Tutorial - Voltage divider - Measuring Voltages greater than 5vDC

greenman100 · 02-29-2008, 01:41 PM

Often times, there is a need to measure a voltage greater than 5volts.

As long as the ground of the device we want to measure is the same as the ground of the Fusion Brain, we can use a voltage divider. WARNING: If the grounds are not the same, damage can be done to the Brain.

Here is a simple voltage divider:

Vin is what you want to measure, Vout goes to the input pin of the brain (towards the edge of the board) and the symbol at the bottom indicates ground. R1 and R2 need to be selected on a application by application basis.

This formula will help select R1 and R2. For the Fusion Brain, it is a good idea to keep R1 and R2 under 1k-ohm.

Example
Lets say you want to monitor the voltage in your car:

We will assume 20vDC maximum, to allow headroom for voltage spikes.

Vout = 100/(300+100)*Vin
5 = 1/4*20

so the voltage divider, with a 300ohm R1 and a 100ohm R2, would have a scaling factor of 25%, meaning we could measure up to 20vDC without overloading the brain... perfect for car applications.

booksix · 11-04-2008, 04:05 PM

what would be needed to allow FB to monitor voltages up to 400 volts? Is this too much? Would an external device (such that a volt gauge would have) made for high voltages be needed?

2k1Toaster · 11-04-2008, 04:14 PM

Quote: Originally Posted by booksix

what would be needed to allow FB to monitor voltages up to 400 volts? Is this too much? Would an external device (such that a volt gauge would have) made for high voltages be needed?

What are you measuring that is up to 400v?

But it is the same idea however I would recommend a buffer on between the divider and the Brain just in case. So if something spikes on either end, buffer blows up, not the FB or what is sending the 400v.

greenman100 · 11-04-2008, 04:25 PM

you need a transformer and a rectifier to measure that safely.

RonS · 11-04-2008, 07:51 PM

Are you using 12 volt batteries to get to that voltage. This is for your car. Right?

booksix · 11-05-2008, 11:01 AM

yeah, it's for the Z3. Planning a 3-phase ac drivetrain that may go as high as 400 volts. But it'll more likely be 3.2v lithium batteries strung together. Just curious if it can do it... My eventual goal is to replace the stock gauges with an LCD display with custom graphic gauges made to mimic the factory stuff but read things like 0-12k rpm, speed, traction pack and 12v accessory batt voltage, amperage (draw from the traction pack) motor temp, etc...

So why the rectifier? Thats to convert ac to dc right? The batteries will be dc already... Maybe you were thinking I was talking about a home solution?

And one more thing... the transformer: So, from what I get, I'd put the transformer inline with the 400v and the secondary circuit will see a voltage which has been stepped down to a voltage the FB can handle and this voltage (say, 0-4v for example) would represent the actual 400v (1v=100v, 2v=200v, etc...)?

greenman100 · 11-05-2008, 01:16 PM

yeah I figured anything that high would be AC.

if you are doing DC a voltage divider would work in theory but be dangerous if implemented wrong or if it fails/shorts.

booksix · 11-05-2008, 02:06 PM

well, the issue is, I think I'd want to measure it on the DC side, not the ac side because I want to measure the voltage of the combined DC pack.

RonS · 11-05-2008, 03:01 PM

I wonder if measuring one of the 3.2's would work. Just watch the voltage on that. Or multiply the voltage of one battery by 125 to get the total. Not sure how good that would work.

booksix · 11-05-2008, 04:39 PM

could work but the cells don't stay perfectly balanced so one cell may be an inaccurate measurement

RonS · 11-06-2008, 09:34 AM

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.

booksix · 11-06-2008, 09:37 AM

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?

RonS · 11-06-2008, 11:15 AM

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.

booksix · 11-06-2008, 11:33 AM

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!

greenman100 · 11-06-2008, 11:50 AM

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

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

User Name		Remember Me?
Password