Log 150v system with a 30v logger?

**verytricky** · 27th November 2008 09:01 PM

I have a wind turbine that generates from 40v to 150v.

I have been experimenting with various blade designs to try and get a blade great for low wind, but which twists in high winds, so the turbine does not overspeed.

To compare results I have a computer logged anometer, and I have been recording the units of electricity generated from the linked inverter, measured hourly. Sort of like picking your nose with boxing gloves. It gives me a rough idea of what works, but there is no direct corelation - which I need.

I have - from a previous project - a 0 to 30 volt data logger. I can link it up to the same computer as the anometer, and get per second readings of the wind speed and the voltage produced, which is perfect.....

BUT

The turbine produces 40v to 150v so that is useless for the data logger.

I am too cheap to spend the 300+ for a data logger in the correct voltage range. I figure there has to be some type of clever thingy you can do with a few resistors or so to get the voltage divided down to the data loggers range, but still representitive of the original voltage.

If it were an AC system I could use a transformer, but the voltage is DC.

My skill level is more mechanical than electronic. ie I can build a powerstation and wire up a house, but can do no more than plug in a computer, so and answers need to be dumbed down to my level.....

**MrAl** · 27th November 2008 09:15 PM

Hi there,

This problem is easy to solve. You use what is called a voltage divider.
It is made with two resistors that you can buy online or maybe Radio
Shack.
You need a divide ratio of 5 to 1, so that means maybe 100k on top and
25k on bottom.

The two resistors connect like this:

o-----RTop----x-----Rbottom-----Ground

Your 150v input goes in at the little o, and the
logger gets its input from the little x point.
Should use 1/2 watt resistors.

Of course this assumes your 150v source can work with
a 125k load without being bothered too much.
You can test it and calibrate as needed.

It's important that the lower resistor connects to ground
by soldering or other means that makes sure it can not
become disconnected. If that resistor becomes disconnected
it puts the full 150v into the data logger, so no alligator
clip for that ground.

**verytricky** · 27th November 2008 09:40 PM

Cool!

Thanks

**Leftyretro** · 28th November 2008 06:44 AM

You might consider using a 20 turn precision pot.

50K MULTITURN TRIMMER POT | AllElectronics.com

You could adjust it for a 10:1 reduction such that 0-300vdc input is sent to the logger as a 0-30vdc voltage. That way all your scaling in your existing logger would just have to be mentally multiplied by 10.

Lefty

**verytricky** · 28th November 2008 11:02 AM

The 50k pot seems like a good idea. I may look at it as a refinement later. Will the pot handle the power without blowing up?

The first answer gives me what I need with stuff I have lying around. Accuracy is not as important as repeatability and fine grain measurement. If the measurement is out by 30% - that is OK - provided the error remains more or less the same each day for comparisons.

As I intend to continue to generate electricity from the turbine while it is being monitored, I intend to take a link out of the existing wire and run the metering from there, with the inverter continuing to generate electricity...

Code:

 

Trbn+-----------------o-------------------------Invrtr+
                                !
                                !------------
                                !               !
                                Rt               LOGGGER
                                !               !
                                !------------
                                !
                                Rb
                                !
                                !
Trbn------------------o-------------------------Invrtr-

( who said ASCII art was dead! )

**MrAl** · 28th November 2008 12:29 PM

Hello again,

Just be careful if you use a pot instead of two resistors because you can
turn the pot up too high and blow out the logger input.

Here's an ascii representation:

Code:

0 to 150v in o------+
                    |
                    |
                    RTop
                    |
                    |
                    +------o Logger input
                    |
                    |
                    RBottom
                    |
                    |
Ground o------------+------o Logger ground

The formula for the input/output relationship is:

Vout=Vin*(RBottom)/(RTop+RBottom)

where
Vin is the 0 to 150v input
and
Vout is the output to the logger.

The power in any resistor will be:

P=V*V/R

and a good design rule is to use a resistor rated for twice
the power as calculated above, or:

Pr=2*V*V/R

With a max input of 150v the upper resistor will see
120v with a 5 to 1 divide ratio, and the lower
resistor will see a max of 30v. If you want to use
1/2 watt resistors then you should keep RTop above
57.6k, and keep RBottom above 3.6k.
With RTop=100k and RBottom=25k this works out pretty
well.
If you want to go with a 10 to 1 ratio, then you can use
either 100k and 11.1k, or 90k and 10k resistors for RTop
and RBottom respectively.

If you have a little noise in the system a small capacitor
across the lower (RBottom) resistor helps too when
measuring DC voltages. A 0.1uf is good, up to around
10uf if your measurements dont have to be really fast.
Voltage rating for the cap should be 50vdc.

The repeatability will be very good.

**verytricky** · 28th November 2008 12:52 PM

How do you get your ASCII to work? Mine was rubbish....

I plan to test the circut carefully with a multimeter, then seal everything up tight before adding my logger, so hopefully I will not blow it up.

**Nigel Goodwin** · 28th November 2008 02:40 PM

Originally Posted by verytricky

How do you get your ASCII to work? Mine was rubbish....

He put it in code tags, which prevents your browser destroying the formatting.

**verytricky** · 28th November 2008 06:33 PM

Good thing I will test first, as my code shows the logger in the wrong place....

**MrAl** · 29th November 2008 09:32 AM

Hi again,

Yes, as Nigel mentioned i put in code tags, which maintains all the spaces
and such and also it uses the Courier New font which happens to have
characters that are all the same width so when you type out tables and
stuff like that the tables dont get all messed up as with a proportional
font like Arial. I think you might also be able to use just the Courier New
font by selecting that before you begin to type, so you could try that too.

To insert code tags you can find that on the 'advanced' editing options or
you can insert them manually like this (all these chars without quotes):

"[" "code" "]

then the body of your message here

"[" "/" "code" "]"

Note i had to use quotes because it would force the code tags to operate
and you wouldnt be able to see those words and characters, but when
typing that out for use as actual code tags leave out all the quotes.

You can also upload schematics, if someone would be kind enough to
explain this (i havent used this feature yet either).

**ericgibbs** · 29th November 2008 09:44 AM

hi, [Morning Al]

For inserting images:

Click 'Manage Attachments' button [ lower down the replying page]

This will Open a Window, use the 'Browse' button to locate the image on your PC.

Then click the 'Upload button

Wait till the upload is done and click the 'close this window'

While the Browse window is open, it lists the type of files and their max sizes that can be uploaded.

**MrAl** · 29th November 2008 03:02 PM

Hi again,

Thanks Eric, and one thing i was wondering is...

is there a limit as to how many schematics you can upload or no?

**ericgibbs** · 29th November 2008 04:47 PM

Originally Posted by MrAl

Hi again,

Thanks Eric, and one thing i was wondering is...

is there a limit as to how many schematics you can upload or no?

hi Al,
While the Browse option Window is open there are 3 address boxes, so it is at least 3.
I have have never had cause to upload more than three at anyone time.?

**MrAl** · 30th November 2008 10:39 AM

Hi Eric,

Well actually i meant the total amount allowed to be uploaded
per screen name. In other words, after i upload 1000 schematics
will that be it, no more allowed (or some other number)?

**Nigel Goodwin** · 30th November 2008 11:07 AM

Originally Posted by MrAl

Hi Eric,

Well actually i meant the total amount allowed to be uploaded
per screen name. In other words, after i upload 1000 schematics
will that be it, no more allowed (or some other number)?

As far as I'm aware there's no limit.

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