# Thread: Tutorial - Voltage divider - Measuring Voltages greater than 5vDC

1. I was thinking about that. You could pick 4 cells in different parts of the pack. Then average the results. Then take that number as the condition of the pack. If you think that would work. I would use 4 inputs so I could monitor the voltage directly and not have to use a divider.

2. I could do that but it's have to be 4 batteries one after the other, not random, but this could still work ok... Any other thoughts on options?

3. right now .. No.. I will keep thinking. But 400 volts is a little tough. I was thinking about measuring across a shunt. But that would vary as the load went up and down. Hmmmm How about a voltage to frequency converter going to a frequency to voltage converter. Kind of a strange way to do it but it might work. The LM231 will take 40 volts. So a 10 to 1 divider would work. I will have to look at that. I am not real sure if it would work for you.

4. Varying readings should be ok. I think, even analog volt gauges for this application drop when load is applied. But I could be wrong, I'll check while I'm working at the EV shop tomorrow.

edit: btw the way, thanks for the help! I've never studied anything electrical or computer related so my knowledge of circuit and components is very limited. I've gone to wikipedia twice for this thread already! I know... and I'm planning a high voltage electric car! LOL Please bear with me!

5. Really, the best way is a shunt driving an optoisolator.

http://en.wikipedia.org/wiki/Opto-isolator

6. No problem. But if you read voltage across the shut won't the gauge work like an amp meter? I might have that wrong and it would not be the first time. But the optoisolator would be a very good idea. You would not have to worry about the voltage on the high side getting to the FB.
Oh greenman100. Do you know how the fan speed switch works? Does it just add 12 volts to another set of wires. Or does it turn off the first pair of wires and turn on the second. Sorry to add another question to this thread but I was just wondering.

7. I was wondering about the shunt setup readng amperage as well. But the optiosolator makes sense... Kind of:

So the voltage goes through the optical side (led) and this signal carries to the photocell which allows the unit to act like a variable resistor? So the input of the photocell side would see something like 4v which would be allowed to pass to the FB in varying amounts based on the intensity of the light...? So how does this tiny isolator/led handle 400v?

8. Originally Posted by RonS
No problem. But if you read voltage across the shut won't the gauge work like an amp meter? I might have that wrong and it would not be the first time. But the optoisolator would be a very good idea. You would not have to worry about the voltage on the high side getting to the FB.
Oh greenman100. Do you know how the fan speed switch works? Does it just add 12 volts to another set of wires. Or does it turn off the first pair of wires and turn on the second. Sorry to add another question to this thread but I was just wondering.
I used the term a little too loosely to describe a voltage divider.

Really, you're going to want a voltage divider, about 10:1. Feed this into the optoisolator/optocoupler,

something like this: http://cp.literature.agilent.com/lit...989-2137EN.pdf

9. thanks Greenman! I'll read up after work

10. Looks pretty good, but unfortunately it still goes over my head at this point. Was I at all close in my description of how these things work (above)?

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