Reading a large current into labview

[canoo]

I have a 200W generator (generates around 14V). I'd like to measure how much current it can produce. It needs to be read into labview. I have a NI USB-6008 DAQ reader. Am I correct in saying that I need a known resistance that can handle 200W then read the voltage over the resistance into labview and do I=V/R? Or is there an easier way without having to use a 200W resistor bank.

Thanks in advance.
- Jon

 

[Nigel Goodwin]

If you want to measure it's performance under full load, then you need a suitable load - you can work out the required resistance easily enough, and ensure it will withstand more than the 200W available.

 

[canoo]

Ok, so there is no easier way to do it without buying a large power resistor that can handle 200W?

 

[Nigel Goodwin]

Just find something of the correct resistance, that will stand 200W.

As you're talking 14V, car headlight bulbs in parallel should do.

 

[canoo]

oh wow yeah i didnt even think of that. i have some extra ones laying around too. thanks!

 

[crutschow]

If you use bulbs for the load, you'll need a small shunt resistor of known value in series with the bulbs to measure the current accurately, since the bulb resistance varies greatly with load.

 

[Nigel Goodwin]

If you use bulbs for the load, you'll need a small shunt resistor of known value in series with the bulbs to measure the current accurately, since the bulb resistance varies greatly with load.

No you don't, what do you think your meter does on the amps ranges?.

Be best to have two meters through, one to measure the current, and one to measure the voltage.

 

[crutschow]

He wants to read the values into Labview using a DAQ, thus I assumed he needed a voltage and therefore a shunt. But he could read the small voltage across an ammeter shunt, if he has one that will measure 17A or so.

 

[canoo]

So i can't just read the voltage over the bulbs into labview and have labview calculate a current value based on the read voltage and the measured resistance of the bulbs?

 

[crutschow]

So i can't just read the voltage over the bulbs into labview and have labview calculate a current value based on the read voltage and the measured resistance of the bulbs?

Not with a single point measurement. The resistance of an incandescent lightbulb can change on the order of 1 to 10 when going from cold to hot.

One solution would be to measure the bulb current at various voltages. You then use that table of voltage and current to extrapolate the bulb resistance between the measured points, which a computer program can readily do, or generate a best fit equation that gives the current versus voltage (Excel can perform that function from a table of data).

Edit: If you use Excel to get an equation, generate an X-Y (scatter) chart of the data. Then do an Add Trendline to the chart (try various types to get the best fit) with the Option of "Display Equation on Chart".

 

[canoo]

One solution would be to measure the bulb current at various voltages. You then use that table of voltage and current to extrapolate the bulb resistance between the measured points, which a computer program can readily do, or generate a best fit equation that gives the current versus voltage (Excel can perform that function from a table of data).

Hmm ok. I may have to hook the bulbs up to a power supply first and collect that data. The voltage output from my generator is not very consistent at all.

Thanks for your help I understand what I'm doing now. I'll be back if i run into any issues.