Electronic – Not enough current for voltage divider

sensorvoltage dividerxbee

I'm trying to use a HIH4030 humidity sensor, where the output value is provided to a XBee.

Considering that output sensor max output voltage is around 4V and the XBee ADC only supports 1,2V, I took some time to choose the correct resistors for a voltage divider to get a decent ratio. I end up choosing a 120 ohm and a 270 ohm resistors to get a ratio of 4V to 1V.

And in this case I wasn't getting any values on my XBee. When I measured the signal, I had about 31mV going out the voltage devider when I should have around 800mV.

Now, correct me if I'm wrong (and there's a good chance that's the case), but the reason for these low values are due to the low total resistance value (R1+R2 = 390ohm) and the low current value going out the sensor. The HIH4030 specs say that the current supply varies from 200uA to 500uA, so I'm assuming that from this sensor I won't get higher currents than 500uA. Is this a correct assumption?

Anyway, taking this assumption as the correct one, through ohms law,

V=RI. V=390*0.0005, thus V=195mV which is hardly enought.

I was considering getting higher resistance values, at least to a total of R=1,2/0.0005.

Am I proceeding correctly?

Best Answer

Yes, you're on the right track. The values in your voltage divider should be driven mainly by the characteristics of the ADC on the XBee.

Typically, the ADC on a microcontroller requires a source impedance no higher than about 10 kΩ. You could use 27 kΩ and 12 kΩ in your divider; the resulting source impedance would be about 8.3 kΩ, well within spec.

The load on the sensor would be 39 kΩ, drawing just over 100 µA at 4V.