try adding a large electrolytic capacitor (1000uF or more) to input an then check the results.

I have one single 100uF available. Can try it tomorrow.

you can spend the almost 20€ that a TR1- 2490 will cost

Actually it seems 7,6 euros + VAT will suffice.

Anyway, the same result can be obtained, spending less than 1€, with an external DC/DC converter module. Wiring it instead of a 7809 is very easy.

Thanks for the tip.

Most of the times, the better is keep the things simple!

You will see in the end I will go for the "simple" solution. I just don't take it without questioning/understanding exactly why