0
\$\begingroup\$

I want to use this sensor by using 5V supply voltage and I do the connections as shown in its datasheet. According to he data-sheet the minimum load is 80k which means I should use less than 80k. But when I use different resistor values I get very different outputs and Vsupply is 4.96V in both cases:

enter image description here

For instance, using a 15k load I get 1.08V output; and if I use a 22k load I get 1.48V output. Both loads are less than 80k but I get very different results. What could be the reason?

\$\endgroup\$

3 Answers 3

1
\$\begingroup\$

Probably they mean minimum load resistance. It’s not 100% clear from the datasheet, but they don’t specify a maximum output current or equivalent drive capability so it has to be that.

\$\endgroup\$
1
1
\$\begingroup\$

Duplicating my own answer from an earlier question on the same subject:

I have no experience with the HIH 4000 (or other humidity sensors,) but I thought I'd have a look and see what I could find.

From what I have found, the datasheet isn't telling you that you need to put an 80k resistor from the output to ground.

What the datasheet trying to say is that the load impedance connected to its output must be higher than 80k ohms.

This thread on the Velleman forums refers to an ADC module with a 20k ohm input imopedance. An HIH 4000 connected to that module (the Velleman K8061) would consistently show a wrong output (as measured with a voltmeter) when connected to the K8061, but would show a proper voltage when disconnected from the K8061. The recommended solution was to remove the 20k resistor in parallel with the input on the K8061. Alternatively, an op amp could be used to buffer the HIH4000 output.

There's a similar problem and solution with the Velleman K8055 mentioned in this forum post.

Something similar happened with an ADC for a Raspberry Pi. Again, the solution was to increase the input impedance of the ADC module.

Summary: The 80k is the minimum input impedance of the ADC or buffer amplifier you connect to the sensor.


Given that you have a high impedance output coupled to a high impedance input, you have a very good chance of picking up stay noise over the wires from the sensor to the ADC. The solution to that is to use the minimum load allowed in all cases (80k in parallel with the ADC input) or to install a buffer right at the sensor output so that you have a lower impedance line going to the ADC.

\$\endgroup\$
1
  • \$\begingroup\$ I flagged the other question to be closed as a duplicate of this one. \$\endgroup\$
    – JRE
    Commented Jul 20, 2020 at 18:32
0
\$\begingroup\$

It is apparent from your test that, the lesser the load resistance relative to 80kΩ, lower the output voltage.

Hence it would not be incorrect to presume that the value of the load resistor should be 80kΩ or greater.

You may confirm that by checking the output voltage at 68kΩ, 82kΩ and 100kΩ.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.