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Link Ring Trip Circuit Ommission


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While browsing through my collection of Link circuits online the other day, I noticed an ommission of the Ring Trip cct on the Link A2B+1 DTMF design. Shown below the main cct is the RT cct, composed of the relay RTR/2 a diode and the AC voltage source (between 30 and 40 VAC is usually OK for local intercom ccts,) and the relay contacts.

What should also be there, is a 2.2uF/250V capacitor (a large greencap/browncap or suitably voltaged MKT cap. This is because the relay itself will only trigger and operate on a DC circuit (such as a called party's handset going 'off-hook') while passing AC, and not operating. The 2u2 cap ensures that only AC current will bypass the relay coil, giving a more stable ring trip circuit which only operates when the relay coil senses DC current.

Austin Hellier

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