1. Home
  2. Electronics Design
  3. Pilot relay on HVAC control board

Pilot relay on HVAC control board

I have a Viconics HP727 controller board in a swimming pool heatpump. The circuit I'm concerned with is the one that controls the 'pilot relay' for the compressor contactor. The pilot relay uses a 12vdc coil. It's contacts are 15amp / 24vac.

This board monitors thermostat, pool temp, water flow sensor and compressor high/low sensors. IF the water is flowing AND the thermostat is calling for heat AND the freon sensors are reporting OK, THEN the board will energize the 'pilot relay' coil. When the coil energizes, it closes the load-side contacts which are 24 VAC and those in turn energize the contactor coil and the compressor starts.

This board has gone through two pilot relays in a month. I replaced the original with the exact same part. I've seen other threads on this board talking about driver circuits of these miniature relays. It looks like there's more to the driver circuit design than you'd think. I'm not sure if a capacitor is used to assist the circuit to provide enough power to energize the relay's coil or not. But something on that board has caused the coils of two relays to run hot enough to open up. Both relay coils were open when tested with a multimeter.

I don't have the schematic of this board. Should I just chock it up to 'something failed' and buy a new board?

Sorry if this is off-topic. I'm a novice (can you tell?).

Reply to
jimlavella
Loading thread data ...

What does the coil voltage measure when it is energized (just a quick measurement on a cheap digital meter is close enough)?

Is the environment where the relays are located particularly hot?

What is the relay part number and manufacturer?

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Snip aggravating extraneous crap...

Open coil usually is due to overvoltage for the DC relay. This implies that you want to measure the so-called 12VDC ...

Reply to
Fred Bloggs

...

Well, even though it's apparently out of warranty (it must be or you'd have already taken it to the store (or got a service call)) it should still do no harm to ask the dealer about it.

Good Luck! Rich

Reply to
Rich Grise

Hide quoted text -

Spehro, My relay is a Hasco Relay part number = KLT1C12DC12

The voltage going to the relay was 13.20 volts. I soldered an LED across the coil connections to verify when I had power. I also soldered a switch across the load-side contacts. When the LED illuminated, I turned the switch on. The compressor started and I noticed the voltage to the coil at that time was 12.72.

One more thing. The only resistor that I could see that was wired in series to the coil was one with Brown/Black/Brown - Gold stripes (100ohm). I check it. It measured 100ohms.

Reply to
jimlavella

Hide quoted text -

Change the resistor to 220 ohms, 1/2 watt. That may give you time, once the replacement relay is installed, to find the real problem. The 100 ohm resistor is intended to drop about 3 volts, implying a 15 volt supply. (The computed value comes out to 15.24 volts, with the resistor dropping 2.54 volts.) The 220 ohm resistor will drop about

5.4 volts leaving about 9.84 volts for the relay, whose specs state that it must operate at 9 volts. 12.7 volts is high, but should not cook a 12 volt relay in a month, so I suspect something else is wrong. Look for an electrolytic capacitor connected to the same line that goes to the relay coil - the other side of the capacitor will be connected to ground. The capacitor will be marked with xx uF where xx is the capacitance, and it will also be marked with the voltage. Betcha the &(^$*# thing is marked with 16 volts. If it is, stop the repair action, take time out to curse the manufacturer of the circuit, and replace it with a cap of the same xx uF but at 35 volts. Pay attention to the polarity marking on the cap.

I'm taking a guess that the cap cooked and is allowing ripple voltage to smack the relay. You can use your DMM to see if that is the case. Put it on an AC scale, connect one lead to the relay coil and the other lead to a .1 uf capacitor. (The actual value of the cap doesn't matter - anything from .001 uf to .47 uf will do.) The other side of the capacitor goes to the other side of the relay coil. Whatever voltage your meter shows is added to the 12.7 volts. Your meter can't see that extra voltage when it is on the DC scale. If you see "big" voltage (like 15 volts) the diode that produces DC for the relay is shorted.

You also mentioned a 13.2 volt measurement (which makes sense with respect to the 12.7 when the compressor is running) which means the relay is energized at a higher voltage than I used in the computations (I used 12.7).

Finally, put a 1K resistor in series with the LED you added, if you don't have a resistor in there now. A typical red LED will be damaged by 12 volts with only 100 ohms to limit the current.

Ed

Reply to
ehsjr

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.