electronic txv
 

Anyone played with building an electronic txv using surplus regular bulb txv?

E.g . NTC varistor sensors and feedback circuits with op amps driving a small heater on the txv bulb to drive SH to lower than txv mechanically set value.

Thought I'd ask, why re-invent the wheel, etc...

Sounds like a good way to sabotage a working unit to me.

Brad_C (from Australia) did some interesting stuff with one he salvaged from a scrap unit. He controlled it with a PIC controller, and wrote a sort of fuzzy logic algorithm to control it.

There was another Aussie, who posted on the 'Manifesto' quite a while ago, who was intending to use a stepper motor to directly dial the valve. Don't know how far he got.

But I think your idea sounds pretty good. There might be an energy-loss issue from the heater, but I couldn't begin to estimate what it could be.

-AC

Ummmm, why when an EEV is common and relatively cheap. Use the Arduino or Raspberry to drive it.

I'm doing that on my experimental unit, by simply leaving the TXV set to a high (20F or so) superheat and using a resistor as a small heater. The resistor must make good thermal contact with the bulb or there will be lag that can destabilize the loop.

Another approach is to replace the sensing bulb with copper bellows (like the ones found in some refrigeration thermostats) filled with oil) and a stepper motor and gear assembly to electronically adjust the valve. That's best for reusing a TXV that has lost its sensing charge.

BTW, opamps won't give you that much advantage over a plain TXV. A microcontroller is the way to go.

EEVs are hardly cheap from what I've seen. TXVs regularly go for a few dollars each on surplus sites.

Using a power resister such as a 10w rectangular one?

It's a 100 ohm, 2W sealed in heat shrink. It doesn't take much to change the superheat. Just connecting 5V makes a surprising difference.

Very cool idea. I'll stay tuned to this bat channel for further updates (I'm dating myself with that comment)