24Vac to 5Vdc Power Supply Analog Humidistat

Thread Starter


Joined Apr 11, 2022
24 Vac Analog Humidistat.JPG
I've tried powering this circuit with 2 configurations: one with 120vac where the power module (RAC03-05SK) converts it to 5vdc, 600mA, and another with 24vac (from a transformer) connected to a full bridge rectifier + a DC/DC stepdown (EPM78V2-05R0-01R0R, 5vdc, 1A). The circuit with the power module works (as I adjust the potentiometer the relay switches from NC to NO) with a 24vac hydraulic actuator I tested.

As I was switching between 120vac to 24vac to test the circuit, I forgot to connect the power module to 120vac (85-264vac input voltage range based on spec sheet) and left it at 24vac. I saw that the output with the power module was still 5vdc and the circuit was producing the desired result. Is it appropriate to a power module below the input voltage range? Is there a power module that takes in 24vac up to 120vac or so?

In the 24vac configuration with the DC/DC stepdown, I still get the 5vdc output, but as I adjust the potentiometer the voltage of the DC/DC stepdown drops to as low as 2.5vdc or so. I don't get this issue with the power module. I'm not entirely sure why this is happening, but ideally I would like to get this circuit to work with a 24vac source where the full bridge rectifier + DC/DC stepdown powers the rest of the circuit.

R1 and R2 were used to coincide with the humidity sensor output voltage (0% RH = 0.1652*Vsupply, 100% RH = 0.7952*Vsupply)



Joined Nov 6, 2012
What is this Circuit supposed to do ?
Why is it so complex ?

What is the box ..... "Terminal" ?,
it appears to be another Relay, or a part of a different piece of equipment.

Why does your "Potentiometer" have 5-Connections,
( what is it really, and why are You using it ).

What is "Trimming-Resistor" for ?

Do You have a Spec-Sheet for your "Humidity-Sensor" ?

Even though your Power-Supply may still seem to work on ~24-Volts,
it may not be reliable at that low-Input-Voltage, ( minimum is ~85-VAC ).

Your chosen Comparitor will drive your small Relay directly, without an external Transistor,
but there are quite a few different ways to accomplish what You want.

Thread Starter


Joined Apr 11, 2022
This is a circuit for an analog Humidistat. From researching online I found that roughly 90% of all humidistats are powered under 24vac since the solenoid valve in typical HVAC systems are powered with 24vac. All this circuit is doing is turning the humidifier (or more accurately the solenoid valve) on or off. The voltage comparator takes the humidity sensor output voltage (anywhere from 0.826vdc (0% RH) up to 3.76vdc (100% RH)) and compares it with the potentiometer voltage (currently the range is around 1vdc to about 4vdc).

Vin>Vref = humidifier on
Vref>Vin = humidifier off

The box labeled "Terminal" is simply a terminal block. I was considering having two 3 pin terminals, one for power (24VAC, 24VAC, GND), and the other for the relay (COMM, NC, NO). A user would simply connect the 24vac terminals for power, and then COMM + NC or NO from the solenoid valve.

The spec sheet for the humidity sensor can be found here: https://prod-edam.honeywell.com/con...ries-product-sheet-009017-5-en-ciid-49922.pdf

The trimming resistor is a hysteresis resistor. You can see the effect with and without it below (multiple vs. single transition states).

Trimming resistor.JPG

The potentiometer is being used with a dial to easily adjust the voltage. I'm also using this because it's relatively easy to incorporate this into an enclosure using a dial.


Joined Jan 23, 2018
If the power module you are using is intended to accept either 120 volts AC or 24 volts AC with only a switch to select the input voltage, I wonder about it. If it is providing a 5 volt DC output while set for a 120 volt input but only getting 24 volts in, my guess is that the current being drawn is not much.
So yes, it is reasonable that it can still function, but probably it will not be able to supply the rated current until the correct input voltage setting is used.


Joined Jul 29, 2018
Your trimming resistor at 20K is way too low a value, and is causing a huge amount of hysteresis. For an LM311 you need no more than maybe 20 mV hysteresis (unless the sensor is really noisy). The "trimming resistor" is fed with a 5V swing, and sees a load in the 2K-8K range. 20 mV across 2K is 10 uA, to get a step of 10uA from a 5v swing requires 500K Ohms. If you need more hysteresis drop the resistance some, but I doubt you will need to go below 100K.