12VAC smal ice cube relay 1

[wrsharper]

We have some new equipment that is using 12VAC relays powered by 16.5VAC. This is over 30% designed coil voltage. The relays are all hot. The wiring matches the drawing. Is this in the range of voltage tolerance? I searched the internet but was unsuccessful to find the answer so here I am where I get factual and accurate advice.

 

[Skogsgurra]

The standard specification for maximum AC coil voltage is 110 % at 25 C ambient. I doubt that your relays has any different specification. Is the equipment running off a non standard voltage? Or other frequency? What make are the relays? Sometimes the control voltage transformer was designed for a lot more load than actually connected. Poor regulation makes voltage go high when idling.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[geekEE]

That sounds awfully high. 38% more voltage means that the power dissipated by the relay coils is going to be 89% higher than at 12V.

 

[itsmoked]

Yeah, good question. Where is the 16.5 volts coming from?

We can cough up some solutions for helping the relays but I would imagine other devices,(not relays), are suffering in this scheme also. So it may make more sense to fix the 16.5V once, instead of five or ten times.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[wrsharper]

The circuit performs a ground sensing safety function. If a spiral conveyor binds up, this causes it to lift up and push a metal rod against ground causing the circuit to produce a fault signal to stop the equipment.

How the circuit is installed:
The circuit starts at a unregulated 24VAC transformer and one side of the secondary is tied to ground. The 24VAC travels through a diode and current limiting resistor. It then goes to the probe and then back to the 12VAC relay and then to ground. This causes the relay to energize which then pulls up a set of holding contacts which puts 24VAC in the same circuit described before but it bypasses the diode. If the probe touches ground then ground is on both sides of the relay and it drops out sending the fault and opening the holding contacts.

This is a sub-assembly not manufactured by the equipment OEM and I have written them but did not get a response.

I think the circuit is designed wrong and that the diode should never by bypassed. Does this sound right?

 

[waross]

As you describe the circuit it sounds as if you are supplying the relay with 1/2 wave rectified DC to get a definite pull-in. Then the resistor limits the current to the relay. The resistor also limits the current through the probe when it is grounded. What type of meter did you measure the voltage with? Did it respond to average or RMS values? Is your supply voltage correct? You may have a measuring error. You may have a limiting resistor off spec.
Does this unit have any approvals on it?
respectfully

 

[wrsharper]

I used a true RMS Fluke 83 voltmeter and backed it up with another meter. The voltages are correct and the high voltage is reflected by the high temperature of the relay coil. You are right waross, they use half cycle to energize the relay and then hold it with full voltage that is only limited by the 75 ohm resistor and relay coil. I per chance had a 22 ohm 5 watt resister in my tool box that I put in series with with the hold voltage and that was the right number because relay coil voltage fell to 12.5VAC. I think I will try this out and check for nuisance trips. That is the only reason I can think of that the designing engineer would design the circuit too hot.

 

[OperaHouse]

Sounds like a nice aplication for a 7805 three terminal regulator running in constant current mode instead of the resistor. That would give you the added sensitivity to prevent false trips without over voltage on the relay. Calculate resistor for 5V at relay current and a couple of .1 uF on input and output. Power supply can remain unfiltered half wave. Suitable heat sink required.

Another option is to put a zener somewhat over 13V (because of half wave) on the relay coil. This will throw the excess heat into the resistor instead of the relay. Theb you will have resistors burning out!

 

[wrsharper]

operahouse, I am at home and so do not have any access to documentation. Will a 7805 output an AC voltage? They only pick up the relay with half wave AC and hold it up with 16.5VAC. I tried installing a general purpose diode where the 16.5VAC enters but the relay only chatters. I, so far have not figured out the reasoning behind the design of this circuit. We have another spiral that has the same type of design but different manufacturer(for probe faults) but that spiral is down and locked out for rework so I can not check to see if the relays are getting hot. This type of circuit in that spiral has never given us any relay problems. The one I am working on is a fairly new installation and the probe fault sensing is vital. There must be some reason for the wacky design.

 

[itsmoked]

Best solution would probably be a 20VAC transformer. Resistors and such are all band-aid type fixes. (and band-aids all fall off eventually - if you get my drift..)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[wrsharper]

itsmoked, Got your drift and that was my original plan before I started this thread. I was searching for theoretical reasons for the circuit design. I found a 20VAC transformer that would fit my application. My concern is nuisance trips. There must be some reason for overdriving the relay coils. I measured relay temperature and the base of the relays are above 200F and the highest temp I measured is 228F, thus my concern. The resistor is only a testing vehicle and so far it is working great.

 

[itsmoked]

Sure be fun to see a picture of this crazy sounding contraption..

There could also be NO good reason... LOL Maybe they had 24V transformers and went with them. Frankly a failure prone safety system is not a safety system if things start burning or failing. And things will - as you have no doubt surmised.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[wrsharper]

Love to send you a drawing but that is beyond my capability. It is an extremely simple circuit with one transformer, one relay, one diode and one resistor. This circuit is duplicated many times for each probe but that is it. That is the reason I am having difficulty understanding the rhyme or reason. I would have discounted it as only a quirk but the circuit is duplicated on another piece of equipment with the same purpose. I will continue with the modification if the resistor test passes.

 

[OperaHouse]

With the diode out of the transformer, you have half wave DC. The 7805 would work with that waveform. With the current limit action the relay the relay would see more of a square wave. The diode on the relay shouldn't have made much difference.

 

[Skogsgurra]

Hey! I think they just plugged the wrong relays in. Try 24 V AC coils. The DC produced by the diode should pull them in safely. And then the diode is bypassed by the contact. Current limited by the resistor. Might be as simple as that.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Good call Gunnar. I like it.
Respectfully

 

[waross]

Re the wrong relays, I once got a shipment of about six standby generators with the wrong voltage relays in the control cards for the automatic transfer switchs. The fix was simple. I had the proper voltage available if I changed the voltage sensing point, which I did. (Time constraints, I couldn't wait for parts to arrive.) However the supplier sent a complete set of replacement circuits. The original fix has been working for years and I have spares for the next few generations at least.
Does equipment sometimes come with the wrong relays installed?
It happens.
Respectfully

 

[wrsharper]

Bingo waross, The lock out was off the older unit and I was just able to make comparisons where this type of circuit has worked for years. Everything is the same except for the relays. The relays in the old unit are physicaly larger with a lower coil resistance. This drops the 24VAC, after the 75 ohm resistor to 13.5VAC instead of 16.5VAC like in the new unit. The sensible thing to do now is change the transformer on the new unit to either 16VAC or 20VAC secondary.

 

[waross]

Thanks wrsharper, but you're embarassing me. This was skogsgurra's call and I was just commenting on it. LPS for Gunnar
respectfully

 

[wrsharper]

Somehow I missed Gunnars second post. Troubleshooting is so easy when you have a working system to compare measurements with so his insight was exceptional.