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Open Collector Driven Relay Circuit

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Author Topic: Open Collector Driven Relay Circuit  (Read 864 times)
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« on: June 10, 2019, 02:50:07 PM »

I recently had an open collector transistor circuit fail which was used to key a small low current reed relay. The transistor shorted. The relay operated at 12V @ ~15ma. I had a diode,1N4006, and a .1 cap across the relay coil.

Iím wondering if there is any way to improve this common design practice.

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Nico and Chappie (Chappie is the dog...)

« Reply #1 on: June 10, 2019, 05:33:15 PM »

Must have been a weak transistor.
The protection depends upon the application. There isn't much reason for the 0.1 uF except RF decoupling in a high RF environment. A diode should be sufficient.
But the diode makes the relay slow at fall off. If you want a very fast relay, leave the diode away and limit the collector voltage spike with a zener of e.g. 20 - 36 V across the transistor or a transzorb.
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