Compact Logix & Interpose Relays

georgeofthejungle02

Member
G
Join Date
Mar 2004
Posts
60
Hello All:
I'm looking for feedback/input regarding an unusal relay humming problem.
We are using A.B. Interpose relay 700-HLT1U1 on the compact logix output card 1769-OA16.
When we power the cabinet up, all the relays starts humming away with the LED light turning on.
The voltage across the coil in its off state is 30VAC.
We've used these relays before and had no issues with humming other than the LED light does not turn off due to leakage currents.
Any inputs on this is greatly appreciated.
George
 
The OA cards have a triac output. You want to use the 700-HLT1L1 instead of the 700-HLT1U1 with triac output cards such the 1769-OA16 or 1747-OA16. The "L" version of this relay has a built in limiter/supresser which kills the leakage current. The "U" versions work fine with relay outputs such as 1769-OW8's.

Hope this helps.
 
George,

Yea, those interposing relays with the built-in supression circuit are more expensive. I've swithced over to using the 1769-OW8 output cards (cheaper tha the OAs, and use them with the "U" type interposing relays. Can you use a 1769-OW8 card insted, AND would your supplier swap with you on your 1769-OA8?
 
I think diode are used in DC circuits as free wheel diodes to prolong the life of the relay coils. In our case is AC circuits...
Click to expand...
Actually the free wheeling diode is there to protect the solid state switching device within the output module against the back EMF generated by the relay coil.

Instaed of replacing the relays try using a pull down resistor
say 5k, 5w across the relay coil it will pull down the Off state voltage to a level that may not trigger the humming.
Using relay output card is not a good idea. My experience with it is that it does not last very long if you switch outputs
on and off frequently.
 
Jiri, an interesting point you made regarding the pull down resistor, I am just curious as to why the resistor has to be as big as 5W? How are you rating that? Is it the ON state you are rating it at?
George
 
I calculate that you will need 3.38 Watts for the On state at 130 V
and Off state would pull you down to about 10 V at 2 mA of leakage.
These modules usually leak 1 to 2 mA.
I don't know if 10 V will stop the humming, you could go to 2.4k
at 10 W and that would put you roughly at 5 V and hopefully that should do it.
 

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