Analog Input circuitry.

Hi CAI guys. Hope you can clarify for me some things...

You published the A/D input circuit and protection being:


Subsequently, you've said you've changed the protection circuitry which has substantially improved the old crushing problem.

Can you please confirm what the current input arrangements really are, and what the actual A/D input FSD is? I'm guessing its 0 to 3.3V for a count of 0-1023, but the actual voltage will be important for me!

My terminal boards have an amp onboard. Running off the WC internal power (5V), I'd expected to get "close" to a 0-5V, but reality is that its closer to 3.5V with the LM2902.

Option 1 is to run the amp off the webcontrol DC power in at 9 to 12V and get close to 0-10V in.
Option 2 is to change the input resistors and aim for 0-3.5V or thereabouts.

Thanks...

The WebControl8 A/D input circuit is exactly as shown in your posting. R25, R27, and R29 are 91k instead of 82k. You can change R26, R28, and R30 to reduce the full scale range from current 0-10V to 0-5V. With hw rev 2.2.2 board, the A/D input can get very close to 0-1023, protection is clamped to 3.5V. But A/D input max reading is 3.3V or 1023.

CAI_Support said:

The WebControl8 A/D input circuit is exactly as shown in your posting. R25, R27, and R29 are 91k instead of 82k. You can change R26, R28, and R30 to reduce the full scale range from current 0-10V to 0-5V. With hw rev 2.2.2 board, the A/D input can get very close to 0-1023, protection is clamped to 3.5V. But A/D input max reading is 3.3V or 1023.

Click to expand...

OK, so neglecting the input impeadance of the A/D channels themselves, 200K + 91K divider string with 10V will give 3.1V at the divider point, which would only be a count of 961 (not 1023).

To get 3.3V at the input with 10.0V, the high-side divider would need to be around 185K. Or have you also done that?
The high side resistor would need to be about 49K for a 0-5V range.

What's actually installed now? Is it 200K? Paralleling a 65K across the existing 200K would fix it, but looking at the board, that isn't practical. Replacing the SMD resistor is the only real answer.

Maybe I'm not seeing the full picture looking over this quickly, but why not just use a voltage divider before getting to the (A1 in the example below) input of the WebControl channel?

You might have to adjust the A-D conversion, but this would be the same ratio as well, or just calibrate it in-situ by assuming linearity and use a few test voltage points to get a voltage vs bit graph/equation.

rossw said:

OK, so neglecting the input impeadance of the A/D channels themselves, 200K + 91K divider string with 10V will give 3.1V at the divider point, which would only be a count of 961 (not 1023).

To get 3.3V at the input with 10.0V, the high-side divider would need to be around 185K. Or have you also done that?
The high side resistor would need to be about 49K for a 0-5V range.

What's actually installed now? Is it 200K? Paralleling a 65K across the existing 200K would fix it, but looking at the board, that isn't practical. Replacing the SMD resistor is the only real answer.

Click to expand...

if 3.3V across 91K , the current is 3.626e-5A. If you want to have 5V full scale, then 5-3.3=1.7V. V/I = R, so 1.7V / 3.626e-5A = 46.879K
1/(1/200K + 1/Rx )= 46.879K
Rx = 61.231K

62K resistor is probably the most closed one.
http://www.ebay.com/itm/251074889117

BraveSirRobbin said:

Maybe I'm not seeing the full picture looking over this quickly, but why not just use a voltage divider before getting to the (A1 in the example below) input of the WebControl channel?

You might have to adjust the A-D conversion, but this would be the same ratio as well, or just calibrate it in-situ by assuming linearity and use a few test voltage points to get a voltage vs bit graph/equation.

Click to expand...

I think Ross wanted to keep using the existing 16 pin header. Solder one SMD resistor on top of another is not that hard, just make sure do one end first, then next end.

fwd03 said:

I think Ross wanted to keep using the existing 16 pin header. Solder one SMD resistor on top of another is not that hard, just make sure do one end first, then next end.

Click to expand...

Yes, correct. Actually, I didn't want to modify the boards at all. Custom boards makes for a support nightmare.
I'd personally be a lot happier if they were shipped as 0-5V or 0-3.3V full scale in the first place

But we have to work with what we have, so 10V it is.

My terminal/amplifier board can directly pick up the 5V board power via the 16-pin header, so using 5V seems easiest.
Amplifying input signals to make them 0-5V is ok, although we're wasting half the ADC scale (making it really only 9-bits). We can live with that for convenience but it isn't ideal. But a standard opamp like the LM2092 cannot pull output to +5V, only to about 3.7 or 3.8V maximum. So we're even further reducing our resolution.

I have ordered some pin-for-pin devices that will output rail-to-rail (or within about 10mV, so close enough). At least then I can get 0-5V full scale, without having to modify the board.

The alternative to that is to power my opamp off the webcontrol raw input which could be 9 to 12V. If it is running on 12V there is just enough to get a full 0-10V output. But that means running higher voltage to the WC board than I would prefer (which has to be dissipated as heat).

fwd03 said:

if 3.3V across 91K , the current is 3.626e-5A. If you want to have 5V full scale, then 5-3.3=1.7V. V/I = R, so 1.7V / 3.626e-5A = 46.879K
1/(1/200K + 1/Rx )= 46.879K
Rx = 61.231K

62K resistor is probably the most closed one.
http://www.ebay.com/itm/251074889117

Click to expand...

I was trying to keep it around four volts as the OP stated:

I'd expected to get "close" to a 0-5V, but reality is that its closer to 3.5V with the LM2902.

Click to expand...

So I just wanted the full scale to be slightly above 3.5 volts.

Brave,

You are correct. Without external power, 5V powered op amp will only get close to 3.5V range anyway.

Ross,

If you are off grid, using a 5V DC/DC converter to replace the on board LM2940S could allow you use 12V or higher power supply without too much energy loss on the regulator.
I am interested in learning how well your solar panel monitor work, since I brought some solar panels lately, want to have someway to monitor them and/or track the sun. I am thinking to use a DC/DC conveter directly taking power from solar panel to power the WC board. When no power generated, there will be no power consumed.

F.W.D.

fwd03 said:

If you are off grid, using a 5V DC/DC converter to replace the on board LM2940S could allow you use 12V or higher power supply without too much energy loss on the regulator.

Click to expand...

I have been using a DC-DC converter on this particular WC board for about 8 months, and it has worked fine. I should point out that this particular WC board (one of many around my place) does little apart from control the position of all my solar trackers, park them overnight, monitor wind and put them in "safe" position during high wind, etc.

The same board (since its in the solar enclosure) is planned to also measure the solar inputs, hence the amplifier boards and an additional 4-channel A/D input. That gives me a total of 7 channels. Just enough to monitor current from 6 arrays and the input voltage (which varies between about 50 and 150 volts DC)

I am interested in learning how well your solar panel monitor work, since I brought some solar panels lately, want to have someway to monitor them and/or track the sun. I am thinking to use a DC/DC conveter directly taking power from solar panel to power the WC board. When no power generated, there will be no power consumed.

Click to expand...

I have had to disconnect the DC-DC converter and replace it. I need an isolated supply, because in order to measure the amps, I need to get the WC board ground to the current shunt common, and since my solar panels run through a MPPT charge controller and then my inverter/charger there is a variable offset between +0.5 and -0.5V from battery negative. (the solar arrays are typically about 6 amps per array at nominally 105V DC, for a current shunt voltage of around 45-50mV).

I only have 3 arrays logging at the moment, and for the last 3 days we've had substantial broken cloud making the readings hard to interpret. It's 7am (26/dec/12) now and we currently have a clear sky - so with luck I'll have some better data to post later today.

Here's a couple of traces from today. They're pretty grotty, but it'll have to do, I've been busy with other work.

The webcontrol board that records the array current is the same one that does all the tracker control. Here is its view of the tracker position during the day:



And here is the current (remember, this goes into an MPPT controller, the voltage isn't currently monitored so this is purely current, not watts)


You can see for the first 90 mins or so after sunup at this time of year, the "midwest" tracker is in shaddow and its output is far below what it should be. I'll try to grab a picture of the output from the pyranometer for the same time.

Forgot to add the pyranometer output.....


And for the curious... here is one of my trackers, showing the "working parts"

Ross,

You got more than 12 hours generation in summer there, that is really good. Don't know if your tracker also track the sun elevation, which changes a lot between 6/23 and 12/23.

You could have TTL output to control which analog input to read, by using only one A/D input with IC like this:
http://www.analog.com/en/switchesmultiplexers/analog-switches/adg5233/products/product.html
or
http://www.analog.com/en/switchesmultiplexers/analog-switches/adg1612/products/product.html

The WebControl internal logic actually do over sampling, so that A/D resolution is more like 12bit. You do need to adjust R10 to get A/D noise to 0. The best value for R10 is 2.26K, however, your board may be between 2.0K to 2.4K to get the optimal zero noise level. All current shipping board already have the R10 value adjusted to near zero.

You could make a product by providing tracking control to people. From your chart, your solar arrays produce twice more than those fix mounted panels.
If you need any help, we will definitely work with you.

Happy Holidays!

CAI Support

Ross,

If you add a BCD decoder, with 3 TTL output, you can switch 8 analog channels:
http://www.jameco.com/Jameco/Products/ProdDS/12909.pdf

Hope this helps.

CAI Support

rossw said:

Here's a couple of traces from today. They're pretty grotty, but it'll have to do, I've been busy with other work.
The webcontrol board that records the array current is the same one that does all the tracker control. Here is its view of the tracker position during the day:

And here is the current (remember, this goes into an MPPT controller, the voltage isn't currently monitored so this is purely current, not watts)

You can see for the first 90 mins or so after sunup at this time of year, the "midwest" tracker is in shaddow and its output is far below what it should be. I'll try to grab a picture of the output from the pyranometer for the same time.

Click to expand...

Sorry, off topic but what are you using to graph stats from the board?

MobileMe said:

Sorry, off topic but what are you using to graph stats from the board?

Click to expand...

A system I wrote many years ago. It logs, graphs and alarms something like 920 conditions (not just WC boards).