I have built a circuit that uses a flashing LED (flashing function built- in) to control a relay; the relay resistance is the current-limiter for the flashing LED. Do I need to worry about voltage spikes killing the LED, and if so, is there a simple way to prevent that. LED 5V, 12v supply.
This works now, but it would be nice to know if it will not break two weeks from now...
What's the max voltage spec of the led in the off state? If it's less than 12V, you're likely exceeding it. If it's more than 13V, put a diode across the relay coil.
When people state, "12V" they often mean, "I'm gonna plug it into a car." If that's the case, a car ain't 12V. It's 15V or so on its best day and maybe a hundred volts when you let go of the starter button on a bad day.
Thanks.
12V gell cell, and yes I knew it was nominal, and about cars and spikes; Don't know the rating on the LED, a diode just in case then.
The diode can't hurt, but it also won't help if the led breakdown in the off state is less than about 16V. I don't have any experience and don't know the specs on the LED. I'd guess that a 5V flashing led would be stressed at 16V, but that's just a guess. If you need reliability, I'd at least stick a scope on it and see what the waveform does.
Sounds like a high risk design in several areas...but it's hard to knock success.
For the cost of 2 resistors and a NPN emitter follower...well...it's a common-base amplifier, but it looks a lot like an emitter follower when you draw it, you can eliminate the LED problem...but you raise the power consumption and don't solve the potential problems you have with under-driving the relay.
Your potential solutions all revolve around the characteristics of that LED subsystem.
Do you need to see the flash? or is it just a convenient way to flip the relay on and off?
yes
Just a convenient way. Seemed *really* convenient - one part in-line.
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Too many unknowns and undocumented performance to be worthwhile. The curren= t is internally limited to some small value- could only activate really tin= y relays, the blinking LED has to keep a minimum voltage drop for glitch fr= ee operation like 3V in some cases, the blinking frequency is voltage depen= dent and on the high side for a relay- like up to 3Hz, behavior is not char= acterized for sudden voltage drops across the LED like what you get when yo= u put in series with something else.
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