Large boiler Controls of Yesteryear;
Mine use a mechanical computer with Bourdon tubes, springs, balance beams, cams, levers, etc., and a mercury/oil filled dashpot to make a mechanical signal consisting of a silver plated brass wire "finger" moving up or down to make contact with other wires in order to increase or decrease fuel to the boiler.
This electrical (28 VAC) signal triggers a pair (one runs fuel up, one down) of timing relays (similar to http://www.grainger.com/Grainger/items/1A983 )for a .2 sec off delay. There is a 220 VAC feed powering the switched circuit. They are interlocked from each other (one relays output disables the other as long as it is called in) because the 220v signal (from the timing relays) calls in a 3 phase reversing starter. (3 phase 450 on this line) this activates a small motor hooked to a wormgear and cam arrangement that opens or closes fuel control valves, thus regulating boiler pressure.
And yes, this was a widely used system years ago and fairly well proven reliable. If anyone can name model and manufacturer, I'll mail them a beer.
So if anyone got through that, my question stems from the problem that the original starters used vacuum tube technology to generate the low voltage required for the silver plated brass "finger" contacts (remember them?). It sensed the v-e-r-y light contact pressure well. In a pinch, after the last original starter was burned, a refit was made. This includes the 28vac power supply, the timing relays (high impedance input, a 28 vac input runs about 3/4 of a milliamp) and the reversing starter.
So here's my problem:
The fingers (about 1/16 diameter) don't always trigger the timing relays. They get dirty and sometimes a jiggle fixes it, sometimes electro clean and a coffee filter. Sometimes you have to bend them a bit to a new contact spot. A higher voltage would work, but with all that poking and prodding would result in folks getting zapped.
Would a different metal be better (gold plate the fingers)? Would DC work better? Is there a small super high impedance circuit I could use that would better sense this light contact pressure and trigger an output but still be safe to touch?
I'm told that a CMOS has the kind of high impedance input I'm looking for. What I'd like to do is to use the 12vdc that is available in the enclosure. (along with 28, 120, 220, and 450 vac).
I however, am more of a relay, motor controls, and power guy, I'm not too good when it gets to transistors and such. I'd like to also integrate the time delay with the finger sensing circuit and get the whole mess from the fingers to the reversing starter fairly bulletproof, as the rest of it is.
Thanks for reading,
Mine use a mechanical computer with Bourdon tubes, springs, balance beams, cams, levers, etc., and a mercury/oil filled dashpot to make a mechanical signal consisting of a silver plated brass wire "finger" moving up or down to make contact with other wires in order to increase or decrease fuel to the boiler.
This electrical (28 VAC) signal triggers a pair (one runs fuel up, one down) of timing relays (similar to http://www.grainger.com/Grainger/items/1A983 )for a .2 sec off delay. There is a 220 VAC feed powering the switched circuit. They are interlocked from each other (one relays output disables the other as long as it is called in) because the 220v signal (from the timing relays) calls in a 3 phase reversing starter. (3 phase 450 on this line) this activates a small motor hooked to a wormgear and cam arrangement that opens or closes fuel control valves, thus regulating boiler pressure.
And yes, this was a widely used system years ago and fairly well proven reliable. If anyone can name model and manufacturer, I'll mail them a beer.
So if anyone got through that, my question stems from the problem that the original starters used vacuum tube technology to generate the low voltage required for the silver plated brass "finger" contacts (remember them?). It sensed the v-e-r-y light contact pressure well. In a pinch, after the last original starter was burned, a refit was made. This includes the 28vac power supply, the timing relays (high impedance input, a 28 vac input runs about 3/4 of a milliamp) and the reversing starter.
So here's my problem:
The fingers (about 1/16 diameter) don't always trigger the timing relays. They get dirty and sometimes a jiggle fixes it, sometimes electro clean and a coffee filter. Sometimes you have to bend them a bit to a new contact spot. A higher voltage would work, but with all that poking and prodding would result in folks getting zapped.
Would a different metal be better (gold plate the fingers)? Would DC work better? Is there a small super high impedance circuit I could use that would better sense this light contact pressure and trigger an output but still be safe to touch?
I'm told that a CMOS has the kind of high impedance input I'm looking for. What I'd like to do is to use the 12vdc that is available in the enclosure. (along with 28, 120, 220, and 450 vac).
I however, am more of a relay, motor controls, and power guy, I'm not too good when it gets to transistors and such. I'd like to also integrate the time delay with the finger sensing circuit and get the whole mess from the fingers to the reversing starter fairly bulletproof, as the rest of it is.
Thanks for reading,