My home HVAC systems include electronic air cleaners that utilize air flow sensor modules to determine when to turn on and off. I have separate HVAC systems for upstairs and downstairs, so I have two modules. A module is a small daughter cards that plug into the power supply. Unfortunately, the thermistors that detect the air flow have literally fallen off of their leads!

The problem I'm having now is I don't know the specs for the replacement thermistors I need to buy, or how to ultimately recalibrate the modules. In the interest of trying to answer my own questions, I have been trying to analyze the circuit. While I can see a resemblance to some of the examples in the op-amp documentation on this site, I don't think any of the examples quite equate what I'm seeing. Any guidance on how to repair the modules or continue with my analysis is greatly appreciated!

As far as what I know about the original thermistor, the documentation indicates that it self heats to around 130F. I know that the thermistor is an NTC type since the resistance increases as the temperature drops. Also, I was able to measure the resistance of one of them at 115 ohms at ~25C, and 150 ohms at ~0C. Unfortunately, I unsuccessfully tried to resolder the leads on the other one and think I damaged it in the process, as it provides really bizarre readings.

As far as the circuit, I have included my Spice net list below. Basically, there are two separate voltage dividers that are used to feed the Vin+ and Vin- pins on an LM358 op-amp. The op-amp is configured with positive feedback. I can't quite figure out what the op-amp is measuring the voltage difference between though. I have figured out that if I make the resistance of the thermistor above 60 ohms then Vout turns on (except exactly when the thermistor equals 68 ohms, which I don't understand at all).

Here is my Spice net list:

Air Flow Sensor Module

# Actual circuit has 24VAC input converted to 18VDC
vdc 4 0 dc 18

# First voltage divider connected to Vin- on op-amp
# R2 is thermistor; ~115 ohms @ 25C
r2 4 5 60
r3 5 0 68

# Second voltage divider connected to Vin+ on op-amp
r4 4 6 5.1k
# R5 and R6 are two halves of 5.0k ohm potentiometer
r5 6 7 2.5k
r6 7 0 2.5k

# LM358 Dual Operational Amplifier
# Pins are (Vin+, Vin-, Vcc+, Vcc-, Vout)
x3 7 5 4 0 8 LM358
r7 7 8 2.76m
# X4 appears to be unused part of op-amp despite it is not floating
#x4 0 9 4 0 9 LM358

# D2 is infrared LED part of a 4N32 optoisolator
r8 8 10 1k
d2 10 0 dref

.INCLUDE LM358.LIB
.MODEL dref d
.TRAN 0.01m 100mm
.END

Thanks!

Chris

 

How about you force the modules "on" and wire their power to the fan motor leads? It doesn't hurt an air cleaner to be on for a few seconds until the air gets some velocity developed.

 

The obvious answer is to contact the manufacturer. You probably already tried this route, but you didn't say anything about it (and you might not even be able to identify the manufacturer if you're just looking at a PCB).

You seem to have two temperature points to the thermistor's resistance curve; that may be enough to identify it (you also have the self-heating statement that may indicate the current through it). I'd suggest contacting Measurement Specialties, as they may be able to help you identify the thermistor (they recently helped me identify some 30 year-old YSI thermistors from the specs and they bought YSI a while ago).

 

How about you force the modules "on" and wire their power to the fan motor leads? It doesn't hurt an air cleaner to be on for a few seconds until the air gets some velocity developed.

Yes, it is a possibility. I'm a little squeamish about this idea though as I found a tag in the attic with my air conditioning equipment specifically warning against doing this if you have a multi-speed blower, which I do. Of course, the circuit board that controls the system has leads for doing this, so maybe it would work fine.

 

What? Is the manufacturer afraid the electronic air cleaner will come on when the fan is on a low speed? or high speed? That doesn't make sense. What brand is this air cleaner?

I've been installing electronic air cleaners for 30 years, and I've never heard of one that fails at certain fan speeds. Is the air cleaner's supply voltage connected to the fan motor? Does it use 120v or 240v?

 

Bychon,

The air cleaners are branded by Carrier, but I believe they are actually manufactured by Trion. While the face plate is a little different, everything else is the same as the Trion HE 1400 model (see http://www.trionparts.com/he1400-trion-whole-house-electronic-air-cleaner.html). Both air cleaners currently have a regular power cord and currently plug into a dedicated 120V outlet.

Could the concern be that if I attach directly to the blower that I might not get the proper voltage if it is not running at full speed, or that the voltage will fluctuate? I'll have to measure to see if the voltage stays constant when the blower is on at the place I could wire in an air clearner.

 

The obvious answer is to contact the manufacturer. You probably already tried this route, but you didn't say anything about it (and you might not even be able to identify the manufacturer if you're just looking at a PCB).

Yes. I tried contacting the manufacturer but the best they did was to offer to sell me some new modules at around $50 a piece. I figure if I could determine what part I need it should cost less than $5 per module to fix. In one sense, I'm sure the $50 is a good deal given how much time I have spent messing with this. However, this has been a good chance for me to revisit my electronics knowledge and learn some new things. It has even inspired me a bit to maybe try designing and/or building some circuits on my own.

I'll try getting in touch with the vendor you mentioned to see if they have any ideas.

 

This is my first time using Spice or any of its variants (i.e. I'm using LTSpice). While I think it is an impressive tool, I found that once I start adding ICs to my model that I can get very inconsistent results. For example, I see quite different behavior in the circuit I provided if I use the National Semiconductor LM358 model vs. the On Semiconductor LM358 model vs. the Linear Technologies LT1413 equivalent model. I see even different results if I incorporate the real power circuit that converts 24V AC to 18V DC using a TIP122 power transistor in the process. Is this a limitation of Spice or the models I'm using? Or, is it something else altogether?

 

Bingo. "Contains optional air flow sensor". The key word being OPTIONAL.

Now, go find out if you have a multi-winding fan motor that always gets 240 volts. Whether it does or not, there is someplace in there that switches power when the fan comes on. If you can make spice models, you can wire an outlet.