# 4 Channel Input Isolator and Voltage Divider

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• 08-08-2010, 03:14 PM
PaulF
How many watts does each resistor dissipate?
• 08-08-2010, 03:16 PM
2k1Toaster
Completely dependent on the input voltage.

P = I*R = V^2/R
• 08-08-2010, 03:18 PM
2k1Toaster
And blue resistors? Do you have the 3.3v version?
• 08-09-2010, 01:56 PM
PaulF
Quote:

Originally Posted by 2k1Toaster
And blue resistors? Do you have the 3.3v version?

Maybe! That would make sense, since right now the divider isn't making any sense at all.

I just looked at the Mp3Car store image. The board is different from what I received.

What I received looks like the following, with big blue resistors in the circled locations which I'm assuming is not the default configuration. Especially since moving the jumper barely makes any difference and no matter what voltage I feed (between 12 and 14 volts) it shows roughly 4.34 - 4.44 on the 5v output side and I couldn't figure out why the range was so small.
http://www.mp3car.com/vbulletin/imag...0407b4d0d0.jpg
• 08-09-2010, 02:05 PM
2k1Toaster
Yup, you have the 3.3v version... Not sure how that happened. Just clip the resistors and that should help.
• 08-09-2010, 02:13 PM
PaulF
Quote:

Originally Posted by 2k1Toaster
Yup, you have the 3.3v version... Not sure how that happened. Just clip the resistors and that should help.

Yeah weird, something must have ended up in the wrong box. It doesn't even look like the Mp3Car store sells the 3.3 version though. Hmmm. Oh well, I'll just clip the resistors.
• 08-09-2010, 02:20 PM
2k1Toaster
Quote:

Originally Posted by PaulF
Yeah weird, something must have ended up in the wrong box. It doesn't even look like the Mp3Car store sells the 3.3 version though. Hmmm. Oh well, I'll just clip the resistors.

We do not sell the 3.3v versions to anyone but a very specific couple of clients who they were special made for. That is why this is somewhat confusing.
• 08-11-2010, 08:13 PM
PaulF
Okay I clipped the resistors, much better as far as heat goes haha

Now whats the official equation to get the voltage for 15 volt mode. Testing with a multimeter, I came up with
volts = sensor * 2.96 + 9.63

It seems that the accuracy is good to within about +/-.1 volts. Is there a better equation?
• 08-13-2010, 03:02 PM
PaulF
That accuracy is actually worse than I thought. It's way off for higher voltages like 13.5+

So I'm going to assume there is a much better equation which probably isn't y=mx+b
• 08-13-2010, 04:25 PM
2k1Toaster
http://a.imageshack.us/img208/3199/opto12v.jpg

y = 3E‐06x4 ‐ 0.0004x3 + 0.0183x2 ‐ 0.1488x + 0.2318

y = What the FB reads
x = Input voltage

This is for the 12v jumper setting
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