Blinking LED controlling small relay - issues?

[xpzzzz]

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...

 

[mike]

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.

 

[xpzzzz]

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.

 

[mike]

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?

 

[xpzzzz]

Do you need to see the flash? or is it just a convenient way to flip the
relay on and off?

Just a convenient way. Seemed *really* convenient - one part in-line.