What voltage is too low to activate a 24 relay ?

KCA_2

Hello all,
I have a 24 V coil on a relay activating a number of items on the boiler. The relay seems to chatter and make noise. I'm getting 23 or better volts through the transformer. What voltage is too low to make it necessary to replace the transformer? The relay is new.... I would think that 23V is sufficient for the relay but what have you all learned?
Thanks....
:-)) Ken

Tinman

Typically, plus or minus 10%. 23 should be good. Are you using an analog meter or a digital meter?

hvacfreak2

If I see less than 25 I start looking for issues.

MikeSpeed6030

Are you measuring the voltage when the relay is called to be closed? Or when the xmfr has no secondary load on it? What is the VA rating of the xmfr? What is the VA requirement of the relay coil?

Tinman

Yeah I get that. Closer to 27 is what I usually see.

BobC

23 should be enough assuming that is at the relay terminals while it's trying to pull in. Maybe heavier gauge wire would give you a bit more.

That said relays are not precise so it's possible that one relay wants more, also try orienting the relay in a different plane if you can.

Bob

Jamie Hall

Measure the voltage at the relay coil when the relay is trying to close. If it is much less than 24 volts (the nominal on your transformer) you have a voltage drop in the circuit controlling the relay. A bad switch, bad connection, too small wire or too long, or an inadequate transformer are possibilities.

KCA_2

Thank you All...........
I use a digital meter, the existing transformer is 25 years old but.... question was with regard to minimum voltage to operate the relay.... so, anything less than 24V?

JUGHNE

You could have two issues. Both due to age.
Most transformers show 27 VAC and hardly drop under load.
Older relay coils and their steel core can lose their goodie.
The relay may have worked on your 23 volts previously, but now is just tired.
I would try the transformer first though. The voltage can drop during chatter and recover so quickly that most meters will not see it and show you a solid 23 volts. IMO FWIW

Zman

I would also look at the line voltage on the other side.
If you have normal voltage in and low voltage out, I would suspect a bad transformer.

BobC

If you can measure the voltage at the transformer and then at the relay coil - both while that relay is chattering.

They should be very close, if not either the wire is a bit small or something between them is dropping to much voltage.

Bob

ChrisJ

Depends on the relay.
Usually the data sheet will show the minimum voltage to close, and the minimum voltage for the contacts to open.

Off the top of my head, I thought a 24VAC relay would stay closed as low as 15 volts or so. I can't remember the minimum needed to close it in the first place though.

ChrisJ

Here's an example.

In this case, 19.2 volts is required to close the contacts.

KCA_2

How do you get 19.2 V? Please explain.....

ChrisJ

KCA said:

How do you get 19.2 V? Please explain.....

Rated coil voltage is 24. Minimum Pickup voltage is 80% of rated voltage or less. 24 * .80 = 19.2

Harvey Ramer

Usually when I see relay chatter it is caused by a bad connection. Do you have any wire nut connections or spade terminal connections? If so, you might want to clean them up.

KCA_2

Well I went there today. I couldn't get it to make any kind of noise at all. The homeowner says it's not a chatter it's more of a whiner squeal. That doesn't sound like a relay at all but he swears that's where the noises coming from. So after they're staying there for about 2 1/2 hours I decided to replace the transformer . I even called Honeywell to find out about temperature maximums and minimums voltages. Minimum voltage was 24 V. He said anything less than that and you're gonna have problems I was getting today 27 volts and 26 V fully energized. But replace the transformer anyway. I guess we'll find out over the next few days how everything works I'll let you know

JUGHNE

JAT.... if the relay is switching a 24 circuit sourced from the same transformer that supplies the coil.....then if the contract points are bad, overheating and creating voltage drop by the energy used to burn across the contacts causing the chatter.

If the downstream load coils are faulty and overloading the transformer, that also could create this condition. This might be a rare occurrence but I have seen it happen.
(This may be why Lennox engineers had relays controlling relays with each having their own power source???)

Again it would take a high end meter with sophisticated "capture the moment" feature to show you this on the first relay coil.

hvacfreak2

Lennox is still over using relays . I saw a new Lennox product last week , RTU , the first I have seen in 15 years or more. All electro-mechanical controls and I know I saw at least 3 relays required to energize a reversing valve solenoid. The problem mysteriously vanished after I pushed on the connectors ( about 12 or 14 ) with my meter leads. It will show up again when it's really cold I'm sure.

Major7

23VAC is definitely enough. If you're worried, get the specs for the relay and look at "pull-in" and "release" voltages. Make sure you have a good volt meter that measures true RMS, and make sure that the voltage stays true as it tries to pull in the coil. If your output impedance is too high, your voltage will dip when it tries to pull in the relay.

Jamie Hall

hvacfreak2 said:

Lennox is still over using relays . I saw a new Lennox product last week , RTU , the first I have seen in 15 years or more. All electro-mechanical controls and I know I saw at least 3 relays required to energize a reversing valve solenoid. The problem mysteriously vanished after I pushed on the connectors ( about 12 or 14 ) with my meter leads. It will show up again when it's really cold I'm sure.

I wonder which of those connectors you pushed on was the high resistance one... One of them was!

As to relays vs. solid state. Relays do have their little problems, true -- however, they are a lot sturdier than solid state when faced with peculiar voltages and voltage spikes.