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Quiet Relay?

Ed Phelps

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Dec 18, 2014
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It's time to replace the master relay in my IBM master clock. The replacement that I put in makes a lot of noise, is there a quiet relay that I can purchase? It's got a 24 volt AC coil and carries 24 volt DC with maybe an amp of current.

Thanks,
Ed
 

Ed Phelps

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Dec 18, 2014
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Will that work? All the solid state relays I've looked at, including this one, call for a direct current control voltage. I have 24VAC.

Ed
 

Ed Phelps

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Dec 18, 2014
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Thanks guys for the great suggestions!!!! Hope you had a good Thanksgiving.

Ed
 

Toughtool

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Aug 12, 2016
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I don't think it will work! I had suggested using a solid state relay to replace the mechanical master relay in an IBM master.

I had the spontaneous idea today about doing such a conversion on my own master relay because I have been unsuccessful in getting it to work for more than two weeks. The contacts are bad and I have tried a couple of times to dress and clean them up.

I started looking for how to do that from some of the schematics in the ITR Service Instructions #230 and found that almost all of the diagrams, the master relay is switching the AC side of the bridge rectifier and that the coil is an AC coil to boot.

So you need to verify that your master relay circuit is either switched on the AC side of the rectifier or not. These solid state relays will not work on an AC circuit. A triac may. I am going to continue to look for a solution to use a modern solid state device to bypass my master relay, but leave the original relay in place for aesthetics.
 
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Ed Phelps

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Dec 18, 2014
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Yes mine also is switched on the AC side so that's why I was asking. All the solid state relays I could find were DC.

Ed
 

Toughtool

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Aug 12, 2016
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That is encouraging, they must be using a Triac.. I will see if they have one that will work on 24 volts for the input. The one listed says 80 to 250 volts. We need one that operated on 24 volts input as well as output.
 

Toughtool

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Aug 12, 2016
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Yes, I was.
The first schematic is the general AC master clock relay circuit that seems prevalent on ITR and IBM masters . This is actually out of a distribution cabinet schematic but best represents the circuit for explanation here.

Description of circuit to pick the master relay:
If you follow the path from T1, up to the movements "min cont" (minute impulse contact) through the N/O contact set, then down to the "run" switch, down to the relay coil, then back to T2. This circuit operates the master relay every minute, normally.

The relay contacts then connect T3, through a fuse to one side of the +/- tab of the bridge rectifier. T1 is connected to the other +/- tab of the rectifier. I would have expected that the T3 connection to be at T2, but it could be that T2 is a center tap and that 24 volts is between T3 and T2 as well as between T2 and T1. This scheme allows a low current AC voltage source to pick the relay, which will connect 24 volts at a maximum of 2 amps for the supervised control circuit, thus saving the minute contact from damage from excessive sparking.

The second circuit, grabbed of the web, shows an arrangement where a Triac is used to connect an AC line together, being triggered by an opto-isolated Infrared (IR) LED circuit. This IR LED/Triac pair is driving a larger Triac for more current. Notice the bridge rectifier ahead of the IR LED which can rectify the 24 volt AC and [using a pair of resistors] limits the current of the minute impulse circuit to 20 ma, the required current for the IR diode. This should cause minimum sparking at the minute impulse contact.

I think this scheme will work, can be built somewhat compact, and operate as the mechanical relay without noise, no sparking and can be easily hidden.

GeneralMasterCircuit.jpg Clipboard01.jpg
 

ElectricTime

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NAWCC Business
Sep 28, 2002
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One type of relay that is completely quiet - and not solid state - BUT possibly illegal (mercury) in some states - and expensive.

It's a Durakool Relay - link here - these relays will last forever.

I think MXFRANK has the correct solution - the Crydom relay though.

Note sometimes these relays leak a bit depending on the load - but a 1 amp load will probably drive the output voltage to zero.
 

skruft

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Aug 5, 2002
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Here is what I did with a similar situation, but involving a door chime setup, to use parts I had on hand already, and where AC power was available for the relay coil. I took a small sealed DC relay and powered it with AC through a tiny full wave rectifier, all mounted on a small circuit board. The relay is almost silent and could never be heard through a clock case. Just like Toughtool did above.