measuring dc voltage higher than supply

littlerich

I would like to be able to read a dc voltage that is higher than the supply used for the basic stamp.

Infact they will be separate supplies.

I assume the first step is to tie the grounds together.

I am talking 60vdc or so but I assume the principle will be the same.

I have seen howto's on RC time and voltage dividers but the all seem to revolve around essentially measuring the supply feeding the circuit board. ie for monitoring the battery supplying the project.

Can anyone point me to a howto or give me some pointers. I think it would be nice to have some isolation strategy.

Thanks
Rich

Leon

Just use a voltage divider between the 60V supply and ground (two resistors) to give you 5V maximum. What accuracy do you require?

60V can be dangerous, BTW.

littlerich

Thanks Leon,
Accuracy well 1 volt would do but half or tenth of volt would be nice just because one can.

Does this change the approach?

Cheers
Rich

Leon

It affects the tolerance of the resistors. You need to check whether 1% are OK, or whether you need 0.1%.

ElectricAye

With a voltage divider, I think you also want to consider how much current you'll be drawing out of that 60 volt supply. Use Ohm's Law, V=IR, for that. For example, if you were to build a voltage divider out of two resistors that, in series, totaled only 10 ohms, you would be drawing 6 amps through your voltage divider. Since Power = VI, that would be 360 watts, which would melt, if not vaporize, resistors rated at half a watt.

On the other hand, if you use resistors that are too high, then you might get into troubles with temperature dependency, etc. Using 10 Mega Ohm resistors would NOT be the answer.

You need to find a "safe" level of current you can draw from your source without frying the resistors or sagging the power supply or taking forever for your RC circuit to charge up, etc.

I'm just guessing that something in the 10K to 50K range would probably be a design point to begin with, but crunch through the numbers to see what your system can deal with.

Hope that helps,

bdickens

I was about to build an electric boat "energy gauge" using this from Sparkfun

http://www.sparkfun.com/commerce/product_info.php?products_id=9028

Other projects preempted but you might check out their approach.

I was pumping 48V at 60amps

Thanks