I've got a N/O sensor which is designed to last at least 10 years. I want the switching circuit it drives to last at least that long, plus 50%: 15 years. The sensor and circuit will be replaced every 10 years or when the sensor closes, whichever comes first. The circuit is simple: (schematic below) 24VAC into a 4 diode bridge. NO filter cap. Sensor from + to NPN base through 10K. Base to ground through 330K. 24VDC relay coil (with diode across coil) from + to collector. Emitter to ground. Ice ~ 25mA when sensor closes. The sensor will operate once during the 10 year interval, so the transistor switch will be off. The question: will the long term exposure to unfiltered ripple have any deleterious effects? . +--------|

No, as long as the transistor voltage rating is compatible with possible transients. Like switching the transformer primary off. But it gets interesting when your sensor closes. -- John Larkin Highland Technology, Inc lunatic fringe electronics

Ripple harmful in this circuit?

Jul 23, 2017 25 Replies

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ehsjr 8 years ago

Thanks John.

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John S 8 years ago

How long will the sensor remain closed when it finally closes?

What will be exposed for a "long term"? Do you mean that the sensor and the relay will remain closed for a long time? Like maybe 9 years?

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ehsjr 8 years ago

Thanks, Phil. Good points on the cap suppressing the transient and the use of a sealed relay. The relay I'll be using operates at 16 mA and drops out at 4 mA - I just tested it today. It's a sealed relay, and doesn't chatter in the circuit.

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ehsjr 8 years ago

Periodic testing, using a momentary switch across the sensor connection points.

Thanks, Ed

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ehsjr 8 years ago

It will stay transferred until AC power is removed. The sensor is low current, < 5mA.

Thanks, Ed

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Phil Allison 8 years ago

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** With full wave rectification it will be fine - most DC relays are rated to work this way. Half wave, however, is not usable.

..... Phil

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