Monitoring voltages

I just got a 24volt solar panel.
I want to monitor the DC voltage coming off the panel.
I'll use my HB solar sensor also so I can get a graph with solar readings and solar panel output readings.

I was looking at your store Eric, looks like the HVAC Monitor would be what I want to monitor the solar panel/battery system?

EL34 said:

I just got a 24volt solar panel.
I want to monitor the DC voltage coming off the panel.
I'll use my HB solar sensor also so I can get a graph with solar readings and solar panel output readings.

I was looking at your store Eric, looks like the HVAC Monitor would be what I want to monitor the solar panel/battery system?

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The HVAC Monitor was designed to monitor whether a voltage is present or absent but you can read the actual voltage using the formula VOUT = (VIN - 0.7) /10. I would not use it for precision measurements but to get a close idea of the voltage it would work OK.

Eric

Ah, ok.
Is there any other device that would be better for momitoring 24vdc and lower voltages with say 1/10th volt precision?

We don't have a general AtoD 1-Wire device at the present. There may be others out there that do.

Eric

I would not use it for precision measurements but to get a close idea of the voltage it would work OK.

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I forgot to ask.
How precise voltage wise would the hvac monitor be.

+- 2volts, 1volt, 1/2 volt?

It may be good enough, just curious.

It should be within a volt.

Eric

Hmm, looking at your pdf schematic here
http://www.hobby-boards.com/catalog/links/...20Schematic.pdf

Is there a way to set up the DS2450 for a bit better precision?
resistor changes, etc?

ericvic said:

It should be within a volt.

Eric

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If your HVAC monitor is not right for the situation, how about this?

I found a great arcticle on the maxim site on using the DS2438 for higher voltage battery packs over 10 volts.
There's a nice schematic listed here and easy to follow instructions

http://www.maxim-ic.com/appnotes.cfm/appnote_number/125

Does this sound like the way to go to measure my 24VDC solar batteries?

ericvic said:

We don't have a general AtoD 1-Wire device at the present. There may be others out there that do.

Eric

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As Eric stated, there doesn't seem to be an analog to digital converter compatible with one-wire devices. If you are interested in a general A-D device, there are plenty to choose from (most with serial interfaces though).

If you are interested you might want to post another thread (outside of the Hobby-Board's forum) and also glance at my Analog to Digital Converters Guide (will help determine overall accuracy to see if a solution is adequate for your needs).

As Eric stated, there doesn't seem to be an analog to digital converter compatible with one-wire devices.

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Not sure I understand this.
Isn't a DS2438 a one wire ad converter that can read volatges over one wire?

I'll have a look at your link
Thanks

EL34 said:

As Eric stated, there doesn't seem to be an analog to digital converter compatible with one-wire devices.

Click to expand...

Not sure I understand this.
Isn't a DS2438 a one wire ad converter that can read volatges over one wire?

Click to expand...

Yes, but, unless I'm not looking at this correctly,that is "just" the A-D chip (i.e. not a complete device you just power up and use).

Ah, ok, I see.

I am not neccesarily interested in a ready to plug in device.
It's nice if there is already one available, but if not, I can build electronic devices from scratch.

I am lazy and will pay more for something off the shelf and ready to go.

I am more interested in getting the core basics in place.
That link I posted above to the maxim site seems to be exactly what I am looking for.

AN125 shows how to select the correct resistors for what you want to do. Replace the resistors on Eric's HVAC monitor (and remove the series diode) or roll your own using a DS2450 or a DS2438. This is fundementally what I do to measure my security system 12V power.

Mitch

Ah yes, thanks, that confirms my thinking on that doc sheet.
Appreciate the info.

What kind of accuracy would you guess can be obtained setting that circuit up for 24volts DC measurment?

The AN125 doc sheet seems to indicate that you can get fairly accurate, but does not give any figures?

For higher accuracy measurements, each unit can be individually calibrated with a precision voltage reference and the multiplier data stored inside the DS2438's user EEPROM.

Click to expand...

I'b be happy with 1/10 volt

EDIT:
Just curious if you used the circuit shown below for your 12 volt system and if so what values did you end up using for R5, R6 and R7 ?

Ah, ok, I see where you are going with this now.

I'm not familiar with one wire products at all but I did glance over the application note #125 for monitoring voltages above ten volts, and the specs on the DS2438 analog to digital converter.

Again, I did this very quickly .

What you are really doing is changing the ratio of the input voltage to the voltage measured by the A-D converter with R5 and R6 in order to get a higher measurable range.

The app note suggests calculating R5 (R6 needs to be 10K) as follows:

R5 = R6 * (Vmax - Vvad)/Vvad

So if I am correct in the assumptions for voltages (your Max is 24 and the A-D Max is 10):

R5 = 10,000 (24 - 10)/10 = 14,000 or 14 Kohms (may have to series available resistors to get this value)

The actual voltage can then be calculated from the measured via:

Vactual = Vmeasured * (R5 + R6)/R6

Vactual = Vmeasured * (14,000 + 10,000)/10,000

Vactual = Vmeasured * 2.4

To determine accuracy the DS2438 is a 10-Bit Analog to Digital converter. If you looked at the Analog to Digital Converter Guide you would see ten bits is "2 to the 10th" power or 1024. This means the "measured" voltage (ten volts max) has a bit accuracy of 10/1024 or .01 volts.

Remember that this is the accuracy is for the voltage range into the A-D input, not the measured range so you have to multiply the .01 by the ratio setup with your resistors above, which was 2.4.

So I'm calculating the accuracy for 24 volt measurement to be .01 * 2.4 = .024 volts

I'd really appreciate someone else looking over these numbers since, as I stated above, I'm not familiar with one-wire products.