Re: Modifying Type 109 for single-shot use?

Ed Breya

You should be able to make it into a DC-controlled, one-shot deal, but you may have to adjust the magnetic structure and coil bias such that it won't work in its normal resonant mode. Does it need to work normally too, or just one-shot? Without any magnetic mods, I think it should be able to do it if you can (over) drive the coil hard enough with the DC/step, but I don't know how much, or if the coil can take the extra power dissipation. I don't recall what form the Hg reed switch has - A, B, or C - but I think it's "A" so normally open with no field applied. If you make it so the desired fast edge happens when the reed is turned on, then the coil power needs only to be applied for a short time, I would think, and you can hit it pretty hard without much temperature rise. If the reed needs to stay closed for quite a while, then you may have coil dissipation issues.

I'd say give it a try and see how it works without magnetic mods, just with reasonable DC coil drive hooked in place of the normal signal. If you disable/disconnect the line charger supply, you can confirm switch operation with an ohmmeter, and not have to search for pulses on a scope. You may luck out and it will be fine with fairly low coil power. The reason I suspect that it may need quite a lot though, is that as I recall, the drive is supposed to be just enough to buzz the resonant armature to lightly jump the contacts into the Hg film, but not so much that the solid parts slam together. That would aggravate the wear, and slow things down. For DC drive, you won't have the benefit of resonance, and you will want to slam the contacts together enough to assure positive contact closure for the required time. On the plus side, the Hg reed should last virtually forever in this kind of application, assuming low duty factor.


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